Flexible and Lightweight Solutions for Energy Improvement in Construction: A Literature Review

Abstract

Innovative materials and systems for flexible and lightweight energy-efficient solutions in construction can help achieve the objective of more efficient buildings. This literature review categorizes these solutions into three categories: materials/systems topology, design flexibility from 3D-printing technologies, and innovative solutions for building envelope designs. The review shows a significant increase in interest in this research topic in recent years, with an average annual growth rate of about 73%, with most research focused on the design and thermal aspects, as well as the material typology and 3D-printing technologies. According to the review, flexible and lightweight systems can be applied to all building sectors, and retrofitting existing buildings may become the primary approach. However, there is no specific European regulation for these systems, and a more holistic design approach is needed, involving both designers/constructors and users, to plan for actual social, economic, and environmental impacts.

1. Introduction

Buildings use about 30% of global energy, which increases to 34% when including the final energy use associated with the building construction. In particular, over the past ten years, the demand for energy in buildings has grown at an average annual rate of just over 1%; in 2020, there was a reduction due to the COVID-19 pandemic, but the demand for energy in buildings rose again by about 0.8% in 2022 when compared to 2021 (131 EJ), showing a continuous trend growth [1]. In 2022, space cooling demand increased, compared to 2021, by over 3%, whereas space heating decreased by 4% due to a mild winter in some regions. In the “Net Zero Emissions (NZE) by 2050 Scenario” (proposed by the International Energy Agency—IEA, which outlines a plan for the global energy sector to attain net zero CO₂ emissions by the year 2050), the buildings’ energy consumption should be about 25% by 2030 [2]. Achieving net zero CO₂ emissions globally by 2050 relies on many uncertain factors, such as the citizens’ willingness to change behavior, international cooperation, and the pace of innovation in emerging technologies [3].

On the first and second factors, the European Union (EU), with the Energy Performance of Buildings Directive (2018/844/EU), has updated new features to show the EU’s dedication towards upgrading the buildings industry in response to technological advancements and to encourage more building renovations [4]. Also, other initiatives such as the United Nations Sustainable development goals [5] and the EU Nearly zero-energy buildings [6], Green Deal [7], and Energy Positive Buildings [8,9] have been adopted and promoted in these last years, aiming to have an impact on the citizens’ behaviors and habits.

On the technology innovation factor, the advancement of digital design software and construction technologies is leading to more and more complex building designs [10]. Architects and engineers who deal with these types of buildings face various challenges [10,11,12,13,14], due to unique shapes and complex components. In this context, most of the inefficiency comes from over-specification due to old techniques and materials that are not suited for complex shapes, thus leading to a poorly optimized final product [15]. To achieve efficiency and sustainability, the construction industry must face significant challenges to develop/produce innovative materials that are more durable, lighter, stronger, and environmentally friendly, as well as to develop the appropriate design strategies to exploit the potential of such materials fully [14].

On the first topic, innovative advanced materials, such as smart or functionally graded materials, are being developed (e.g., concrete mixtures, shape memory alloys, phase change materials, polymers, and textiles), and experts in the field are researching the extent of their performance from different points of view, as evidenced by a growing body of literature on the topic [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88]; these materials usually offer clear advantages over conventional construction ones, such as ease of production, installation, durability, low maintenance requirements, lightweight properties, and the capability of being molded into intricate shapes.

On the second topic, optimized design strategies are necessary to exploit the potentials of new construction technologies fully, reducing the amount of wasted materials, thus lowering the energy and environmental impact of the production sector and improving the resilience and overall performance of the building sector. Indeed, the impacts of technologies able to produce optimized structures quickly and accurately are already quite consolidated in other industries such as aerospace and automotive [89,90]. Construction technologies, such as additive manufacturing (AM), are changing the fundamental approach to design; compared to conventional construction methods, technologies such as 3D printing have seen remarkable progress in the last ten years, and they can potentially offer numerous benefits such as better material efficiency, more design flexibility, and decreased construction expenses and time [12,14].

However, as the use of these materials and systems grows in the construction sector, there is still a gap of knowledge in predicting their performance or selecting the most functional design, due to a lack of holistic studies and the challenges in characterizing and studying an ever growing number of solutions [14,91]. Several review papers are already available [10,11,12,13,14,92,93,94,95,96,97,98,99,100,101] but focus on examining each material or aspect separately. In particular, Li and Zanelli [11] provide a review on flexible Photovoltaic (PV) systems, whereas Moskaleva et al. [10] discussed the impact of Fiber-Reinforced Polymers (FRP) on the design and shape generation process of freeform structures; several researchers [12,13,14,92,93,95,96,97,98,99,100,101] focused their attention on 3D-printed applications, processes, and materials, as well as the thermal performances of 3D-printed solutions for buildings [94].

So, based on the authors’ knowledge, none of the reviews on these systems/solutions, naturally flexible in design or physical characteristics, have taken a holistic approach in their studies. Indeed, the aim of this review is to present what is the current state of the art in research on advanced materials for flexible and lightweight solutions for energy improvement in construction, in order to define the current most investigated topics and the possibilities for further studies.

1.1. Aims and Organization of the Review

The main objective of this review paper is to examine the existing literature that involves advanced materials for flexible and lightweight construction solutions to support their usage in the research and design of energy efficient and sustainable buildings. In particular, considering the different building elements, this review mainly concerns the analysis of the studies focused on the improvement of the building envelope: it is the barrier that defines the indoor environment and through which all the exchanges between the indoor and outdoor happen, thus primarily affecting the buildings’ behavior. The main questions that this review aims to answer are:

• What are the current possibilities offered by flexible and lightweight materials and design solutions?
• Which types of analysis are currently considered when assessing their performance?
• Can these materials/solutions be used to design building envelopes with complex and efficient features such as kinetic or responsive facades?

Therefore, this work is structured on the following three categories of study:

• Category 1: papers focused on flexible and lightweight materials and systems;
• Category 2: papers that examined the design flexibility offered by 3D-printing technologies;
• Category 3: papers that investigated new solutions in construction that use flexible and lightweight materials to create more complex and efficient envelope designs (e.g., reversible transformations in response to external stimuli, as in responsive or kinetic façades, etc.).

Section 2 explains the review methodology used to collect the papers. An overview of the gathered articles is presented in Section 3. In Section 4, the selected studies are analyzed in detail and discussed, following the three main categories defined above. In conclusion, Section 5 provides an overview of the present work, comments on the findings, and suggests potential areas of future research.

2. Materials and Methods

This review was carried out following two main steps. The first step was to select a database, and the second was to define a set of queries for downloading relevant metadata. Therefore, after evaluating various databases such as PubMed, Scopus, Web of Science, and Google Scholar, we selected the Scopus database developed by Elsevier. This decision was based on its extensive coverage of indexed journals and its ability to combine PubMed and Web of Science features [102]. In addition, Scopus covers subject areas such as engineering, energy, environmental science, computer science, and materials science, which align with our research goals. In particular, this database covers 51,508 papers from 1937 to the present on the topic of building energy efficiency.

Scopus provides an advanced search feature that allows for specific queries using different operators and codes [103]. To carry out this review, four different queries were defined on the basis of the review topic: “Flexible and lightweight solutions for energy improvement in construction”. The first query (Query 0 in Figure 1) is generated to ensure that the results are aligned with the review’s objective. The green group of keywords was associated with the fabric/textile materials used for the building envelope, whereas the part of the query highlighted in orange was associated with the 3D-printed materials used in the building; in addition, the blue color highlighted the AND operator, whereas the red color represented the OR operator. Query 0 returned 3422 documents published between 2017 and 2024, of which 3046 were associated with the abovementioned subject areas.

Due to the large number of results, the authors divided the query into three different sub-queries, one for each category reported in Section 1.1, and then operated a screening of the results to exclude some works that were not directly related to the review topic. Figure 2 reports the three different sub-queries used to create the final database. In particular:

• Figure 2a shows the query (Query 1) configured to find the paper associated with the fabric and textile materials used to improve the building envelopes, particularly their façades;
• Figure 2b reports the query (Query 2) adopted to find the paper related to the design flexibility offered by 3D-printing technologies;
• Figure 2c shows the query (Query 3) used to find the paper associated with new solutions in construction that use lightweight and flexible materials to create more complex and efficient envelope designs.

By using these three specific queries, the authors were able to conduct a detailed analysis of 45 papers, including 16 documents (of which 2 review papers) related to the first category, 22 articles (of which 4 review papers) associated with the second category, and 7 studies (of which 1 was a review paper) focused on the third topic.

Then, the three databases were implemented with documents found manually by the authors. In particular, these were added:

• A total of 8 articles investigating flexible and lightweight materials and systems (category 1);
• A total of 30 (including 8 review papers) documents considering the adoption related to the design flexibility offered by 3D-printing technologies (category 2);
• A total of 5 research investigating new solutions in construction that use lightweight and flexible materials to create more complex and efficient envelope designs (category 3).

Therefore, a total of 88 documents (73 research articles and 15 review papers) were analyzed in detail. Figure 3 summarizes the screening process, the results for each query, and the final number of documents identified for each category.

3. General Analysis of the Collected Documents

This section reports the analysis of the final databases (73 research articles), carried out to outline the trends in interest of the research topics.

Table 1. Data associated with the selected research articles published from 2016 to August 2023 [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88].

Parameter	Description	No.
Documents	Total number of documents	73
Sources	Number of documents published in international journals	52
	Number of documents published in proceeding of international conferences	20
	Number of documents published in the book chapter	1
Period	Years of publication	2016–2023
Authors	Total number of authors involved in documents included in the review topic	327
Countries	Total number of countries associated with the authors	26
Authors per article index	Ratio between Authors and Documents	4.48

and Figure 4 provide a summary of the documents [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88]. In particular, about 71.2% came from international journals, around 27.4% were published in proceedings of international conferences, and only 1.4% were published as book chapters. The research topics were investigated by over 300 researchers from 26 countries, mainly the USA (~16%), Italy (~12%), China (~7.4%), United Kingdom (~7.4%), Switzerland (~7.4%), and Germany (~6.3%).

Figure 5 summarizes the subject areas covered by the collected documents [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88]. In particular, this figure highlights how more than 50% of the papers are connected to the engineering area, followed by the materials science and energy areas (16% and 11%, respectively); the computer science and environmental science areas are the most minor regarded (10% and 9%, respectively). It is important to emphasize that approximately 60% of the documents can be categorized into three subject areas.

3.1. Documents Focused on Flexible and Lightweight Materials and Systems

This section provides statistics from the 22 papers on flexible and lightweight materials and systems [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. Figure 6 highlights the trend in the published research [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37] that considered this topic. The Average Annual Growth Rate (AAGR), calculated according to [104], is about 56.5%. In particular, the year with the highest Annual Growth Rate (AVG) is 2022 (equal to 300%), with 6 more published documents than 2021. This figure indicates a growing interest from authors in recent years, except for 2020 and 2021, which may be attributed to the impact of the COVID-19 pandemic.

Figure 7 reports the authors’ countries of origin. In particular, if multiple authors were from different countries, we included their countries of origin in the count. This figure highlights that the interest in flexible and lightweight materials/systems is coming from all continents, and the most involved Countries are Italy (6 documents), Germany (4 documents), the United Kingdom (4 documents), and Spain (3 documents). Furthermore, it is evident from Figure 7 that researchers from the Near East, including Iran, Qatar, and the United Arab Emirates, are also interested in this particular research topic.

3.2. Documents Focused on Flexible and Lightweight Design by 3D Printing

This section provides data associated with the 40 documents focused on 3D-printed, flexible, and lightweight designs [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77]. Figure 8 highlights the trend in the published research [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77] that considered this topic. In particular, for this review, we did not find any relevant documents published in 2016 or 2017. The first works that caught our interest were from 2018 onwards. The AAGR, calculated from 2016 to August 2023, was about 82.7%. This figure indicates a growing interest in the last 3 years (2021, 2022, and 8 months of 2023), with 29 documents published on this research topic.

Figure 9 reports the authors’ countries of origin (in the case of multiple authors from different countries, we included all of their countries of origin in the count). Also, in this case, the interest in flexible and lightweight design and 3D printing is coming from all continents, and the most involved Countries are the USA (11 documents), China (5 documents), Italy (5 documents), and Switzerland (5 documents).

3.3. Documents Focused on Flexible and Lightweight Solutions for Complex Envelope Designs

This section reports the statistics associated with 11 published papers on complex envelope designs based on flexible and lightweight materials, systems, and design [78,79,80,81,82,83,84,85,86,87,88]. For this review, no relevant documents were found published from 2016 to 2018. Figure 10 highlights the trend in the documents published in international conferences or journals [78,79,80,81,82,83,84,85,86,87,88] that considered this topic.

Considering the data reported in Figure 10, the AAGR, calculated from 2016 to August 2023, is about 78.6%, and 2023 returned the highest AVG equal to 400%. Figure 11 reports the authors’ countries of origin (if there were multiple authors from different countries, we included all their countries of origin in the count).

This figure highlights that the most involved countries in this topic are South Korea (5 papers), Brazil (2 papers), and USA (2 papers). Furthermore, among the EU countries, only Portugal focused research in the fields of flexible and lightweight materials and systems, as well as flexible and lightweight designs by 3D printing.

4. Results and Discussion of the Review

The following sections report in-depth analyses of the selected studies, organizing them into the three main categories highlighted in Section 1.1. In each category, the studies have been then further classified and grouped according to different aspects, in particular, based on the construction topic they focused on, starting from (i) the entire building envelope or (ii) construction components, to specific envelope sections as (iii) the façade, (iv) the roof, or (v) shading. This topic classification was selected, as it provided a clear and easy distinction between many studies, which could be further analyzed considering other parameters, such as the type of analysis or the materials. Figure 12 reports the results of this classification process, highlighting the most addressed topics (façade, construction components, and whole envelope) and the different contributions of each study category. In particular, in the first category (flexible and lightweight materials and systems), 15 studies were focused on the façades, whereas, in the second category (flexible and lightweight design and 3D printing), the focus shifted towards the construction components (18) and the whole building envelope (13) topics, and no studies was carried out on roofs. Finally, the studies in the third category (flexible and lightweight solutions) covered mainly the shading systems (8).

Each investigated study addressed the five topics following different criteria and carrying out different tests, analyzing a specific aspect or evaluating the performance with a broader analysis through a set of different tests. In particular, the challenges and solutions on the design process were the most investigated aspects (in 44 studies, especially in the studies where 3D printing was involved), followed by the thermal (in 28), energy (in 17), and optical (in 14) analyses. Figure 13 summarizes the most considered analysis type and each category’s contribution.

4.1. Documents Focused on Flexible and Lightweight Materials and Systems

The use of flexible and lightweight materials and systems in construction presents several challenges that have to be taken into account and overcome in the design stage; the first and foremost challenge that researchers and designers are faced with when adopting such materials is how to effectively use them as main elements in a structural component, such as the whole building envelope. However, they offer several opportunities, starting from the possibility of being easily installed on existing buildings without affecting their structures, providing a solution for a significant percentage of the construction sector, as well as being molded in complex shapes and integrated easily with conventional materials and components.

Figure 14 shows the word cloud generated using the keywords of the collected research articles in this category [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37], visually highlighting the most investigated aspects and materials.

All the details of the collected research articles included in this section are reported in

Table A1. Additional details of the 22 collected research articles included in category 1.

Ref.	Year	Country	Keywords	Study Object	Type of Study	Type of Analysis	Material	Envelope Section
Whybrow [34]	2016	United Kingdom	Double-sin membrane cells; Composite of steel and fabric; PTFE-Glass; Daring sculptural form.	Specimen of a tensile fabric shading structure	Experimental	Design (fabric warp and tension distribution); Optical (solar transmission)	Glass-PTFE fabric	Roof
Zhao et al. [35]	2017	China; United Kingdom	Large-span reticulated domes; Membrane roof; Solar radiation; Non-uniform thermal load; Non-uniform thermal effect; Field monitoring	Model of an ETFE membrane dome	Simulation	Thermal (stress on the structure)	ETFE	Roof
Stegmaier et al. [36]	2018	Germany	Silicone; Film; Shading; Stretch; Façade; Window	Specimen of shading device	Experimental	Optical (transmission)	Silicone (Wacker; CENUSIL R360)	Shading
Menéndez et al. [26]	2018	Spain; Germany	Organic photovoltaic; ETFE; Textile architecture; Façade lighting; Energy harvesting; Multifunctional module	Specimen of an ETFE module integrating OPV and LED	Experimental	Design (integration of flexible ETFE membrane; PV and LED); Energy (PV and LED efficiency)	ETFE; OPV; LED	Façade
Flor et al. [27]	2018	United Kingdom	Ethylene-tetrafluoroethylene (ETFE); foil Ray-tracing; Transmittance; Reflectance; Building simulation	Specimen of inflatable ETFE cushion	Experimental	Optical (Transmittance; Reflectance; Absorptance; Useful daylight illuminance); Energy (lighting; heating; and cooling energy demand reduction)	ETFE	Façade
Daza and Zamora [19]	2019	Colombia; Spain	Ventilated air cavity; Airborne sound insulation; Lightweight ventilated façade (LVF); Building refurbishment	Specimen of ventilated façade with lightweight cladding	Experimental	Acoustic (insulation)	Cement Board: cement mortar reinforced with glass fibers	Façade
Fadli et al. [33]	2019	Qatar; USA	Biofaçades; Smart Living Materials; Innovative Technology; Sustainable Design; Smart Buildings (SBs) and Smart Cities (SCs)	Specimen of green façade	Experimental	Thermal (surface temperatures)	Plants	Façade
Moreno et al. [37]	2019	Spain; France	Ethylene tetrafluoroethylene (ETFE); Organic photovoltaics (OPV); daylighting; Thermal performance; Energetic simulation; Building integrated photovoltaics (BIPV)	Model of four different ETFE-based shading panels	Simulation	Energy (space heating and cooling demand; PV production); Optical (Daylight autonomy; Useful daylight illuminance)	ETFE	Shading
Echenagucia et al. [16]	2019	Switzerland; Australia	Flexible formworks; Form finding; Concrete shells; Active control	Specimen of a fabric-reinforced concrete roof-shell	Experimental	Design (use of fabric as a structural element along with sprayed concrete)	Polypropylene fabric; concrete	Building envelope
Pujadas-Gispert et al. [25]	2020	Netherlands	Sustainable; Energy-efficient; Circular economy; Bio-materials; Green; Cradle-to-cradle material	Specimen of a bio-based ventilated façade	Experimental	Thermal (natural ventilation and reduction in indoor air temperature)	Bio-based composite material (Nabasco 8010)	Façade
Tomholt et al. [28]	2020	USA	Adaptable façades; Tunable material; Solar; Near-infrared regulation; Pneumatic façades; PDMS (polydimethylsiloxane); ETFE (ethylene tetrafluoroethylene)	Specimen of pneumatic tunable shading	Experimental/Simulation	Optical (Infrared transmission and reflection); Energy (heating and cooling reduction)	Polydimethylsiloxane (PDMS; Rogers Bisco HT-6240)	Façade
Ciampi et al. [21]	2021	Italy	Comparative outdoor test cells; Full-scale measurements; Tensile façade; Building energy efficiency; Energy savings	Specimen and model of a textile ventilated façade	Experimental/Simulation	Thermal (indoor air and cavity air temperature); Energy (primary energy saving and energy flows)	PVC-coated polyester fabric (SOLTIS FT 381)	Façade
Denz et al. [29]	2021	Netherlands; Germany	Adaptive sun shading; Textile building envelope; Smart materials; Autarkic operation and control mechanism; Shape Memory Alloy (SMA)	Specimen of a three-dimensional textile shading device	Experimental/Simulation	Optical (sun shading)	Textile and Shape memory alloy	Façade
Kahnt et al. [31]	2022	Germany	Building physics; Indoor climatic properties; Textile-reinforced concrete, Façade elements; Sustainable building	Model of textile-reinforced concrete	Simulation	Thermal (energy storage)	Textile-reinforced concrete	Façade
Ciampi et al. (a) [22]	2022	Italy	N/A	Model of a textile ventilated façade	Simulation	Energy (primary energy saving and energy flows)	PVC-coated polyester fabric (SOLTIS FT 381)	Façade
Ciampi et al. (b) [23]	2022	Italy	N/A	Model of a textile ventilated façade	Simulation	Energy (primary energy saving and energy flows); Environmental (reduction of CO2 eq emissions); Economic (simple pay-back period)	PVC-coated polyester fabric (SOLTIS FT 381)	Façade
Mainini et al. [30]	2022	Italy	Building envelope; Optical properties of building envelope components; Shading devices; Building energy performance design; Technology transfer	Specimen of a three-dimensional building skins	Experimental	Optical (angular spectral transmittance)	Stainless-steel mesh; perforated aluminum sheet; perforated zinc-plated steel sheet; expanded aluminum mesh; expanded steel mesh; stainless steel grid; plissé stainless steel; HDPE mesh; polyester 3D textile	Façade
Spanodimitriou et al. [24]	2022	Italy	Building energy efficiency; Simple payback period; Tensile façade; Second-skin façade; Carbon dioxide equivalent emissions; Office building refurbishment; TRNSYS	Model of a textile ventilated façade	Simulation	Energy (primary energy saving and energy flows); Environmental (reduction of CO2 eq emissions); Economic (simple pay-back period)	PVC-coated polyester fabric (SOLTIS FT 381)	Façade
Rahiminejad et al. [20]	2022	Switzerland; Canada	Ventilated air-space; Thermal inertia; Passive cladding; BIPV façade; Numerical model	Model of multi-layer ventilated wall assembly	Experimental/Simulation	Thermal (Temperature distribution; thermal inertia); Electrical (efficiency of PV)	Fiber cement; tempered glass, EVA film, and PV cells	Façade
Karimi et al. [18]	2022	Iran; United Kingdom	Polypropylene nonwoven; Acoustic properties; Thermal insulation; 3D fiber orientation; Porosity Fabric structure	Specimen of a polypropylene nonwoven fabric insulator	Experimental	Acoustic (Sound absorption); Thermal (Thermal conductivity and thermal resistance)	Polypropylene	Construction/Component
Bande et al. [32]	2022	United Arab Emirates	Parametric architecture; Energy simulation; LEED; Rhino; Grasshopper	Model of dynamic parametric façade	Simulation	Design (Parametric design; design optimization); Optical (view-out; shading effect; lighting daylight factor); Energy (thermal energy load reduction; energy production from PV); Economical (electricity cost)	PTFE	Façade
Mazzola [17]	2023	Italy	Structural membranes; Temporary architecture; Indoor comfort Condition; Technological solutions; Shading elements; Transparent thermal insulation	Specimen of an ultra-lightweight membrane-based pavilion	Experimental	Optical (shading); Thermal (indoor air temperature)	ETFE; low-density polyethylene; expanded polyethylene; polyurethane foam	Building envelope

.

4.1.1. Materials and Systems for the Whole Building Envelope

In both [16] and [17], the solution has been found in adopting a tensile structure to fully exploit the structural characteristics of the selected materials while allowing to keep the overall lightness in the final result. In [16], the authors present the design of a shell structure to cover large span areas efficiently, significantly reducing the on-site construction complexity and the amount of materials compared to conventional construction solutions and potentially decreasing the costs and embodied emissions. In their designs, the tensile structures, made of steel ties and nodes on which a woven polypropylene fabric was fixed, were used as bases for a layer of concrete to realize thin carbon-fiber-mesh-reinforced concrete shell structures. Instead, in [17], the tensile ETFE-based envelope already existed as a temporary pavilion, and the study focused on improving its thermophysical properties to provide a more comfortable indoor environment in both summer and winter. In this study, the properties of the materials (ETFE, low-density polyethylene, expanded polyethylene, and polyurethane foam) were exploited, combining them on a set of priorities based on feasibility, environmental performance, and spatial/aesthetic quality while preserving the transparency of the envelope, and evaluating their performances by monitoring the solar radiation transmission through the envelope and the indoor air temperature.

4.1.2. Materials and Systems for Construction Components

Lightweight materials can easily be used to improve the characteristics of existing envelopes or be implemented in the construction of conventional structures. In these cases, the study carried out by [18] showed how the use of lightweight polypropylene nonwoven fabrics could affect both the thermal and acoustic properties of the envelope, two characteristics that were addressed separately in conventional construction; a total of 31 samples were produced using a melt spinning machine to produce the polypropylene fibers (1.4 to 1.8 dtex), followed by a needle punching line to fabricate nonwoven samples with three thicknesses (2, 3, and 4 cm), four porosities (0.83, 0.88, 0.93, and 0.96), and different in- and through-plane fiber orientation distributions. The tests carried out using impedance tube and guarded hot plate techniques revealed that in-plane fiber orientation affected the sound absorption behavior at frequencies between 4000 and 6300 Hz, whereas there was no difference in the thermal insulation properties. The best acoustic (sound absorption average—SAA) and thermal performances (thermal conductivity—λ) were achieved by samples made of 1.4 dtex fibers with thicknesses of 3 cm (λ = 0.0278 Wm⁻¹K⁻¹, SAA = 0.552) and 4 cm (λ = 0.0277 Wm⁻¹K⁻¹, SAA = 0.675), a significant improvement over commercial products.

4.1.3. Materials and Systems for Building Façades

A broader range of aspects has been covered on the topic of flexible and lightweight materials and systems implemented in or used as building façades. On this topic, the most investigated system was the ventilated façade, as seven works [19,20,21,22,23,24,25] studied the use and performance of lightweight and flexible materials added to the exterior of the building envelope in a ventilated façade system. In [19], the authors analyzed the behavior of ventilated façades realized with fiberglass-reinforced cement boards upon varying the air cavity depth between the building wall and the external cladding (Figure 15a). In particular, the authors focused on the improvement of the acoustic insulation performance, finding out that the lightweight ventilated façade system provided better acoustic insulation, up to 5–7 dB, than a traditional wall at medium and high frequencies, with no significant difference from the air cavity depth. However, [20]’s focus was on the thermal performances of the system, monitoring the temperature distribution and its thermal inertia and calibrating a numerical thermal model to replicate the behavior of the whole system, which allowed them to calculate a potential in energy-saving rates of up to 50% in summer and 30% in winter when compared to a traditional wall. Instead, in [21], the authors focused on monitoring the behavior and calibrating a thermal model of a ventilated façade integrating different materials, a PVC-coated polyester fabric, which allowed them to carry out further numerical research in different climates [22,23,24]. In particular, in the experimental phase, the PVC-coated polyester fabric proved to be as effective as cladding in a ventilated façade system, resulting in a good chimney effect in the air cavity with natural ventilation despite being a highly porous material [21], whereas the numerical analysis carried out in various climates using the thermal model developed in TRNSYS allowed them to exploit the main characteristics of the material (rollable and semi-transparent) and test the logic behind the control of the rollable section in front of the windows in a variety of boundary conditions [21,22,23,24]. The results of the numerical studies showed a potential in primary energy saving up to 9.12% in temperate climates [21,23], up to 13.6% in hot climates [24], and up to 35% in cold climates [22] while also suggesting the need for more incentive policies to promote the endorsement of such energy efficiency measures on existing buildings [23,24]. Lastly, in [25], the authors reported the design, construction process, and thermal performance evaluation of a bio-based ventilated façade on a 1-story building in Dubai (UAE), highlighting the high sustainability of the whole construction process, from the sourcing of the bio-based composite material (made from renewable and recyclable sources) and the improved thermal performances of the envelope, which dissipated more heat during the day and preserved a comfortable indoor air temperature at night (Figure 15b).

Then, other than ventilated façades, several studies investigated using lightweight and flexible materials in various innovative façade solutions, integrating membranes, metals, fabrics, or bio-based materials.

Three studies [26,27,28] investigated the use of membranes in the building façades. In [26], the authors reported the results of developing an intelligent solution for multifunctional façades (energy harvesting, glazing, thermal isolation, and lighting), addressing the design challenges in prototyping a laminated module based on Ethylene Tetrafluoroethylene (ETFE) and incorporating LED strips, organic solar cells, and flexible electronics while also highlighting the advantages, such as applicability to planar and curved façades, a modular building appearance, and better natural cooling. Instead, in [27,28], the membranes were used in inflatable cushion systems, carrying out extensive analyses mainly on the optical properties of the final module. In particular, in [27], the authors used the possibility to customize the appearance of the ETFE membrane by developing a triple-layer cushion with a reflective frit print. The system allowed them to control its optical performance by moving the inner layer; the results, while showing a strong dependence on the angle of incidence of the solar radiation, also returned better daylighting performance compared to conventional glazing, thus, a reduction in energy demand for heating, cooling, and lighting by up to 48.5%. A similar result was achieved in [28], where the authors exploited the flexibility of a membrane made of gold-coated silicone elastomer (polydimethylsiloxane—PDMS); in this study, the light transmission of a single-layer module was regulated by controlling the reversible wrinkling and cracking in the thin, rigid, gold coatings on the PDMS membrane. The results highlighted a maximum variation in solar transmission equal to 34% and the final product’s high stability, reversibility, and rapid response time.

Two studies [29,30] investigated using metals or alloys in the building façades. In [29], the authors presented the development of adaptive textile sun shading systems integrating Shape Memory Alloy (SMA) to control the optical characteristics of the façade system; two functional prototypes were developed and evaluated, and the feasibility of these systems was compared to state-of-the-art sun shading systems (Figure 15c). The first system used a wave-shaped textile band interwoven with an SMA, whereas the second system consisted of two perforated textile layers that moved against each other to change the density of the structure thanks to the SMA directly integrated into the non-woven mesh; the samples were activated by a customizable temperature change, and the prototypes showed good optical performance and integration into façade construction. A broader analysis was carried out by [30], focusing on alternative materials for shading systems, including metal grids and meshes, 3D-expanded metal meshes, metallic plissés, plastic grids, and 3D textiles, and their potentials to provide solar protection to building façades. Due to the challenges in using conventional spectrophotometers on 3D skins, the samples’ normal and angular spectral transmittances were measured using a built-in spectrophotometer equipped with a large-diameter Spectralon-coated integrating sphere. The measurements revealed a strong influence of the openness factor on solar reflectance, as metal meshes and grids have low normal transmittance but can provide significant angular selectivity, thus resulting in performance similar to conventional shading devices at a reduced cost.

Two studies [31,32] differently addressed the use of fabric materials in innovative façade systems in various degrees of complexity. In [31], the authors proposed a more conventional approach, designing a textile-reinforced concrete to be used in non-load bearing walls, to develop a lightweight system able to guarantee the same thermal storage capacity of traditional heavy walls while also providing for less raw materials and a lower overall environmental impact. The proposed system was tested against conventional wall constructions, exceeding the required storage capacity and performing the best among the proposed solutions. The lightweight textile-reinforced wall’s low impact and high performance made it suitable for sustainable nearly zero-energy buildings. Instead, in [32], the authors exploited the lightness of the material to develop a model of a dynamic façade module (Figure 15d). The material implemented in the dynamic module was polytetrafluoroethylene (PTFE), a coated fiberglass fabric. The module’s design and movement were designed using parametric software, which allowed the optimization of the module characteristics and resulted in a potential energy demand reduction of about 25% and a 44% improvement in daylight analysis.

Finally, in [33], the authors presented an innovative bio-green wall system prototype to propose a solution to the challenges provided by the continuous development and the prevailing hot climatic conditions in Qatar, where creating a comfortable urban built environment has become a difficult task, as high-rise buildings and broad concrete masses generate a widespread Urban Heat Island effect. To overcome these challenges, the proposed façade system uses vegetation to improve the solar reflections in the urban canyons and the absorption of many air pollutants. The vegetation, the growing substrate, and the irrigation system were integrated into a lightweight prefabricated structure to realize an innovative green panel. This smart living construction material could be easily incorporated also in high-rise building façades. The preliminary on-field tests returned promising results, reducing exterior and interior surface temperatures, air humidity, and heat flux through the walls while also requiring low maintenance and including a user-friendly monitoring system.

4.1.4. Materials and Systems for Building Roofs

On the roof topic, Whybrow [34] described the challenges behind the design and construction of the hexagonal textile shadings in the King Abdullah Petroleum Research Centre in Riyadh (SA) designed by Zaha Hadid Architects, featuring a unique structural skin made of translucent membrane cells that provided shade and ventilation without blocking light (Figure 16), whereas, in [35], the authors reported the results of a measurement campaign on the tensile structure of the Yujiabu Station dome in Tianjin (CN), with the aim of exploring effects on the thermal stress and thermal deformation induced by the ETFE membrane installed on the steel structure; harsh daily variations in temperature could heavily affect the stability and durability of steel structure, and it is a stress that must be taken into account when designing a large span steel structure. However, the long-term monitoring of the surface temperatures and on the major deformation sections showed that the ETFE membrane was quite effective in reducing the daily temperature variations, positively affecting the stress on the structure.

4.1.5. Materials and Systems for Shading Systems

Lastly, on the shading topic, Stegmaier et al. [36] presented the design and testing of a dynamic shading device based on the biaxial bent of a silicon film (Figure 17), able to reduce by stretching the solar transmission by up to 50%, doubling the Near-InfraRed light (NIR) absorption, and increasing the reflection by a factor of three to six. Moreover, Moreno et al. [37] presented a numerical study on the energy and luminous performance of an ETFE glazing element integrated with organic photovoltaic (OPV) cells for building applications; four configurations were presented, a reference case with no shading element, a case where a tinted layer was added, and two cases where OPV cells were added and acted as shading elements, with area coverages of 50% and 100%. The configuration returned the best results with 50% OPV coverage and a window-to-wall ratio of 0.45, able to achieve an illumination level from 100 lux to 2000 lux for the majority of the annual hours while also maximizing the space heating and cooling energy demand reduction as well as the OPV electricity production.

4.2. Documents Focused on Flexible and Lightweight Design by 3D-Printing Technologies

Additive manufacturing heavily impacted the construction industry, opening the path to more complex and efficient design concepts. Using an enormous number of different materials allows for shaping every component, be it technical or decorative, directly on-site and deeply optimizing it for its specific application in each project. The studies described in the following sections prove this flexibility in design provided by 3D-printing technologies, where freeform shapes become the new normality, and non-conventional materials, like polymers or foams, are used to realize building components or even whole buildings. In this context, modularity often becomes a priority to improve the use of materials and construction times or allow fewer human resources to assemble an entire building. It represents the apotheosis of modern architecture movements, such as metabolism and deconstructivism, and it is leading the way for contemporary digital architecture movements. Unsurprisingly, most of the studies in this category (29, see Figure 13) carried out extensive analyses of the design aspects, intended as the study of the shape and its structural characteristics, as the first step in making something new corresponds intuitively to verify its feasibility. However, with 3D printing becoming an everyday commodity, more and more studies are investigating further aspects, trying to understand how to fully exploit the opportunities provided by new materials and the freedom in calibrating their physical characteristics (outer and inner shape, weight, infill, and so on) from a multi-physical point of view in a holistic design approach.

Figure 18 shows the word cloud generated using the keywords of the collected research articles in this category [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77], visually highlighting the most investigated aspects and materials.

All the details of the collected research articles included in this section are reported in

Table A2. Additional details of the 40 collected research articles included in category 2.

Ref.	Year	Country	Keywords	Study Object	Type of Study	Type of Analysis	Material	Envelope Section
Bhagat et al. [52]	2018	USA	Natural ventilation; Wind velocity; Rapid prototyping; 3D-printing; Nozzle profiles	Specimen of full-scale 3D-printed sectional nozzle profiles to modulate the air velocity in a façade system	Experimental	Design (air velocities measured at specified distances from the wind source through nozzles)	PLA	Construction/Component
Kaszynka et al. [44]	2018	Poland	N/A	Specimen of a concrete 3D-printed wall with insulation	Experimental	Thermal (temperature distributions; thermal conductivity and resistance; water vapor pressure distribution; heat bridges)	Concrete mixture with 70% of cement, 10% of silica fume, and 20% of fly ash	Building envelope
Sarakinioti et al. (a) [70]	2018	Netherlands	Additive manufacturing; 3D-printing; PETG; Heat storage; Thermal insulation; Façade module	Specimens of full-scale FDM 3D-printed façade elements with elongated cells	Experimental/Simulated	Design (internal geometries); Energy (cooling and heating energy demand)	PETG	Façade
Sarakinioti et al. (b) [71]	2018	Netherlands	Façade; Additive manufacturing; Fused Deposition Modelling technology; Plastic filament; Movable heat storage; Thermal insulation	Specimens of full-scale FDM 3D-printed façade elements with elongated cells	Experimental	Thermal (thermal conductivity and heat storage)	PLA/PETG	Façade
Alghamdi and Neithalath [57]	2019	USA	Three-dimensional printing; Fly ash; Surfactant; Geopolymeric foam; Porosity; Rheology; Yield stress; Buildability	Specimen of a 3D-printable geopolymeric foam	Experimental	Design (mixture; expansion; shape retention; buildability); Thermal (thermal conductivity)	Concrete foam mixture of Fly Ash; Silica fume; ordinary Portland cement; Limestone	Construction/Component
Grassi et al. [69]	2019	Italy	Shading system; Additive manufacturing; Durability testing; 3D-printed façade; Computational design	Specimens of 3D-printed full-scale façade shading elements for desert climates	Experimental	Design (durability of materials in a climatic chamber)	ASA (Acrylonitrile Styrene Acrylate); PP (Polypropylene); PLA (Polylactide); HT-PLA with 20% wood fibers (High-Temperature Polylactide); PETG (Polyethylene Terephthalate Glycol-modified)	Façade
Lipsky et al. [51]	2019	USA	Component; Humanitarian; 3D printing, relief; Development supply chains	Specimen of a resilient 3D printer for humanitarian relief scenario	Experimental	Design (portability; durability; reliability)	Aluminum frame	Construction/Component
Sanguinetti et al. [41]	2019	USA	Net Zero Energy Building; 3D printing; Additive Manufacturing; Affordability	Model of a 3D-printed net-zero energy building	Simulated	Design (type of added insulation); Energy (heating and cooling energy demand; electric consumption; energy use intensity); Economic (predicted electricity cost; utility bill; affordability)	Concrete	Building envelope
He et al. [50]	2020	USA; China	Digital construction and 3D printing; Green wall; Thermal network model; Thermal comfort	Specimen of a 3D-printed Vertical Green Wall building	Experimental/Simulated	Thermal (surface temperatures; heat flux; comfort); Energy (energy demand reduction; energy flows)	Concrete mixture + plants and insulation	Building envelope
Korniejenko et al. [43]	2020	Poland	Geopolymer; 3D printing; Additive manufacturing; Large-format 3D printer; 3D printing in civil engineering	Specimen of a geopolymer for large-scale 3D printing	Experimental	Design (composition of the geopolymer mix and resistance of the samples)	Metakaolin and sand; Fly ash and sand	Building envelope
Taseva et al. [73]	2020	Switzerland	Large-scale 3D printing; Freeform façade; Functional integration; Complex 3D assembly connection.	Specimen of 3D-printed façade elements with an integrated snapping panel-to-panel connection system	Experimental	Design (infill geometries; discretization of size and weight for transportation and assembly; snapping panel-to-panel connection system)	PLA	Façade
Bedarf et al. [77]	2021	Switzerland	Robotic 3D printing; Mineral foam; Lightweight construction; Concrete formwork; Façade shading panel	Specimen of a 3D-printed lightweight composite façade shading panel	Experimental	Design (3D-printed PLA nozzles for the mineral foam casting; print height; internal stress)	Mineral foam made from industrial waste-based fly ash particles mixed with water and modifiers; fiber-reinforced ultra-high-performance concrete (UHPC)	Shading
Ciampi et al. [75]	2021	Italy	Ventilated façade; Second-skin materials; 3D-printed materials; Additive manufacturing; TRNSYS; Full-scale facility; Retrofit action; Energy saving	Analysis of polymers for ventilated façades	Experimental/Simulated	Energy (primary energy savings and energy flows); Thermal (indoor and cavity air temperatures)	ABS; PETG; PLA	Façade
Dielemans et al. [61]	2021	Germany	Additive manufacturing; Lightweight concrete extrusion; Computational design; Thermal performance; Functionally graded materials	Specimen of a concrete 3D-printed wall element with a closed-cell structure	Experimental/Simulated	Design (toolpath optimization; printability); Thermal (heat flow; thermal conductivity via conduction; radiation; convection; thermal transmittance)	Lightweight concrete	Construction/Component
Maier et al. [58]	2021	Singapore; USA; United Kingdom	Phase change materials; Thermal energy storage; Cenospheres Concrete 3d-printing; Macro-encapsulation	Specimen of 3D-printed polymer lattice with encapsulated PCM	Experimental	Design (Density; compressive and flexural strength); Thermal (heat flux; surface and indoor air temperatures; heat storage)	ABS	Construction/Component
Marais et al. [45]	2021	South Africa	Three-dimensional-printed concrete; Lightweight foam concrete; High-performance concrete; Thermal performance; Cavity wall; Void ratio	Specimen of 3D-printed concrete wall elements	Experimental	Design (toolpath optimization; finite element analysis); Thermal (thermal resistance; heat flow)	Foam concrete and polypropylene fiber-reinforced concrete	Building envelope
Meibodi et al. [67]	2021	Switzerland	Binder jet 3D printing; 3D-printed formwork; Reusable formwork; Minimal surface; GFRC (GRC)	Specimens of 3D-printed interlocking formworks for prefabricated freeform façade panels	Experimental	Design (Reusability; Precision of interfaces; Level of details of surface features; Quality of freeform surface; Fabrication time of the process)	Sand (Binder jetting); PLA (FDM)	Construction/Component
Nazarian et al. [38]	2021	USA	Robotic construction; Additive construction; 3D-Printed concrete; 3D printing at architectural scale; Design and automated construction; NASA 3D-printed habitat challenge	Method to print a fully enclosed, 3D-printed, self-supporting, architectural-scale concrete structure using robotic arms	Experimental/Simulated	Design (printing system with robotic arms; toolpath optimization; structural analysis)	MarsCreteTM (a concrete formulation with basalt rock; kaolinite; sodium; and silicon; all of which can be harvested on Mars)	Building envelope
Sun et al. [39]	2021	China	Three-dimensional-printed concrete (3DPC); In-situ measurement; Infrared thermography; Thermal defect; Thermal performance	Specimen of a 3D-printed concrete prototype house	Experimental	Thermal (surface temperatures; heat flow; thermal conductivity; resistance and transmittance); Design (thermal bridges and surface inconsistencies)	Concrete	Building envelope
Zhan et al. [65]	2021	China	Three-dimensional concrete printing; Prestressed concrete; robotic fabrication; structural optimization.	Specimen of a concrete 3D-printed prestressed bridge	Experimental/Simulated	Design (toolpath optimization; finite element analysis)	Fiber-reinforced high-strength cementitious mortar with an alkali-free accelerator.	Construction/Component
Atkins et al. [49]	2022	USA	Energy storage; Buildings; Model predictive control; Active insulation; Additive manufacturing; Peak reduction	Specimen of a smart 3D-printed concrete wall with thermal energy storage and active insulation system	Experimental	Thermal (surfaces and internal temperatures; heat flux); Energy (energy consumption reduction); Economic (electricity reduction)	Concrete	Building envelope
Ayegba et al. [46]	2022	China; USA	Three-dimensional-printed concrete; Numerical optimization; Sustainability; Energy efficiency; Building insulation	Model of 3D printable building envelopes	Simulated	Thermal (thermal transmittance; thermal comfort); Energy (total energy demand; energy transfer); Environmental (carbon emissions reduction)	High-performance fiber-reinforced fine aggregates (HPFRFA) mix; fiber-reinforced cementitious concrete (3DPFRCC) with polyvinyl alcohol (PVA) fibers; fiber-reinforced engineered cementitious concrete (3DPFRECC) mix combined with three admixtures for improved thixotropic and rheological properties	Building envelope
de Rubeis [62]	2022	Italy	Three-dimensional printing; Additive manufacturing; Insulating materials; sustainable materials; Hot box analysis; Infrared thermography; Heat flux meter	Specimen of insulating 3D-printed block	Experimental	Thermal (surface temperatures; heat flow and conductance)	PLA filled with Polystyrene or wool	Construction/Component
Ebrahimi et al. [47]	2022	Iran	Three-dimensional-printing construction; Life-cycle assessment; Energy efficiency; Calcium sulfoaluminate cement; Reactive magnesium oxide cement	Model of concrete 3D-printed buildings integrating insulation and PCM	Simulated	Energy (total energy consumption); Environmental (LCA)	Magnesium oxide cement (RMC) and calcium sulfoaluminate (CSA) cement	Building envelope
Gribniak et al. [53]	2022	Lithuania	Aluminum profiles; Low-modulus stiffeners; Web buckling; Strengthening 3D printing; Numerical analysis	Specimens of polymeric stiffeners for aluminum profiles	Experimental/Simulated	Design (tensile, flexural, and compression resistance of different infill densities)	PLA	Construction/Component
Hao et al. [59]	2022	China	Three-dimensional concrete; Recycled fine aggregate; Paraffin; Phase change materials (PCM); Thermal conductivity	Specimen of PCM-infilled concrete 3D-printed elements	Experimental	Thermal (heat storage; thermal conductivity; surface temperature); Design (density; porosity)	Paraffin; Concrete	Construction/Component
Murad and Alseid [54]	2022	Jordan	RC beam-column joints; Quasi-static loading; Retrofitting three-dimensional; 3D-printed bars; Near-surface mounted; Elevated temperature; Carbon fibers	Specimens of PLA and carbon-fiber 3D-printed bars for seismic retrofitting	Experimental	Design (3D-printed bars insertion; finite elements analysis; material behavior)	PLA 20% mixed with carbon fibers	Construction/Component
Na et al. [68]	2022	Republic of Korea	Additive manufacturing; 3D printing; Irregular façade; Offsite construction; Node system	Specimen of a 3D-printed formwork for a structural smart node	Experimental	Design (Finite Element Analysis; casting roughness; dimensional fitting)	Sand (binder-jetting)	Construction/Component
Salandin et al. [40]	2022	Spain	Additive manufacturing; Acoustic insulation; Thermal transmittance; Life cycle assessment	Analysis of performances of a 3D-printed building	Experimental/Simulated	Acoustic (Sound reduction index); Thermal (outdoor air; indoor air; and surfaces temperatures; U-value); Environmental (LCA of environmental footprint)	Micro-concrete by CONCRETE S.L.	Building envelope
van den Heever et al. [63]	2022	South Africa	Three-dimensional concrete printing; Hardened state mechanical performance; Design and fabrication rules; Numerical simulation; Finite element analysis; Reinforcement	Model of concrete 3D-printed structural elements	Simulated	Design (toolpath optimization; finite element analysis)	Polypropylene fiber-reinforced printable concrete	Construction/Component
Volpe et al. [60]	2022	Italy; Spain; Portugal	Three-dimensional concrete printing; Prefabricated components; Magnesium potassium phosphate cement; Building envelopes; High-performance	Specimen of a concrete 3D-printed building envelope	Experimental	Design (toolpath optimization; Assembly; Modularity and Adaptability; structural resistance); Thermal (thermal transmittance)	Concrete (magnesium potassium phosphate cement)	Construction/Component
Cheibas et al. [74]	2023	Switzerland	Additive manufacturing; 3D printing; Façade; Thermoplastic connections; Add-on 3D printing	Specimens of 3D-printed façade panels connections	Experimental	Design (toolpath optimization; connection shape; deformation)	PETG; TPU	Façade
Cuevas et al. [56]	2023	Germany; Poland; South Korea; United Kingdom; Jordan	Three-dimensional printing; Lightweight concrete; Wall; Thermal insulation; Building envelope	Specimen and model of a 3D-printable insulating wall element	Experimental/Simulated	Design (toolpath optimization; load capacity; strain); Thermal (thermal transmittance; surfaces’ temperature; heat flux)	Concrete mixtures	Construction/Component
Emami [76]	2023	USA	Precast façade panels; Topology optimization; Additive manufacturing; TPU molds; Additive flexible molds	Specimens of 3D-printed formworks for freeform façade panels	Experimental	Design (material; reusability; flexibility; durability)	TPU	Façade
Kamel and Kazemian [42]	2023	USA	Building information modeling; Building energy modeling; Construction 3D printing; Lightweight concrete	Model of concrete 3D-printed buildings	Simulated	Design (BIM integration); Thermal (heat flux; thermal resistance); Energy (total production energy use)	Concrete	Building envelope
Laghi and Gasparini [66]	2023	Italy	Additive manufacturing; Directed energy deposition; Wire-and-Arc; Digital fabrication; Computational design	Model of structural wire-and-arc 3D-printed metal components	Simulated	Design (toolpath optimization; finite element analysis)	Steel (wire-and-arc additive manufacturing—WAAM)	Construction/Component
Pessoa et al. [55]	2023	Portugal; Netherlands	Three-dimensional printing; Hygrothermal characterization; Thermal conductivity; Water vapor permeability; Capillary water absorption; Sorption isotherm; Cementitious mortar	Specimens of 3D-printed cement-based mortar	Experimental	Thermal (specific heat capacity; thermal conductivity; water permeability)	Weber 3D 145–2 dry mortar	Construction/Component
Piccioni et al. [72]	2023	Switzerland	Three-dimensional printing; Building façades; Polymers; Solar transmission; Thermo-optical properties	Specimen of 3D-printed translucent façade panels	Experimental	Optical (transmission; refraction; reflection); Design (printing parameters)	PETG	Façade
Ragab et al. [64]	2023	Sudan	Honeycomb structures; Voronoi tessellations; 3D printing; Mechanical properties; Energy absorption (EA)	Specimens of 3D-printed hexagonal honeycomb structures	Experimental	Design (toolpath optimization; infill design; tensile strength; Crush force efficiency—CFE; energy absorbed—EA; and specific energy absorbed—SEA)	PLA	Construction/Component
Tari et al. [48]	2023	Iran	Additive manufacturing; 3D printing; Life cycle assessment; Construction industry; Magnesium potassium phosphate; Energy-efficient building cement; Energy-efficient building	Model of a 3D-printable building	Simulated	Energy (heating and cooling loads; electricity and natural gas consumption); Environmental (LCA)	Magnesium potassium phosphate cement	Building envelope

.

4.2.1. Design by 3D Printing of the Whole Building Envelope

On this topic, all 13 selected studies [38,39,40,41,42,43,44,45,46,47,48,49,50] investigated the use of concrete 3D-printing technologies in constructing whole buildings.

In five works [38,39,40,41,42], the authors designed or analyzed whole 3D-printed building systems. In particular, in [38] the development of a concrete additive manufacturing system and materials for NASA’s 3D-Printed Habitat Challenge was reported: the authors developed innovative 3D-printable mixtures of cementitious and non-cementitious mortars, called MarsCrete^TM, using materials similar to those that can be harvested on Mars, and experimented on printing angles, surface conditions, and nozzle parameters. The mortar was used to print 1:3 scale habitats for the Martian environment using a robotic extrusion platform, optimizing the printing toolpath on the constraints of the printing system and the material’s behavior based on finite element analysis. In two other works [39,40], the authors analyzed the performance of the envelope of a concrete 3D-printed building (Figure 19a), highlighting that the density of the 3D-printing mortar directly affected the thermal insulation performance of the envelope [39] and that it was already possible to meet the acoustic and thermal performance requirements of most European countries with current concrete 3D-printed technologies, especially when considering the possibility of easily integrate functional layers or materials in the printed envelope while offering comparable environmental impacts to conventional alternatives, with a slight reduction in carbon dioxide emissions and lower impacts in most other categories [40]. Then, in two studies [41,42], the authors developed numerical models of concrete 3D-printed buildings, investigating the design challenges and the thermal and energy performance of the models and highlighting a research gap in proper design tools and techniques, such as simulation software able to replicate 3D-printed structures and solutions to infill the 3D-printed cavities with smart insulation materials, such as phase-changing materials (PCM) or microparticles of ceramic insulation. In [42], the authors estimated that a properly designed all-electric 3D-printed residential building could achieve a potential 26% reduction in energy demand and CO₂ emissions.

Indeed, the remaining eight studies selected in this topic addressed the problem of improving the concrete 3D-printed envelope by modifying the concrete mixture [43,44,45] or the printed structure by integrating an insulation material [46,47,48] or additional systems [49,50].

Korniejenko et al. [43] reported the challenges in developing a new 3D-printable geopolymers mixture, starting from the definition of the mixture and the consequent basic properties to be considered in the 3D-printing process: viscosity and time of bonding. Both characteristics were tuned by adding conventional concrete additives, such as glass fibers, to the metakaolin and fly ash-based mixtures to reduce the brittleness of the final samples.

In both [44] and [45], the authors focused on the lightweight aspect of the 3D-printed envelope samples, comparing their performances to conventional solid and heavyweight constructions. In [44], a mixture (40% Portland cement, 40% crystalline silica, 10% silica fume, and 10% limestone filler) was used to print a sample integrating mineral wool insulation (Figure 19b), which was then characterized in terms of thermal conductivity and temperature and vapor pressure distributions; the envelope sample showed good thermo-hygrometric characteristics while being less thick than traditional walls, especially when considering having a continuous structure that could prevent the occurrence of a linear thermal bridge. Similarly, in [45], the authors verified that the opportunity provided by controlling both the density of the mixture (foam concrete) and the printing (geometry of the cavities) could lead to the easy manufacturing of lightweight panels that allowed for more specific and granular control over the heat transfer in the envelope.

In [46], the authors developed a model of a 3D-printable fiber reinforced engineered cementitious concrete with polyurethane spray foam infill that provided better energy and environmental performance when compared to other infill materials (expanded polystyrene, extruded polystyrene, and fiberglass). Also, in [47,48], the polyurethane infill returned the best results in terms of performance of the 3D-printed envelope over PCM (up to 50% [48]).

Finally, the opportunity of integrating complex active systems is investigated in both [49] and [50].

Atkins et al. [49] reported the development and testing of an active insulation system that combined additive manufacturing and model predictive control to create a more sustainable building envelope; the system was based on the concept of combining thermal energy storage and active insulation systems by including a two-pipe circuit connected to the HVAC and active insulation systems in the 3D-printed assembly to minimize energy consumption during peak demand hours. The results showed that the wall effectively charged and discharged; only a tiny amount of energy was last while idle; and this inconvenience could be further improved by integrating passive insulation or optimizing the geometrical parameters of the assembly in relation to the placement in the envelope.

Lastly, He et al. [50] developed a 3D-printed Vertical Green envelope system, integrating a green vertical wall system into a 3D-printed panel instead of conventional aerated concrete blocks to reduce the weights, materials, labor costs, resource demands, and construction times. The proposed system was integrated into a commercial building to test the challenges in installation and management. At the same time, its performance was evaluated using a thermal network model; the numerical results highlighted that the system effectively reduced the peak heat flux through the exterior wall, especially during summer daytime, through high solar-induced evapotranspiration, improving the potential energy-saving and indoor comfort.

4.2.2. Design by 3D-Printing of Construction Components

In comparison to the previous topic, the studies that investigated the design and 3D printing of construction components covered a wider variety of aspects, such as materials (plastic, cement, and metal) and 3D-printing technologies (layer deposition modeling, wire-and-arc soldering, and binder-jetting), while showing a narrower set of analyses, mainly focused on design aspects (shape, toolpath optimization, and structural performance). The extreme flexibility of such technologies is clearly expressed in [51], where the authors developed a model of a 3D printer for harsh emergency environments, such as post-disaster areas. By using a collapsible and sturdy aluminum frame and an isolated XZ actuator system to protect electrical connections and dynamic components, the 3D printer could be easily deployed off-grid to help in providing assistance by quickly fabricating tools and equipment (Figure 20a).

Three studies [52,53,54] investigated using 3D-printed components to enhance the performances of existing systems. Bhagat et al. [52] developed a retractable nozzle design to install in the external walls to quickly and easily improve natural ventilation in buildings. The prototypes were printed in polylactic acid (PLA) upon varying nozzle forms, nozzle extensions, and collapsible tubes to observe their impact on air velocities. The straight nozzle provided the best result in terms of air velocity at the outlet, suggesting a tangible effect on enhancing natural ventilation and cooling in developing countries. Gribniak et al. [53] explored the advantages of integrating low-modulus 3D-printed PLA stiffeners into aluminum profiles to improve the structural resistance, also investigating the influence of the infill density of the printed stiffeners. The compression tests demonstrated that the application of the low-modulus stiffeners increased the ultimate bearing load more than four times, altering the deformation behavior of the specimens, and that further studies should be carried out on developing a reliable adhesive connection between the aluminum profile and polymeric stiffeners. Lastly, in [54], the authors proposed the use of 3D-printed bars made of PLA and carbon fibers for seismic retrofitting of reinforced concrete beam-to-column joints subjected to cyclic loading and elevated temperatures; the results showed that the 3D-printed bars provided significantly improved performance in the heat-damaged joints compared to the control specimens in terms of elasticity (up to 206%), drift ratio (up to 216%), and load capacity (up to 51%), resulting in a promising solution due to their flexibility, accuracy, and low cost.

As pointed out in [55,56], cement 3D-printed elements have a hygrothermal performance comparable to traditional elements, suggesting that additional thermal insulation materials are required to obtain appropriate thermal performance through the building envelope (Figure 20b).

In the following studies [57,58,59,60], the authors investigated the possibility of improving the thermal performance of the building envelope by enhancing the construction materials. In [57], the authors presented the synthesis and characterization of 3D-printable geopolymeric foams (fly ash, limestone, and cement) for thermal insulation in building envelopes as a fire-resistant and sustainable alternative to conventional insulation materials; the measurement results showed thermal conductivities of the foamed matrices ranging from 0.15 to 0.25 Wm⁻¹K⁻¹ and compressive strengths up to 6.5 MPa. Instead, in [58,59], the integration of PCM was investigated. Maier et al. [58] designed a 3D-printed lattice in ABS to structurally support the PCM in the wall assembly, proving also effects in acting as structural support for the wall panel itself, whereas Hao et al. [59] incorporated paraffin in the concrete mixture by impregnating the recycled fine aggregates, resulting in a final concrete 3D-printed sample that showed significantly lower thermal conductivity than the mold cast concrete ones. Lastly, Volpe et al. [60] proposed the design and prototyping of a concrete 3D-printed module for building envelopes; the module used a prefabricated 3D-printed block to integrate insulation and technological and green wall systems into a single element that can be assembled without interruptions, leading to a more straightforward construction process and better thermal performance (Figure 20c).

Another solution to improve the thermal characteristics of the construction elements through 3D printing is to implement local variations in geometries, creating complex structures that can provide better performance without affecting the production process or integrating other materials. Dielemans et al. [61] developed a fabrication-aware design tool to generate close-cell and printable infill geometries for lightweight concrete 3D printing; a truncated octahedron shape was used as the basis for modeling the closed-cell structures and evaluated their thermal properties through simulation and prototype manufacturing, showing high thermal performance despite the still limited cell shapes and sizes (Figure 20d). In [62], the cavities of the PLA 3D-printed samples were also filled with recycled polystyrene and wool, further improving the control of the surface temperatures.

The mechanical properties of complex 3D-printed geometries were investigated in [63,64]. Van den Heever et al. [63] proposed two finite element modeling strategies to analyze complex concrete 3D-printed geometries; different beams were printed using polypropylene fiber-reinforced printable concrete to test their behaviors under different loading configurations. The experimental and numerical results suggested that vertical shear reinforcement should be considered to improve the performances of 3D-printed concrete beams. Instead, Ragab et al. [64] focused on PLA 3D-printed honeycomb structures, testing the load-carrying and energy absorption capacities upon varying the infill geometries. All the samples were printed with a 10% infill volume, making them lightweight while providing good energy absorption properties. The best results were achieved by an infill geometry optimized with Voronoi tessellation (Figure 20e).

The structural properties of 3D-printed elements were also investigated in [65,66]. Zhan et al. [65] developed a tensioning system to overcome the limited tensile and flexural strength of prefabricated concrete 3D-printed components. The system allowed them to combine the tensile properties of steel bars with the compressive performances of the concrete 3D-printed parts; to thoroughly test the feasibility of the integrated tensile/3d-printed system, the authors assembled a 3D-printed prestressed bridge, designed starting from the compressive strength and elastic modulus and printed in a 90° cross pattern to prevent transverse splitting, then divided into six units for printing and assembly (Figure 20f). If the previous study combined the conventional use of steel with concrete 3D-printed part, the study carried out in [66] fully exploited 3D-printing techniques to design full-metal structural components. Wire Arc Additive Manufacturing (WAAM) is the technique of soldering along lines or dot-by-dot continuous layers of metal on top of each other to create complex shapes hard, or even impossible, to achieve with conventional fabrication methods. Through the WAAM process, the authors designed different structural components, optimizing their shapes through load analysis and achieving a new class of resource-efficient structural elements (Figure 20g). The study considered multiple loading conditions and different printing orientations, with the 45-degree printing inclination resulting in the best structural performance and weight minimization.

Finally, two studies [67,68] investigated the opportunity to use 3D-printed elements to build casted components using traditional construction materials in freeform shapes impossible to model through conventional formworks. In [67], the authors designed reusable 3D-printed formworks for thin-shell freeform concrete structures (Figure 20h). The formworks were 3D-printed using sandstone binder jet 3D-printing, resulting in faster production times, smooth surfaces, sharper details in the formworks, and a more efficient and less wasteful production process of the final thin-shell panels. The demolding process was optimized through a computational analysis during the design stage, suggesting the best splitting sections of the structure panels. In a similar way, Na et al. [68] reported their results in using sand binder-jetting 3D printing to realize the formworks to cast 647 different nodes of a freeform structure; the 3D-printing of the formworks allowed them to accurately produce the nodes’ shapes modeled by the parametric software, allowing for low the production times, weights, and construction errors, and enhancing the aesthetic of the final products.

4.2.3. Design by 3D Printing of Building Façades

Most of the selected studies on the building façades topic report the design and performance assessment of 3D-printed façade panels [69,70,71,72]. In [69], the authors investigated the resistance of 3D-printed materials in harsh desert conditions, with the aim of creating a second-skin system for a pavilion in Dubai. The design of the second-skin façade system took into account the thermal expansion, heat propagation, light filtering, and sand accumulation caused by the desert environment, which led to the modeling of a modular and lightweight shape that could be efficiently built, at such a low production scale, only by 3D printing. The authors carried out a stress test in a climatic chamber to simulate the aging in hot conditions of the samples made of ASA (Acrylonitrile Styrene Acrylate), PP (Polypropylene), PLA (Polylactide), HT-PLA with 20% wood fibers (High-Temperature Polylactide), and PETG (Polyethylene Terephthalate Glycol modified). While ASA and PP also showed good resilience, HT-PLA with 20% wood fibers offered the best overall results considering sustainability and durability. In [70,71], the authors reported two stages in the design and characterization of the same PETG 3D-printed panel prototypes; the façade panel integrated insulation and heat storage properties by a careful design of the inner cavities that provided improved thermal insulation, especially when filled with liquid, which allowed for the heat storage capabilities (Figure 21a). The panel was tested in a controlled box, characterizing its thermal characteristics, which were then used to calibrate its thermal numerical model; the dynamic simulation study was carried out in different locations, highlighting a high potential in reducing the cooling energy demand and the correlated greenhouse gas emissions. The optical properties of different façade panel samples, 3D-printed in PETG, were investigated in [72]; the authors designed several specimens to test the variations in optical properties due to different inner and outer geometries (Figure 21b). The measurement results allowed them to define a correlation between printing settings (layer height and print speed) and the transmission, refraction, and reflection coefficients. This allowed them to design a whole façade assembly where every section could be finely tuned to receive the desired effect on the indoor environment.

The modularity involved in using polymer 3D-printing technology (due to the limitation related to the maximum printable volume of current filament 3D printers) was the concern of two studies [73,74], which addressed the design challenges from this requirement. In [73], the authors explored the design topology and translucency, working on three different scale prototypes: small, medium, and full-scale. Parametric design software was used to create infill patterns along the whole façade assembly and to optimally divide the design into single printable panels. This computational approach allowed them to integrate the seams and connections in the final design while also improving transportability and assembly efficiency, developing a two-major-section façade system: the bottom incorporated an integrated functional seat, whereas the top was angled in such a way to receive direct light for most of the year (Figure 21c). Instead, in [74], the focus of the study was exactly on the panels’ connection system, in particular on the combination of rigid (PETG) and elastic (thermoplastic polyurethane—TPU) 3D-printable materials to create snapping panel-to-panel connections. The connection samples were tested for air permeability, water-tightness, and mechanical strength, highlighting some criteria that must be taken into account, in particular, the thermal dilatation of the materials at the design stage, the maximum overhang angle (45°), and the horizontal printing direction during the 3D-printing stage.

From a material point of view, in [75], the authors defined the characteristics of several 3D-printable polymers (ABS, PETG, and PLA) to be integrated into second-skin façade assemblies, exploring the potential benefits from each material in terms of primary energy saving and reduction in space heating and cooling energy demands. After calibrating the thermal model of the façade system composed of 3D-printed panels with 10% infill using experimental measurements, a numerical analysis was carried out upon varying the material used in the external cladding; the results showed that all the considered materials allowed for positive results, slightly outperforming conventional second-skin cladding materials such as the porcelain grés. The PLA 3D-printed panels returned the best overall results.

Lastly, similarly to other studies on the components topic, in [76], the author described the process of realizing a 3D-printed flexible formwork while designing a freeform concrete façade system. In this study, the formworks were printed in TPU, which allowed for the most effortless demolding process.

4.2.4. Design by 3D Printing of Shading Systems

In this topic, only one study was selected. In [77], the authors addressed the design and fabrication of a lightweight composite façade shading panel integrating mineral foam 3D-printed sections and fiber-reinforced ultra-high-performance concrete (Figure 22). The 3D-printable mineral foam, a fly ash mix with water and modifiers was selected as a sustainable alternative to synthetic foams, which allowed them to create complex lightweight shapes on the building envelope. The shading geometries were printed, dried, and hardened through a sintering process, optimizing the print parameters (layer height, nozzle size, and printing speed) to improve quality and reduce internal material stresses. Finally, the resulting elements were suspended in a metal frame and reinforced with a high-performance concrete cast, allowing for the installation in the final assembly designed to control the solar radiation in freeform structures due to their high thermal and structural resistance.

4.3. Documents Focused on Flexible and Lightweight Solutions for Complex Envelope Designs

This section collected the selected studies from the last category, in which lightweight and flexible materials and systems were integrated into flexible design concepts where another layer of complexity was added to the envelope design (e.g., reversible transformations in response to external stimuli, as in responsive or kinetic façades, etc.) to address actual and future needs and changing the way the building interacts with its surrounding environment. In this context, the challenges were identified in developing a design workflow or analysis methodology that could predict the final behavior accurately. Time and external stimuli represented the significant variables addressed in this section.

Figure 23 shows the word cloud generated using the keywords of the collected research articles in this category [78,79,80,81,82,83,84,85,86,87,88], visually highlighting the most investigated aspects and materials.

All the details of the collected research articles included in this section are reported in

Table A3. Additional details of the 11 collected research articles included in category 3.

Ref.	Year	Country	Keywords	Study Object	Type of Study	Type of Analysis	Material	Envelope Section
Naqeshbandi and Mendonça [78]	2019	Portugal	Digital design; Digital fabrication; Crafting; Building wall components; Component design; Rapid Prototyping	Specimen of a non-structural and modular flexible wall	Experimental	Design (geometries; prototyping)	PLA (prototype)	Building envelope
Yoon [84]	2019	South Korea	Thermo-responsive; Building skin; Shape memory polymer (SMP); 3D printing	Specimen of a thermo-responsive façade element	Experimental/Simulated	Design (printing of shape memory filament; geometry of the kinetic cell; reversibility); Optical (solar radiation control); Thermal (indoor air temperature)	ABS; polyurethane (PU)-based shape memory polymer by SMP Technologies Inc.	Shading
Yi et al. [81]	2020	South Korea; USA	Responsive architecture; 3D printing; Shape memory alloys; SMA actuator; Kinetic building	Specimen of 3D-printed kinetic shading device	Experimental	Optical (Illuminance distributions); Thermal (indoor air temperature); Design (finite element analysis)	Poly-carbonate (PC) and polylactic acid (PLA)	Shading
Zupan et al. [85]	2020	USA; United Kingdom	N/A	Specimen of a shape-changing smart material building surface tile	Experimental/Simulated	Design (deformation control; reversibility)	Polyurethane (PU)-based shape memory polymer by SMP Technologies Inc.	Shading
El-Dabaa and Salem [86]	2021	Egypt	Hygroscopic properties of wood; Passive actuation; Adaptive facades; Programmable materials; 4D wood printing; Fused deposition modeling (FDM)	Specimens of 4D-printed wooden actuators for façades adaptive to humidity levels	Experimental	Design (toolpath optimization; temporal deformation response to humidity levels)	Wood PLA	Shading
Guillén-salas and Silva [80]	2021	Brazil	Bionic; Digital fabrication; 3D printing; Laser cutting; Industrial robotic arm	Specimens of freeform building envelope modules	Experimental	Design (complex form digitization; shapes optimization; fabricability; scalability)	PLA	Façade
Yi and Kim [82]	2021	South Korea	Adaptive architecture; Self-shaping façade; Shape-memory polymer; 4D printing; Shape-memory composite	Model and scale specimen of a 4D-printed building skin	Experimental/Simulated	Design (thermal deformation control; displacement; reversibility)	SMA fiber filaments	Shading
Kim et al. [87]	2023	South Korea	Adaptive responsive architecture; Kinetic façade; Soft robotics; Smart material; Hybrid actuator	Specimen of a flexible module for façades	Experimental/Simulated	Optical (useful daylight illuminance; daylight autonomy); Design (deformation control; displacement; reversibility; finite element analysis)	PLA; Lycra; Tarpaulin; helical shape memory alloy	Shading
da Silva and Veras [88]	2023	Brazil	Kinetic architecture; Shading device; Kinetic design; Environmental control	Specimens of kinetic shading devices	Experimental	Design (toolpath optimization; structural resistance); Optical (dynamic shading)	PLA	Shading
Yi [83]	2023	South Korea	Self-shaping architecture; Shape memory polymer; SMA; Smart material building; Sensitivity analysis	Specimen of a kinetic thermo-responsive building skin module	Experimental	Design (thermal deformation control; displacement; recovery force; morphing validation)	Shape memory polymer (digital elastomers DM9850 and 9885) design with shape memory alloy wires	Shading
Zhang et al. [79]	2023	China	Three-dimensional concrete-printed wall; BEH loop; Flexible FRP textile; Digital rebuilding; Seismic performance	Specimen of a textile-reinforced concrete 3D-printed structure	Experimental	Design (3D geometric deviations; loads and failure patterns; Hysteretic response; Energy dissipation capacity; Stiffness degradation)	Carbon fiber reinforced polymer fabric; glass fiber reinforced polymer fabric; 3D-printed concrete	Façade

.

4.3.1. Flexible and Lightweight Solutions for the Building Envelope

On the topic of solutions for the building envelope, in [78], the authors reported the research and development of a lightweight, dynamic, and interactive building envelope system using digital fabrication techniques (Figure 24). The suggested solution consisted of a modular non-loadbearing wall tensioned between ceiling slabs and pavements, which served as anchoring components. It enabled several benefits associated with weight reduction to achieve strong sustainability indicators, including reducing building expenses, energy use, material use, high recyclability, and simple assembly. The core of the flexible wall was represented by a modular grid, which created a system of inner cavities, in which the water, gas, and electrical network could be integrated; the cavities also served as thermal and acoustic insulation of the integrated service networks and between the other modules. The final wall was entirely self-supporting, without the need for any additional scaffolding.

4.3.2. Flexible and Lightweight Solutions for Building Façades

The two studies included in this section addressed the topic of façades in two distinct ways: focusing on integrating materials and techniques with different properties [79] or pushing the boundaries of digital fabrication [80].

In [79], the authors proposed a deep integration between a textile material and a concrete 3D-printed façade element to overcome the limits presented by using a 3D-printed part alone, such as the lack of tensile reinforcement and corrosion of steel bars. Although concrete 3D printing offers exciting opportunities to create intricate curved designs, it still faces significant hurdles that need to be overcome. These include finding ways to reinforce structures, evaluating their geometric properties and ensuring their mechanical performances meet the required standards. Their 3D-printed wall was reinforced following its freeform shape by using an innovative automatic reinforcement method, a management concept for automatic construction that involves the design (by human), measurement (automatic scanning), and construction (by robots) stages and aims to achieve rapid, unmanned, and low-cost construction using robots. The reinforcement involved spraying a layer of cement compound, followed by layers of flexible textile, and, finally, another sprayed cement layer, without difficulties following curved shapes or disrupting the printing process. All the specimens developed during the studies showed excellent geometric accuracies and reliable final qualities, and their normalized mechanical capabilities were higher than conventional construction elements, suggesting a great potential for fully automatic construction of large-scale complex buildings.

In [80], the authors conducted a feasibility study on three scales of a complex bio-inspired modular membrane system. The design of the modular system highlighted both its inspiration in nature and the potential for automation in the fabrication process. The construction was carried out at three scales: the small-scale prototypes were 3D printed in PLA; the real-scale structure was fabricated using laser-cut cardboard, white glue, and rubber; the large-scale model was divided into structural cells manufactured through aluminum die casting in Styrofoam formworks cut by an industrial robotic arm with a hot wire cutter. The cardboard structure showed deformation due to the material’s low plasticity and limited hardness. At the same time, the aluminum casting had limitations in thin parts and sizes and in cutting complex cell shapes through the hot wire. The whole process highlighted that it was impossible to achieve complete automation. Still, a healthy integration with craftsmanship was observed, and the highest potential in fabricating complex shapes on a large scale was expected from the integration between 3D-printing technologies and industrial robotic arms.

4.3.3. Flexible and Lightweight Solutions for Shading Systems

Smart materials can be utilized in adaptive and responsive building skins as intrinsic control systems, making them one of the most efficient state-of-the-art technologies in architecture. These building skins have the potential to significantly reduce the energy consumption of buildings by minimizing the mechanical system operation and maximizing the natural and passive adjustment of façade components for shading, airflow, daylight, and view. The studies in this section can be organized into three groups: systems that exploit (i) thermally or (ii) hygrometric responsive materials, or (iii) lightweight systems moved by actuators.

In the first group, five studies on the use of shape memory alloys (SMAs) and shape memory polymers (SMPs) were reported [81,82,83,84,85].

In [81,82] are studies on integrating SMAs in 3D printing and developing and analyzing different kinetic shading systems. In particular, in [81], the authors proposed a two-way SMA actuator in an origami shading system, able to be flexibly fabricated, deployed, and controlled by users, as well as attached or removed from existing fenestrations; the performances of different origami patterns were studied through digital and physical simulations, and the symmetric waterbomb pattern was chosen as the most suitable. Two solutions for the system’s movement were tested, a spring SMA-based actuator and a DC motor mechanism; the motorized mechanism had a narrower range of strains and strengths but was easier to design and implement, whereas the SMA spring was more efficient, responsive, and reliable. The two systems were tested against a base unshaded scenario and a static shaded one. The results showed that the SMA-actuated origami shading provided the best overall results in terms of both indoor air temperature (about 2–3 °C less compared to the base case) and daylight performance. However, as the SMA spring could only respond to temperature variations without considering sky illuminance and/or indoor comfort, the integration of light sensors and an auxiliary motor system was suggested to improve further the design and performance of the origami SMA system. In the following study [82], the authors analyzed the mechanical performance and the deformations of the SMA shading system through macroscopic 3D finite element method analysis. While the mechanical analysis highlighted some challenges in predicting self-shaping behavior accurately due to the large recoverable deformation, SMA allowed for unique opportunities when integrated into kinetic shading systems, like developing complex applications and control logic and fabricating more complicated and efficient building skins.

In [83], the same research group explored the development of a combined SMA and SMP 3D-printed system, allowing for a more performant design of the moving parts (Figure 25a). The Sobol Global Sensitivity Analysis (GSA) was used to evaluate the sensitivity of different design parameters, which indicated that the sectional geometry of the bending part and the intensity of the SMA pre-strain were critical factors in predicting the maximum displacement and recovery force of the moving element. The combined SMA and SMP design showed that a well-designed one could overcome previous limitations, creating a stable and reliable force for shape recovery with a maximum deformation of approximately 3.5 mm at temperatures between 30 °C and 65 °C.

In [84], the behavior of a 3D-printable polyurethane-based SMP was investigated; circular 3D-printed samples were used to verify the characteristics of the printed material, calibrating the type/amount of movement of the movable sections and the activation thresholds (Figure 25b). The experimental campaign also proved the ability of the material to roll back to the initial state reliably. Then, the measurements were used to calibrate a numerical model of thermoresponsive façade elements, able to adjust their properties to minimize energy consumption. Various façade arrangements were tested, and the results proved that calibrating the temperature threshold at 55 °C returned the best results regarding both building envelope surface temperatures and indoor solar radiation distributions. The same polyurethane-based SMP was investigated in Zupan et al. [85], in which the design of a dynamic 3D-printed tile for responsive building façades was presented. A digital model of the tile was developed and calibrated to carry out a numerical analysis to investigate its potential in dynamically managing the solar heat gains through the building envelope in response to its surface temperature. The results highlighted the influence of specific criteria in defining the envelope performance, such as temperature, radiated energy, and conducted energy. Also, the performance of the tile, when arranged in an array, was considered to cover a whole building façade and to verify the interactions between neighboring tiles. The results showed that the tiles array performed more efficiently, requiring less energy to receive the same effect on the indoor environment, with a significant decrease in the area exposed to solar irradiance compared to controlling them dependently.

Instead of temperature, the dynamic actuators presented in [86] could respond to humidity variations, particularly developing 3D-printed wooden actuators as a passive approach for adaptive façades with dynamic shading configurations. The experimental campaign explored the effect of 3D-printing parameters of wood PLA filaments, such as printing patterns and layer heights, on the deflection and response speeds of the final printed specimens. The results showed that it was possible to print patterns that resulted in single and double-curved surfaces and that variations in layer height could control each part of the wooden actuator’s response separately; a smaller layer height of 0.3 mm resulted in higher deflection values compared to a layer height of 0.6 mm, whereas the angle of curvature was found to be related to the percentage of wood in the printing filament, with higher rates of wood in the filament resulting in more significant deflection. The results showed a clear potential in utilizing this solution to enhance responsive façade design with zero-energy consumption, allowing to accurately and efficiently program passive motion mechanisms.

Lastly, in [87], the authors discussed using shape memory materials, silicon rubbers, and magnetic/electromagnetic materials in construction. In particular, air fluidic elastomer actuators (FEAs) were investigated as kinetic systems able to move heavy loads and provide instantaneous responses to external stimuli. The authors proposed the integration of such actuators in a bio-inspired hexagonal honeycomb shading system, activated based on environmental temperature and outdoor conditions to maintain indoor thermal comfort and control sunlight. Preliminary mockup specimens were 3D printed to understand the FEA’s geometric deformation, resulting in a minimum air pressure of 50 kPa required to achieve the desired displacement and force. The integrated SMA springs were 3D printed, and their phase transition temperatures were determined through differential scanning calorimetry. The functionality of a prototype assembly was tested in a mockup freeform façade, where the actuators responded well to input pressure and exhibited robust actuation. The system’s limitations were clearly defined, such as constraints on the maximum opening area of the panel and the need for further evaluation of environmental sensors, pressure supply systems, and daylighting performance.

Finally, in [88], the design and analysis of advanced dynamic shading systems were reported, an information-based operated kinetic shading device system for building envelopes, developed considering variations in envelope shape, daylight performance, and digital fabrication prototyping. The performance of different geometrical parameters was verified through parametric structural design and simulations. The proposed 3D-printed final design was an umbrella-like origami-inspired kinetic device compatible with surfaces of positive Gauss curvatures that eliminated sensors and relied on pre-programmed activation schedules based on the desired performance (Figure 25c). The shading performance was evaluated in different arrangements and activation threshold values using hourly sun vectors to maximize sun blocking, minimize device activations, optimize maintenance and energy consumption, and improve indoor environmental control. The analysis results showed that different threshold values should be considered for different façade orientations to balance solar radiation management with the need for indirect daylight and natural ventilation.

5. Conclusions and Future Research Opportunities

The innovative materials for flexible and lightweight solutions for energy improvement in construction are emerging technologies that could help to achieve the NZE Scenario proposed by IEA. In particular, this paper reviewed these solutions to facilitate their development and use in building construction. To provide a deeper analysis, we analyzed flexible and lightweight solutions by categorizing the literature into three categories: (i) materials’/systems’ topologies, (ii) design flexibility from 3D-printing technologies, and (iii) innovative solutions for complex and efficient building envelope designs. The literature review of the 73 research articles showed a significant increase in interest from over 300 authors coming from 26 countries across the world in recent years. In addition, as the field of research is constantly evolving with advancements in materials, technologies, and systems, it is important to highlight that this review only covered the period up until August 2023. However, it is easy to verify, update, and compare the main part of the database used in this review by using the queries provided in Section 2 to check for any new articles published on the research topic. Also, this review focused on a single section of the building: its envelope. A similar holistic approach should be carried out on the other macro sections, such as plants and structure, in order to define and assess the complete potential of new technologies/solutions and their different impacts on the building sector.

Considering the analyzed articles, we can conclude that:

• In many studies, the possibility of using flexible and lightweight materials and design solutions in the building envelope was investigated by means of a modular design;
• Most of the research was focused on the design and thermal aspects, only a few on optical aspects, highlighting a lack of software and, therefore, numerical models able to reproduce the multi-physical behavior of complex materials and systems;
• Analysis of membranes and fabrics was more advanced when compared to studies on 3D-printed and flexible solutions, which were mainly focused on design matters;
• Similarly, this lack of numerical models strongly limited the preliminary designs of such systems; this limitation was strongly emphasized by the extreme design opportunities provided by these systems and the advanced capabilities of software for digital designs;
• The flexible and lightweight systems/solutions could be applied to the entire building sector in new constructions, or in retrofitting of existing buildings; retrofitting buildings may become the primary approach due to the benefits that come with these solutions, extending the lives of the existing constructions, thus lowering the environmental impact of the whole section;
• The environmental impact of these materials and solutions was rarely investigated, especially when considering in-depth analysis such as embodied energy and Life Cycle Assessment;
• Similarly, the economic aspect (production costs, capital costs, maintenance costs, etc.), especially when scaling up the dimensions, was still a strong limitation of the considered technologies, and it was recognized as such in several studies;
• Considering the objectives of nearly zero-energy and energy-positive buildings, these systems could provide a functional substrate to integrate several energy production technologies, but the research is still focused on flexible photovoltaics, whereas other sources are still scarcely exploited (e.g., wind, rain, kinetic energy recovery, etc.);
• Nowadays, there is no specific EU regulation for these systems; to ensure quality and provide a reference for future regulatory decisions, it would be helpful for future works to provide a list of relevant regulations and define guidelines from a multi-physical point of view in a holistic design approach.

In conclusion, this review highlighted how these materials and systems could play a fundamental role in rethinking the entire construction sector, shifting towards a more sustainable and personalized future. However, the human factor, from both sides (designers/constructors and users), should be more involved in the research process to plan actual social, economic, and environmental impacts. This review highlighted that there is a need for further studies on 3D-printed components while membranes (in particular, ETFE) and textiles are widely investigated; the continuous development of new 3D-printable materials, as well as the design variety offered by the additive manufacturing process, are the main elements of assessment, profoundly influencing the thermophysical, optical, and acoustic behavior of these elements. Similarly, new materials and new printing processes need to be evaluated from the economic and environmental points of view, two aspects that are strongly influenced by the advances and availability of the technology.