A Life Cycle Cost Analysis of Structural Insulated Panels for Residential Buildings in a Hot and Arid Climate

Reviewer 1 comments

Authors’ thoughts

Action taken

1.       In page. 15 lines 416-417 the sentence “I have revised the values as the prices formula was wrong. I will send you a voice note to explain it.” should be deleted.

Thank you for this comment.

We apologise for not deleting this sentence in the first submission.

We have deleted this sentence.

Reviewer 2 comments

Authors’ thoughts

Action taken

1.       The length of the manuscript shall be reduced, cutting non-relevant information (e.g., lines 145-152 do not add anything to the methodology scheme in Fig.2). Conclusions are also interesting, but shall be condensed.

Thank you for this comment.

We have removed these lines and trimmed down the conclusion section.

2.       The manuscript should highlight the original contributions of the study to the body of knowledge; LCC calculation and sensitivity analysis are well-known practices. Literature review is quite plain and shall be enriched to better support the goal of the paper.

Thank you for this comment.

We have improved our literature review.

The contribution of the study to the body of knowledge is clearly described in a Section 5.1, titled ‘Contribution of the study’.

The literature review has been revised and enriched to better develop the argument and knowledge gap.

3.       Table 1 provides relevant information, but the current layout makes it hard to read. I suggest using a horizontal layout.

Thank you for this comment.

We have changed the page containing Table 1 to Landscape.

4.       The research methodology provided shall be strengthened, as it now appears to be very case-study centered. This approach reduces the applicability of the method; I suggest dividing methodology and case study application.

Thank you for this comment.

Given that this is an exploratory study in a data-poor context, we believe that a case study approach is very relevant to conduct this research, as recommended by Yin [1]. As such, we have further clarified the need for case study research, rather than restructuring our method.

We have clarified the need for case study research for this study, given its exploratory nature and the data-poor context of Bahrain. The following paragraph has been added to Section 3.1.

As recommended by Yin[18], a case study approach has been implemented in this study due to its exploratory nature and lack of existing data regarding the use of SIPs in the Bahraini context. This case study is described in detail in Section 3.2.

5.       The reason the authors choose a 50-year long period is not clearly expressed. It is a long period of time for residential buildings, considering that no maintenance or renovation is included in the calculation. Furthermore, climatic conditions are likely to deeply change in that period of time.

Thank you for this comment.

We have aligned our study on similar life cycle cost studies of buildings from around the world.

Having a 50 years timeframe also allows the reader to evaluate the results at any given point beforehand using our supplementary information and the time series graphs that we provide.

We have provided further explanations about the choice of a 50 years period of analysis.

As mentioned in Table 3, the period of analysis was set as 50 years for two reasons. First, this aligned our study with similar LLC studies of buildings and building components that adopt a period of analysis between 40 and 75 years (e.g. [20-23]). Secondly, a timeframe of 50 years allows the reader to evaluate the results and potential benefits at any given point beforehand using the results outlined in Section 4. This helps understand the effects of using SIPs as compared to HCB at various points in time to better assess the feasibility of implementing such a construction methodology in the Bahraini context.

6.       Lines 157-159 are not clear: please rephrase.

Thank you for this comment. We have rephrased the sentences in question to better explain the benefits of our case study approach.

The sentences in question are “Having the case study house modelled and modified throughout the study helps compare the results obtained across the different house types. This helps the researcher minimise the impact of variables on the results.”

These sentences have been paraphrased to

Firstly, the study relied on a single representative case due to its exploratory nature and the lack of data in Bahrain[18]. Secondly, having a case study modelled into a simulation tool is beneficial for this type of approach. It allows us to modify individual variables to assess their effects on the overall results, as well as to combine two or more changes to produce a more comprehensive understanding of the effects of combined variable changes

7.       Line 170: please check the text style.

Thank you for this comment.

We acknowledge that the text style was different in some parts of Line 170.

The text style has been changed to ensure consistency with the remainder of the text.

8.       Line 172: table 3 is not easy to read. Some information are not relevant (e.g., number of rooms)

Thank you for this comment.

Table 3 provides a holistic description of the case study house and the values of different characteristics as these affect the individual results specifically and the life cycle cost in general. The number of rooms in the case study house was mentioned as well as this directly affects the thermal profiles created and subsequently the annual operational energy.

We have arranged the layout to have Table 3 on single page to make it easier to read. The content of Table 3 is important and provide often omitted metadata about the case study house.

9.       Line 194: the “three house types” are mentioned here for the first time. Probably this issue could be solved improving the methodology section. You mean three scenarios involving different materials? Please, consider adding the results of energy simulation, if relevant.

Thank you for this comment.

We understand that this might be confusing to the reader being the first time it is mentioned.

The third sentence in Section 3.1 has been modified to clearly indicate that the HCB, HCBI and the SIPW are the three house types such that any reference later in the text can be referenced to this description.

The research considers three types of external materials for the case study house thereby forming the three house types – the first type uses hollow concrete blocks for the external walls and is referred to as “HCB” (and is considered as the baseline case scenario), the second type is using insulated concrete blocks for the external walls and is referred to as “HCBI” and the third is where SIPs are used instead of concrete blocks for the external walls and is referred to as “SIPWR”.

10.    Line 312: please add the $ symbol in the figure.

Thank you for this comment.

It is unclear where the $ symbol should be added in Line 312. If the reviewer is referring to Figure 7, the $ symbol was not added to the individual costs as it would make the figure less readable. Instead, the Y-Axis title states all the values are in USD2020. A $ sign can refer to Australian dollars (AUD), New Zealand Dollars (NZD), Canadian Dollars (CAD) and not only USD, which is the official abbreviation of the currency.

No action taken.

11.    Line 322: I suggest to re-arrange the figure in two rows and two columns to widen them (very small text).

Thank you for this comment.

We have re-arranged Figure 8 as suggested.

12.    Line 328: I suggest improving this section, as uncertainty in LCC calculation is related to several factors (financial, environmental, user-related aspects). Please clarify what criteria guided you in choosing uncertainty factors for sensitivity analysis.

Thank you for this comment.

We acknowledge that the reasons for selecting these factors was not clearly indicated.

We have added new text in Section 4.3. to explain the reason for our selection.

It is worth noting that the variables described in 4.3.1, 4.3.4 and 4.3.5 were selected due to their significant effects on the LCC results as per Equation 1. Section 4.3.2 was included to assess the effect of the orientation of the house on the energy use specifically and the overall LCC results. Lastly, Section 4.3.3 was included to analyse the occupant’s behaviour regarding their preferred setpoint temperatures as well the performance of the cooling units installed as these two factors can affect the total energy used to cool an indoor space, and the associated LCC.

13.    Line 416: be careful what you include in the final version of your manuscript.

Thank you for this comment.

We apologise for not deleting this sentence in the first submission.

We have deleted this sentence.

14.    Line 486: please rephrase “various variables”.

Thank you for this comment.

We have rephrased that to multiple variables, consistently with the sensitivity analysis section.

Reviewer 1 comments

Authors’ thoughts

Action taken

1.       The results of the LCC should be documented clearly by describing the LCC model and application, e.g.: have been considered factors like Capex, Opex, Riskex, Envex etc..?

Thank you for this comment. We have further described the LCC model in the paper.

A new paragraph, describing the LCC model has been added in Section 3.6. It reads:

To determine the life cycle cost, a streamlined Net Present Cost (NPC) model is used that incorporates the capital expenditure along with the ongoing operational costs (and potential savings) associated with the energy use. It is important to flag that we do not include the cost of risk nor the cost of environmental damage in our model as these fall outside the scope of this research

2.       The paper should follow a more rigorous approach. 

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Thank you for your comment. The rigour of the paper has been improved by taking into account al the comments of Reviewers 2-4.

See all actions taken below

Reviewer 2 comments

Authors’ thoughts

Action taken

The main aim of the paper is clear, but the concretion has some points that this referee considers have to be improved to obtain a quality article.

Thank you for your appreciation of our work.

We have further improved the quality of the manuscript as detailed below.

1.       -First, only a case study is selected, a "single-story house" (line 158). This is a strong restriction (specially for a simulation study), something should be said about multistorey buildings, and even single-family two story buildings. This point should be improved to merit publication (see the second difficulty).  Further on, on the plans of the case study, figure 3, it appears a space devoted to "staircase". Is it a staircase to acces the roof? it seems quite generous in space. Please, clarify.

Thank you for your comment.

The main focus of this paper is single-storey houses as these are most representing housing type in Bahrain. We decided to focus on the most representative typology in our exploratory case study approach, as advised for by Yin (2016). Double-storey houses will be considered in future research on the same topic to cover a more holistic approach on the residential standalone houses in Bahrain.

The staircase has been built as provision for a  potential future expansion of the house with a second storey. We flagged that in the manuscript and we understand that it is confusing otherwise.

We have further clarified our choice of single storey buildings and discussed how results of this study might be applicable to other housing typologies. We have also clarified why a staircase is included in the floor plan in the caption of figure 3.

In Section 3.2:

The staircase has been built as a provision for a potential future expansion of the house with a second storey. A single-storey house has been selected for the purpose of this study as it is the most representative housing type in Bahrain. Limitations associated with this selection are discussed in section 5.3. Future research on the topic will ad-dress double-storey houses as discussed in section 5.4

In section 5.3:

While the sensitivity analysis has broadened the applicability of findings, results are valid for the case study house, representing single-storey houses. However, it is worth noting that capital expenditure results are applicable to multi-storey residential houses with a potential reduction in costs as larger quantities are purchased. Moreover, regarding multi-storey buildings, there will be no losses through the roof (except for the top apartment) and this reduces the overall insulation benefits of SIPS

In Section 5.4:

Future research directions include conducting the research to include double and multi-storey residential houses to provide a more holistic approach on the residential housing sector in Bahrain

2.       -Second, the authors do not mention that large heat transfer will occur through the roof. In relatively low latitude regions, the roof receives a high quantity of solar radiation (and, during the night, if skies are clear, allow also for considerable radiative cooling). In fact, the same norms that the authors cite (line 136 and table 2), have increased strongly the need for roof thermal insulation. Then, due to the fact that the case study the authors choose (because the roof has more surface (in sq. meters) than the whole walls enclosing the house (table 3)), it is VERY important to consider the roof separately. Please, give separately  the heat fluxes through the roof and the walls.

Thank you for this relevant comment.

The roof has been modelled in IES-VE to match the type of material used for the envelope. The HCB types were modelled with a reinforced concrete roof and the SIPs type was modelled with a SIP roof. Moreover, the climate was adjusted accordingly to reflect the local climate in Bahrain and the simulations were run over the entire year to cover both the dominant summer season and the winter season.

We clearly indicated in the text that the roof was modified for each construction type (HCB) and (SIPS).

3.       The different choices for the walls should better be accompanied with different choices for the roof, to make a good profitable article.

Thank you for this comment. Please see our comments and actions above.

Please see our actions taken above.

Reviewer 3 comments

Authors’ thoughts

Action taken/to be taken

The Work propose an interesting evaluation of the life cycle cost and operational energy of a representative case-study house in Bahrain, using SIPs and hollow concrete blocks (HCBs) for the envelope over a period of 50 years.

The work is interesting, but some revisions are needed before it is recommended for publication.

Thank you for your kind comments.

We have further improved the quality of the manuscript by addressing the comments of all reviewers.

1.       -Originality/Novelty is not well defined, it is not clear how the results provide an advance in current knowledge. Which important long-standing question do the authors addressed with this work? Aim and scope (Line 70) need to be better specified.

Thank you for this comment.

We have further clarified the aim and scope and further clarified the contribution of this work in Section 5.1. The Aim and Scope now reads as follows.

The aim of this paper is to quantify the life cycle cost (LCC) and operational en-ergy savings associated with using SIPs for external walls and roof, as an alternative to hollow concrete blocks, in residential houses in Bahrain. This study considers three main aspects – operational energy, operational cost and construction cost.

This paper provides insights into the potential cost and energy use benefits asso-ciated with using SIPs, to the housing sector in Bahrain and neighboring countries with similar geographical, climatic and economic conditions.  

The following paragraph was added to Section 5.1:

From a life cycle cost perspective and as shown in Figure 8, the study has demonstrated that the SIPW type would perform better over the analysis period as compared to the HCB and HCBI types, except in the SHPRO17 model, which is the worst-case scenario. This implies that the using structural insulated panels for walls and roofs in single-storey housing in Bahrain can be cost-competitive, as compared to current housing practices. Further discussions on the life cycle operational energy are present-ed in section 5.2.

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2.       -In Lines 114 - 116 authors write: “A very limited number of studies has investigated the life cycle cost of SIPS and none have done it comprehensively in a hot and arid climate. There is a need to conduct an LCC and operational energy analysis of SIPs as compared to traditional hollow concrete blocks in the hot, arid climate of Bahrain.” It would be useful to provide guidelines and practical suggestions as a result of the study carried out. Otherwise many results are presented but the purpose of these results it is not understood.

Thank you for this comment.

We have provided additional practical guidance in Section 5.2, by rewording some sentences. We have also added the following pargraph at the end.

It is worth noting that all the stakeholders involved in the construction of a residential house in Bahrain – owners, designers, builders, etc. – are restricted by the material options available on the construction market and the construction practice. This re-striction and inertia in the practice can be problematic as they do not promote potential construction alternatives that can have both cost and environmental benefits for the end user and the government.

3.       -Line 151: A photo of the case study house would be useful.

Thank you for that comment.

We have now included a photo of the house taken from the street, as well as a section of the house.

A section of the house has been added. Refer Figure 3

4.       -The results of Lines 487-498 are interesting. I suggest to include these comments (but also improving this content) in a new subsection to provide useful guidelines and suggestion to researchers, practitioner, architects and engineers. It might help the reader in understand the importance of the research contribution in the construction sector.

Thank you for this comment.

We agree that these are useful guidelines. We have renamed section 5.2 to: 5.2.       Reducing the overall life cycle operational energy demand of residential houses: practical guidelines to better reflect its contents.

We have revised the content of Section 5.2 in line with comment 2 above.

5.       -In conclusion authors stated “The study aimed at targeting these factors by performing a broad sensitivity analysis on the results obtained from the base analysis. This helped in understanding the impact of the individual factors on the overall life cycle cost and possible strategies that can improve the overall outcome”. A subsection in the discussion to exhaustive argue the resulting impact of the individual factors would be important. In adition, what are these factors? In lines 317 the authors talk about “sensitivity analysis is performed on multiple variables” variable and factors are the same thing?.

Thank you for this comment. We agree that there is some confusion in the terminology of the paper. Indeed, variables and factors are the same thing. The influence of each variable has been discussed in detail in the dedicated sensitivity analysis section.

We have proof-read the paper to consistently use the word variable. We have included signposts in the conclusion section that refer the reader back to the sensitivity analysis section for a detailed insight into the influence of variables on the results.

Add sentence that includes signposts here.

I referred to Section 4.3 as per the highlighted part in the Conclusion to refer the reader back to the sensitivity analysis.

Variables are now consistently used through the paper instead of factors.

6.       -It is important to uniform the terminology in all the manuscript. All these inconsistencies are confusing.

Thank you for this comment.

We have proof-read the manuscript and consistently used terminology to avoid confusion.

7.       -If factors are: “shipping costs”, “Orientation”, “Tariff Rates” etc. It is important to clearly define them.

Thank you for this comment.

We have added definitions of these variables in the text to be more precise. These are listed below for your reference.

Shipping costs refer to the total costs associated with importing the structural insulated panels from China (supplier) to Bahrain (user)

Orientation refers to the position of a house with respect to seasonal variables like the sun’s path, wind patterns, etc.

Temperature setpoint is the temperature/point at which an area is set. The HVAC system in that area would work to bring the room temperature to the setpoint. Coefficient of Performance refers to the ratio of heating/cooling provided by a unit to the amount of electrical input required to generate it.

A tariff rate refers to the price a user is charged for the energy they use.

Discount rate refers to the interest rate at which the central bank lends money to commercial banks to meet their liquidity needs

8.       -Supplementary material, Line 549: the link is not working.

Thank you for pointing that out.

However, we have tested the link from Australia and Belgium and it works.

9.       -General conclusive comment: The work sounds interesting but work needs to be better structured and objectives clarified. In the current form of the work there are a lot of information and results provided without a specific purpose.

Thank you for this comment.

We have addressed all comments and we have improved the scientific rigour of the manuscript. We hope that it now meets your expectations.

Reviewer 4 comments

Authors’ thoughts

Action taken/to be taken

The topic of Life Cycle Cost (LCC) assessment and operational energy savings associated with the use of SIPs for external walls, as an alternative to hollow concrete blocks, is relevant and interesting for extreme climates such as hot and humid weather.

However, the methodology is not clearly applicable and extendable to similar cases, the results are based on inappropriate boundary factors, and the proposed data are not verifiable. Not all references are clearly identifiable

Thank you for this comment.

1.       - The ISO 9001:2015 standard does not accredit software for building performance analysis projects. Explain what is written in paragraph 3.3. Indicate more precisely the internet reference of reference 18 (https://www.iesve.com/software/software-validation)

The authors agree with Reviewer 4.

The reference to ISO 9001 has been removed. Reference 18 now points to the BEST TEST standard (https://www.energy.gov/eere/buildings/downloads/test-procedures-building-energy-simulation-tools).

2.       If the analysed house is a single-storey house, it is not clear where the staircase leads: is the garage underground? Is the roof practicable?

Thank you for this relevant observation.

The staircase has been built as provision for a  potential future expansion of the house with a second storey. We flagged that in the manuscript and we understand that it is confusing otherwise.

We have indicated why the staircase is included and provided details about the garagein section 3.2?

3.       It is not clear why the orientation of the house changed for SIPW to investigate the impact on the annual cooling energy demand (cfr. Par. 4.3.2).

Thank you for your comment.

We modified the orientation of the house to improve the representativity of the results for other houses with different orientations, and test the influence on the results.

We have provided further explanation as to why we explored different orientations.

Orientation refers to the position of a house with respect to seasonal variables like the path of the sun, wind patterns, and other variables [24]. The house orientation is changed for SIPW to investigate its effect on the annual cooling energy demand [25]. This helps in improving the representativity of the results to similar houses with different orientations.

4.       The methodology used to define and calculate the scenarios for calculating tariffs is unclear, and it is not clear the usefulness of comparing such different contexts as Japan, Australia and the United States (section 4.3.4).

Thank you for this comment.

Our point of comparing the rates with other countries was to show how Bahrain stands in comparison to the international context in terms of electricity tariffs. This is further explained in section 5.2.

We have provided further explanation as to why we compare the electricity tariffs of Bahrain to those of Australia, the USA and Japan.

We have added the following sentence to Section 4.3.4:

These countries have been selected as examples of locations with much higher electricity tariffs and their respective rates per kWh are shown in Table 7

We have also provided the source of the tariff for each country in Table 7.

5.       It would be useful if in Figure 1 the wall were characterised by materials and thicknesses

Thank you for this comment.

We have added the thickness of the OSB and EPS layers and the total thickness of the SIPs wall in Figure 1.

6.       It is advisable to convert the Table 1 (Existing studies relevant to the scope of this paper) into a descriptive text.

Thank you for this comment. However, we believe that a table provides a much clearer and synthetic manner to summarise the literature to our readers.

N/A

7.       In figure 2 it would be better to talk about tools instead of methods (green legend).

Thank you for this comment.

We have replaced the word ‘Method’ next to the green box with ‘Tools’.

8.       It might be useful to indicate at least one section of the house, in addition to the floor plan.

Thank you for this comment.

We have included a section of the house, alongside a photo of its street façade.

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9.       It would be good if a summary analysis of the main details of the thermal profiles and the values assigned to the various parameters in IES-VE were provided in the text and not only in the final supplementary data in the annex.

Thank you for this comment. However, we believe that this would take a lot of space and break the reading flow of the paper. This is why we included it in the appendix.

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10.    Section 3.3 only describes the method but does not provide any information on the quantization of the operating energy. Therefore, the incipit of the following paragraph 3.4 is unfounded. It would be appropriate to quantify the operating energy.

Thank you for this comment.

We have included further details about how IES-VE quantifies operational energy use.

The cooling load is calculated based on the occupancy schedule of the house, internal gains, solar gains and ventilation and infiltration/exfiltration rates.

11.    The quantization of operational energy adjusted to 50 years of analysis is highly dependent on the variation of external factors.

Thank you for this observation.

We have flagged this in our discussion section 5.3. The new content reads:

Firstly, it relies on a case study approach to obtain the results, over an assumed analysis period of 50 years. During that period, external factors, such as the inflation and discount rates, material prices and shipping costs can vary, affecting the results. While the sensitivity analysis has broadened the applicability of findings and considered changes of these external variables over time, results are valid for the case study house, representing single-storey houses.

12.    The methodology for determining construction costs is unreliable and the parameters used are not homogeneous for comparison. Construction costs and operating costs cannot be combined because they are not homogeneously expressed

Thank you for this comment. We understand that construction costs do not include labour costs.

However, it is not clear why reviewer 4 thinks that the two expenditures cannot be combined, as they are expressed in the same net present cost unit.

Added a line to the limitation on the potential underestimation of construction costs.

13.    It is not possible to verify the correctness of data of appendix (see Data Availability at the end) and Table 11: Tabular representation of various parameters results.

We thank reviewer 4 for this comment. We provide the data in line with practice on transparency, so that other researchers can reproduce our results. With quasi-experimental research that combines simulation and real-word costs obtained from suppliers, variability can be high, and that’s why we conduct such a broad sensitivity analysis.

N/A

14.    The proposal to make structural insulated panels cost-competitive implies variations in boundary factors that are not directly related to the material or its use, and therefore cannot be considered in the assessment.

We thank Reviewer 4 for this comment.

We have discussed this in Sections 5.2 and 5.3. Relevant new paragraphs are:

It is worth noting that all the stakeholders involved in the construction of a residential house in Bahrain – owners, designers, builders, etc. – are restricted by the material options available on the construction market and the construction practice. This restriction and inertia in the practice can be problematic as they do not promote potential construction alternatives that can have both cost and environmental benefits for the end user and the government.

This study, like any other, is restricted by a few limitations. Firstly, it relies on a case study approach to obtain the results, over an assumed analysis period of 50 years. During that period, external factors, such as the inflation and discount rates, material prices and shipping costs can vary, affecting the results. While the sensitivity analysis has broadened the applicability of findings and considered changes of these external variables over time, results are valid for the case study house, representing single-storey houses