Comparative Cradle-to-Grave Life Cycle Assessment of Low and Mid-Rise Mass Timber Buildings with Equivalent Structural Steel Alternatives

Round 1

Reviewer 1 Report

The paper is aimed at comparing the environmental impacts among alternative designs of low and mid-rise buildings, using two different systems, namely, a traditional structural steel frame or a mass timber design. 

• Introduction: the authors have a tendency to mention several references together in one general sentence (e.g. Line 41, 57). I think that the literature review shall be improved and the authors need to discuss relevant references in detail.   
• Line 62: "Peterson et al. (2006) conducted..." -  the citation refers to authors not the study itself. Please correct this through the manuscript.
• Line 70-71: I think it is rather a common sense that "as building height increased 70 so did the embodied energy emissions" due to increased mass. Please check more thoroughly the reference for an actual contribution.
• Section 2.1: Why did you use two types of buildings (low-rise and mid-rise)? A comparison can be done with one type (mid-rise). There was no clear justification for that. How may that potentially affect the results?
• The structural design using steel is not well justified and well explained in the paper - more sketches are needed and more clarifications. 
• The authors did not show a main step in an LCA - that is the life cycle inventory (LCI) analysis. The input\output analysis should be explained (in terms of energy, water, resources, emissions, etc.). The authors skipped this step to life cycle impact assessment using TRACI. This might not enable the readers to later check the environmental impacts using other methods. 
• The results of the study are inconclusive - in the abstract the authors showed a potential advantage for CLT, but then in the conclusions section it showed otherwise. It is not clear for the reviewer, which material is deemed more environmentally friendly for such buildings, steel or CLT? 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

1. CO2, NO2 --> superscript should be changed to subscript. 2. The authors did not provide the detailed information in terms of life cycle inventory database. In general, processed wood materials have large impact factors compared to other materials. In this paper, the authors did not show any life cycle inventory data. 3. The background of this study should be improved and the literature review should be added to show the gap of knowledge related to this research.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The article presents an interesting contribution to the LCA case studies field focused on two different structural system comparisons. The manuscript is well written and well organized. However, some recommendations and comments are given to the authors, which can improve the paper quality and provide the reader a more transparent and clearer overview of the research.

Introduction: This section is well organized and provides an appropriate presentation of the topic. Further attention could be given when presenting the knowledge gaps and goal of the study (lines 72-83) and consider reorganizing part of this content in section 2. Materials and Method.

Method: You should include a paragraph describing the process you followed. It is recommended to move some contents (lines 72-83) that you included in the introduction to the method, to better describe the steps and decisions you have taken to achieve the results.

Moreover, I also recommend including a sub-section to describe the case studies (now 2.1), which could be for example “Case studies: Low-Rise and Mid-Rise Buildings). In the case study description, you can also include the LOD (Level of Development) of the BIM object -elements you included in the BIM model, which you used in the Bill of Material Quantities. I also suggest including a table to summarize and compare the main characteristics of the 4 (2 in steel and 2 in timber) compared options.

Results: Line 186-192, you should include more information about the assumptions and scenarios you defined for modeling modules A4, A5, C1, C2, C3, C4, D (transport distances, means of transports, energy consumption, etc.).

Line 195-201: you should be more transparent in explaining which information modules you extracted from the EPD’s specific products?, generic EPD? and which one from other sources. However, in case all the information was exacted from EPD’s specific products you should provide more information about which specific product you used or at least some general information or include it as supplementary data.

In steel construction, additional processes such as cutting and welding of steel plates may account for up to 15% and 10% of the energy consumption and CO2 emissions, respectively, in comparison to the whole life cycle impacts of the steel products. Thus, in order to include these statements please check these references: https://publications.jrc.ec.europa.eu/repository/bitstream/JRC110082/report_d1_online_final.pdf. and https://op.europa.eu/en/publication-detail/-/publication/25e4be8e-97c1-4e79-b37b-a51b7634ef7c.

Transport can also affect the results for timber construction (Please check these references: https://doi.org/10.1016/j.jclepro.2020.121958; 10.1016/j.rser.2016.09.107; 10.1007/s00107-017-1236-1), give special care to include more information about the modeling assumptions you used there.

The end-of-life stage scenario is not properly described. You should include this information because can strongly affect the LCA results, and especially for the timber building and the biogenic carbon account. Please check this reference: https://doi.org/10.1016/j.buildenv.2015.03.022.  There, for example, you can also consider accounting as a benefit of the energy recovery produced during the wood incineration (module D).

Discussion: I recommended reconsidering the calculation of module D, there you should include the potential of recycling of scrap steel, please check this reference: https://publications.jrc.ec.europa.eu/repository/bitstream/JRC110082/report_d1_online_final.pdf. You can include those results in Table 2. That fact can also affect results and reduce the disadvantages of steel structures compared to timber.

Please reconsider enrich the first part of section 4.2 (line 328-346). I suggest you check these relevant references in the field: DOI: https://doi.org/10.5334/bc.46, https://doi.org/10.3390/su10062020; https://doi.org/10.1016/j.buildenv.2015.03.022;

Conclusions: The conclusions are supported by results and provide information about future research. However, you can describe more precisely the comparison of the results, for example by giving percentages (lines 391-405).

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The effort spent by authors to address review comments is appreciated. However, a clear input/output analysis is still missing. The authors need to provide, in detail, the inventory flows that include inputs of water, energy, and raw materials, and emissions to air, land, and water. 

Author Response

Reviewer comment:

The effort spent by authors to address review comments is appreciated. However, a clear input/output analysis is still missing. The authors need to provide, in detail, the inventory flows that include inputs of water, energy, and raw materials, and emissions to air, land, and water.

Reviewer response:

Thank you for your comments and time spent reviewing our manuscript. The authors have attempted to address reviewer comments aimed at providing greater detail about the inventory flows of the analysis. Unlike some other LCA software programs, the Athena IE4B is specifically designed for building projects in Northern America. Therefore, the program accepts inventory inputs as fully manufactured construction products and uses its internal LCI database to generate specific raw material, water, and energy inputs. Table 2 in the manuscript represents the exact material quantities modeled in the IE4B program. All other inputs needed for IE4B to replicate our results, such as location, usage type, and design life, are included between lines 241-263. Therefore, the authors believe we have described, in detail, the methodology and inputs used in our analysis to a degree where they would be reproducible by any reader. A clarification has been added on lines 242-247 of the manuscript to further describe this process and explain to users that detailed input schedules can be generated by IE4B for those researchers who would like additional specifics.

For detailed outputs and emissions to air, land, and water our analysis describes the emissions using the 6 impact categories described in the manuscript. We analyzed these categories in several ways to provide a detailed picture of how each building component and material was contributing to the overall building impact. This methodology of describing emissions and building impact is in accordance with the TRACI methodology and follows similar scope as several other research papers in Sustainability and other journals focused on whole-building LCA. Adding additional details and discussion about output flows under the categories of energy use, water emissions, and air emissions is outside of the intended scope of this paper and would unnecessarily add additional length to the manuscript, which is already 16 pages long. Respectfully, the authors do not feel that additional description of analysis outputs are necessary for the results of this study to support our conclusions and to have impact on future studies aimed at comparing steel and mass timber building alternatives for sustainable construction.

We again thank you very much for your time and detailed review.

 

Round 3

Reviewer 1 Report

I recommend accepting the manuscript.