Optimizing Multi-Family Building Massing for Affordability and Envelope Performance: An Investigation of the Trade-Offs Implicit in Low Rise Residential Buildings

Round 1

Reviewer 1 Report

I think this is an interesting and important paper, however, I have a few suggestions that the authors should consider in a revision:

1) With regard to the envelope analysis, the authors take for granted the discussion that the envelope efficiency translates directly into lower energy usage. More discussion is needed on this. How does a higher envelope efficiency translate into fewer produced metric tons of CO2 equivalents? What kind of fuel usage is assumed (natural gas or oil) and what kind of fuel is needed for electricity? I don't think a whole new set of analyses are needed but if the reader is to understand the importance of envelope efficiency then more discussion needs to be done about how this translates into reduced energy usage. Relatedly the authors say that cheaper is the "greenest" (line 549, p 17)--but this seems to assume that fewest materials are used and thus less CO2 is produced. The relationship between costs, energy efficiency and "greenness" needs to be expanded and better explained.

2) While the detailed treatment of costs is very important, the authors ignore both rental income and land values (which are understandable in this case). However, the notion of "optimal" building size and configuration are not independent of occupant preferences, and willingness to pay. I would recommend the authors add a case study section toward the end, where they try to estimate a best apartment building size and height given some estimates for rent--say median U.S. urban rent and say a rent that is 50% of the median and typical land values. The issue about affordable housing  in the U.S. is that low rents do not often justify the expense of construction in big cities. While showing the cost side is important, some of kind of basic return on investment analysis can be helpful to put the cost side into context

3) The authors might want consider a regression analysis where costs per square foot and envelope efficiency are the dependent variables, with length width, area, height, # buildings a independent vars. (and with higher orders on the right hand side). This might provide a useful equation that gives a more systematic and concise set of relationship.

Some minor things:

• 3D graphs might be worth exploring
• A little more discussion is needed about RSMeans data and database the authors use. Perhaps not everyone is familiar with how they create the database and what data is available.
• To what extent are the assumed building codes applicable to local conditions? It's reasonable to assume one set of codes, but some discussion should be added to convince the reader they are are truly representative of what happens in cities across the country.
• The paper reads now as written for an engineering audience, so more care should be taken to make sure definitions and assumptions are a little more clear for the average reader of Buildings.

Author Response

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Reviewer 2 Report

 

 

The introduction is weak. The affordability data should better describe the rental burden of US tenant households. Referring to home price appreciation is misleading, because tenants do not buy houses and even if the price of rental property has gone up, which needs to be shown, that does not imply that rents increased in the same proportion, as interest rates have come down over the last years. It is really safer to look at rental indices and to compare these with income or wage indices, preferably for tenant or lower-income households.

The introduction of the environmental challenge (lines 59-69) is even less convincing, as it mixes energy use with climate change (even "environmental disaster" in the abstract). The paper only looks at (potential) energy consumption of buildings, i.e. the thermal insulation performance of buildings' envelopes, so it can contribute at most to reducing heat loss through the envelope. This is quite far from total energy use and even farther from CO2 emissions. Even if the model does not make these distinctions, it should at least be mentioned that wasting coal-generated electricity, heating oil, natural gas or district heating does not have the same consequences for the climate. In the limitations of the paper, and possible extensions, one should also mention that the grey energy is not taken into account when comparing building solutions. Nor is the impact of energy efficiency on the costs for the tenants considered (a more energy efficient building saves on heating costs for its occupants).

In the (brief) review of the literature, this paper is presented as novel by suggesting features that the model does not have. E.g. "few [studies] are cross-sectional geographically" (110-112); this paper is not. "few studies have considered multiple shape variables (e.g. both height and aspect ratio)", which is absurd, because changing the height necessarily changes the floor area for the same total surface area.

Where the paper is strong is in estimating carefully construction costs … but only for standard US American buildings. As a consequence, the results of this paper cannot easily be carried over to other countries, with different building practices and regulation, in particular as regards the insulation of buildings and heating systems.

Different building shapes are compared in terms of construction costs and thermal insulation performance of the envelope. For the comparison to be valid, all shapes compared must provide the same living area. The authors look at total floor area, which is geometrically much easier, but the share of floor area that is "wasted" for circulation depends on the building's shape. The comparison should also be enriched by considering total footprint. After all, high rises are often chosen despite poorer cost and energy performance (and a greater share of volume lost to circulations) because they save on land. In many real-world projects for multi-family buildings, the developer does actually have the choice between one high rise and several lower buildings, because the plot is large enough for both, but he may opt for the high rise because it offers ample outside space for the tenants. And if that space is well managed, it can actually contribute to biodiversity and a healthy living!

Of course, not everything can be addressed in such a paper. What is addressed is fair but not excessively original or profound. Core relationships between shape and construction costs and between shape and total thermal performance of the envelope are demonstrated, as well as trade-offs between costs and thermal performance are demonstrated and well-illustrated. Not many developers will be surprised by these results, so this paper is more useful for students of architecture … provided they care about construction costs and thermal performance.

The text needs to be carefully reread for typos and curious statements. E.g. lines 39-40: it is hard to believe that half of all US households pay more than 30% of their income in rent, as there are only about a third renter households. Or lines 66-67: "growth in total residential energy consumption has continued to climb…". In equation 1, the multipliers got lost.

The indicator for environmental performance, the "envelope efficiency number", is a bit misleading, as a higher value of this number means less efficiency and poorer performance. May be "thermal loss value" would be more appropriate and still comprehensible.

Author Response

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Reviewer 3 Report

General comments

As a professionally working architect for many years, I cannot agree with the theses and legitimacy of the research carried out. In my opinion, they are too selective. When designing low-rise multi-family buildings (and any other type of building), not only internal but also external aspects are taken into account, and they are equally important. Building design cannot be discussed in such a narrow sense! In addition, the performance aspects of the facade are directly related to the surrounding space in terms of all environmental, landscape and urban elements. This cannot be ignored.

 The authors emphasize that the research carried out refers to the United States, but to what specific part? In this country, we have several completely different climatic zones, the impact of which has a direct influence on the design method (even if we narrow the topic only to the one mentioned in the article), so it is definitely too broad a research field for such a narrow research topic.

Another controversial issue is the assumptions presented in the article, which state that when narrowed to rectangular buildings (L 236) - which is definitely too narrow a research field - plans based on a wider distribution of functions in corridor buildings are more economically efficient. The presented examples to confirm this thesis show remarkably non-ergonomic solutions (Figure 2), which, even if they are based on legal guidelines in force in the United States, require at least a discussion, and certainly a reference to commonly known and accepted standards when presenting the research background. Mainly for: Neufert E., 2019, Architect’s Data, 5th Edition, John Wiley and Sons Ltd, Hoboken, United States

Contemporary architectural design is associated with a whole spectrum of dependencies that should be taken into account. The presented small section of the issues related to this topic is too small for the research to be considered reliable and likely to influence the principles of building planning.

Specific comments

The introduction presents the research background of the presented research, focusing on the legal regulations in force in the United States. There are also examples of strategies for building economics and energy saving principles in China. Such a narrow and somewhat illogical state of research is worrying. There are many examples of American research cited, and it is not fully understood why the example from Hong Kong suddenly appears and why there are no more such examples. It is even more surprising that there are many examples of similar regulations (even in Europe itself, where apart from EU guidelines there are also legal rules that apply to each country separately). The advantage of the chapter is the clearly presented purpose and structure of the article.

The structure of Chapter 2 is clearly presented. The adopted criteria and the purpose of the research, which are far too narrow, raise serious doubts.

The results presented in chapter 3 based on statistics refer directly to the assumptions of the article. The statistical study does not raise any doubts, apart from the adopted starting factors, which consistently are too few.

Chapter 4 is more of a conclusion than a discussion. The authors in no way refer to the existing studies and examples from other countries. This chapter would require a very thorough editing and completion.

References are scarce and miss many key items.

Author Response

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Round 2

Reviewer 1 Report

I think the paper is fine. Couple of minor things:

• Figures need to be updated to reflect new use of phrase "Thermal Loss Factor"
• Ideally, I would like to know, on average, how many metric tons of CO2  equivalents are increased when the Thermal Loss Factor becomes worse by one unit or 1%.  I think it would be interesting and useful for readers to know this, if such relationship can be provided.

Author Response

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Reviewer 3 Report

The text looks much better in its current form. The work that the authors put into redrafting and supplementing the article should be appreciated. The aim of the research still raises my doubts, but the positive aspect is that the authors of the study broadened the impact of a wider spectrum of impacts on facades by external factors, which was emphasized in the present text.

The first chapter has been significantly expanded to include references to more examples from around the world relating to similar research topics. Sources referring to both scientific research and legal regulations taking into account the diversity of individual countries in terms of legislation and environment were presented.

Chapter 2 was significantly supplemented, where the adopted methodology was also referenced with a wider set of external aspects related to facades. The unchanged Figure 2 still raises doubts, where the presented functional diagram of the level plan is controversial from the ergonomic point of view.

Figure 11 contains a minor bug in the placement of the statistics results.

The changes introduced in chapter 4.1 and 4.2. deserve great improvement. The conclusions were presented in a much more interesting and comprehensive way and, above all, a much more honest discussion was conducted. It is true that I still have the impression that this chapter could be further expanded, but in relation to the previous version, the change is considerable and shows the work put by the authors to improve specially this part of the article.

Author Response

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