After writing a recent post about the benefits of simple building forms, I was on a podcast with Chris Magwood, one of the founders of Builders for Climate Action, which created the BEAM Estimator app. Our discussion made me wonder if I could use BEAM to support the argument that simple forms have significantly lower upfront carbon emissions.
The BEAM Estimator allows you to “compare the carbon footprint of individual material options by providing results according to material type and assembly location in the building.” You can easily select and compare any assembly for its embodied carbon footprint, just by moving a check mark on its extensive lists of options.
They also claim: “Long before there is a detailed set of plans, you can use BEAM to examine the carbon footprint impact of massing choices, such as comparing a one-story building to a multi-story option, or a building with a basement foundation to one built at grade.” This is not so easy; you need to save your model and then open an alternate and compare the two. This is the route I took to compare two different designs.
Two floor plans
To test my thesis I started with GO Logic’s 1600 Model Contemporary two-story home because of its simplicity and boxy shape. Then, from a house plans website, I chose a typical 1600-sq.-ft. single-floor plan of the kind I have always disliked: lots of jogs, bumps, gables and doodads. It had been a while since I had done this kind of thing as an architect, but I found my old beloved W&G continuously divided imperial scale and did a manual takeoff of interior and exterior walls for both designs.
I kept it simple on the specifications. I knew a full basement for the single-story home would blow the associated carbon emissions out of the water, so I chose 4-ft.-deep perimeter foundations and a slab on grade with EPS insulation. I spec’d 2×6 walls and mineral wool insulation. The roof was insulated to R-60 with loose-fill fiberglass. And I used the same window area for both versions, as well as the same interior and exterior standard builder finishes.
The BEAM manual says you can compare massing, but this isn’t quite true. Corners and jogs have more material, as does every dormer and gable, but these are not accounted for. There are no inputs for the form factor.
I am sure it will surprise nobody that the two-story Go Logic design came in way lower in total upfront carbon than the one-story house, with material carbon emissions that were only 64% of the two-story house. This is almost entirely due to the foundation type and foam insulation below grade. Having half as much roof made a difference as well. I didn’t prove that boxy is better, as BEAM doesn’t account for complexity; the closest I got was to show two stories are better than one.
Assemblies and materials
The next question I wanted to answer was: Which makes more of a difference, form or the choice of assemblies and materials?
The real fun began when I started using BEAM on my GO Logic model the way it was designed to be used: to compare assemblies and drive down upfront carbon emissions. The program has myriad options. For example, I switched the main floor slab to an earthen floor with clay binder and the underfloor insulation to foamed glass aggregate. I also changed the footings and foundations to a concrete mix with more fly ash. (I didn’t have the nerve to put the house on helical piles, but the option was there.)
Above grade, I switched to carbon-positive cellulose insulation and changed the flooring on the second floor to linoleum. I left the steel roof, and upgraded the windows to triple-glazed, but otherwise used BEAM to choose materials and assemblies with the lowest upfront carbon emissions. The results were astonishing, dropping materials-generated carbon emissions from 25,630 kg down to 9581 kg of CO2e.
There are some caveats come with BEAM. To start, it measures only what they call “Material Carbon Emissions,” or MCE. It does not include emissions from transportation to the site and the construction phase. Asked why not, Magwood responded: “They are much less significant than might be expected (3% to 6% of total emissions), and it’s impossible to estimate them accurately. I did a deep dive for my thesis and found that the assumptions in the Life Cycle Assessment (LCA) software I was sampling were typically 50% to 150% wrong in their estimations compared to an actual analysis of how materials move around.”
This is probably particularly relevant in single-family housing, where so much can depend on the number of pickup trucks coming to a site. It also doesn’t include mechanical, electrical, or plumbing because of the lack of available data; they could add 26% to 49% more upfront carbon. Fixtures and appliances, all items with high embodied carbon, are also missing due to a lack of data. The authors, like the carbon, are totally upfront about this.
“The exclusions described above mean that your BEAM results are not capturing the full climate impact of the building you are studying,” the team explains. “We want to make it clear that the BEAM results are not absolute results and should not be used to support any claims that the ‘total carbon footprint of this building is XX.’”
It’s a shame that mechanical and electrical systems aren’t included; it might discourage designers from putting in so many bathrooms or enough LED fixtures to do surgery on the dining room table.
The BEAM calculator also doesn’t take operating emissions into account, and there is a danger that one might become so obsessed with reducing embodied carbon emissions that one becomes less obsessive about the operating emissions. We should never forget to put energy efficiency first, beginning with the building enclosure and materials. Then, we can figure out how to hit our numbers with the right mechanical systems.
Lloyd Alter is a former architect and developer. His journalism career includes 15-plus years as design editor at Treehugger.com. Today he teaches sustainable design at Toronto Metropolitan University. His work can be found at Carbon Upfront.
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We should all live in cubes.
Seriously though, it was a good article and the tool sounds useful, within its limitations. I do hope, though, that we don't lose sight of quality of life in our pursuit of reducing every last ounce of carbon.
I don't think we're anywhere near ounces yet. The vast majority aren't even measuring tons at this point.
Where’s the picture of you doing surgery on your dining room table?
That's a good list of the things BEAM can't do, at least not right now. I guess we just keep ignoring the things we CAN measure until somebody releases accurate information for the others? Along the same lines, I'm dismayed that my toaster can't cook a full-sized pizza--what a piece of junk!
I'm not aware of any software that can do what you are asking it to do, and certainly not free, independently created software. Someday I think (or hope) that software will be available, but in the meantime, BEAM is helpful for what it can do, and it's available at no charge.
I am not being critical of BEAM! I thought it important and relevant to note that there are limitations. As you say, it is free, it is easy to use, it's got tons of data comparing assemblies, and did I mention that it's free? Others are also creating add-ons and modules. Skylar Swinford has developed the open-source OCEC tool (Operational Carbon Embodied Carbon tool) that lets you compare operating emissions based on BEAM data. https://passivehouseaccelerator.com/articles/now-available-beta-version-of-ocec-tool-developed-by-skylar-swinford
I didn't realize there were third-party add-ons, very cool. And it is interesting how big an impact changing the materials had on your example projects!
For cost and environmental reasons, we chose a precast foundation for the project I am currently working on. The footing is made of crushed gravel. Because the foundation wall itself is only a couple of inches thick (thicker at the base and at the top), it requires only 1/4 of the concrete of a standard poured footing and formed wall foundation. We had our entire 160’ perimeter 5’ tall foundation delivered on a single (although probably overloaded) flatbed semi-trailer. It also came with insulation. I am not affiliated with Superior Walls and I don’t get any discount or quick back; I just think that it’s a worthwhile product.
What’s even more consequential is how many residents are sharing that same foundation; a 4 story/4 unit building can be built with practically the same amount of foundation and roofing material as a single-story one-family home. That living arrangement comes with other pros and cons, but this is something that those who care about carbon emission and arithmetic ought to take into consideration..
“I kept it simple on the specifications. I knew a full basement for the single-story home would blow the associated carbon emissions out of the water, so I chose 4-ft.-deep perimeter foundations and a slab on grade with EPS insulation.”
A 3200ft2 home is likely to be more carbon intensive than a 1600ft home. A 1600ft2 bungalow with full basement has twice the living space as a 1600ft2 bungalow or 2 story on slab or perimeter foundation. That should be considered in the calculation.
For some reason, even though most people use full basements as living space we still ignore it when designing and selling homes.
I haven’t managed to use BEAM yet (but I will soon, looks interesting), but based on concrete alone the least carbon intensive 1600ft2 home is likely to be a 1100ft two story with full basement (3x550ft2), followed by an 800ft2 bungalow with full basement (2x800ft2) and most intensive would be a 1600ft bungalow on slab.
We need to stop thinking of basements as a way of holding up the house and fully integrate them into the design from the start. That includes proper insulation, good sized windows and well designed window wells and a flow that works with the upper levels.
Alternatively we use different foundation slab/wall materials instead of concrete (like ICE) or do away with slabs and foundation walls all together (helical pile foundations).
That's an interesting analysis. I'm temperamentally opposed to basements because they often have moisture issues, and provide poor quality living spaces. But you are right, they don't necessarily have to be like that. It's funny that double-height spaces are often incorporated into the first and second stories of houses, but rarely between the basement and first floor - which would let light get down there, and help integrate them into the rest of the house. Too often, even if they are fully finished, the access is through as small door and down an enclosed stair, which effectively isolates them. As you say - if the basement is going to be finished, it should be better thought through.
>" but based on concrete alone the least carbon intensive 1600ft2 home is likely to be a 1100ft two story with full basement (3x550ft2), followed by an 800ft2 bungalow with full basement (2x800ft2) and most intensive would be a 1600ft bungalow on slab."
You left out the option: 1,600 sq. ft. 2-story on a slab. Seems that would be in the running. Is your logic that because concrete is being send down 4ft anyways, that we ought to keep going and make it living space?
>"For some reason, even though most people use full basements as living space we still ignore it when designing and selling homes."
I would say this is not true in my experience (perhaps a regional thing). True, some people have nicely finished basements that they utilize, but the vast majority of people treat basements as places of utility, not of the same value as above grade space. Of course your proposition to 'better design' a basement from the getgo could perhaps change that. I think the only realistic way for that to happen is with daylight basements (hillsides required).
I am writing this from my "lower level" home office that I built when I duplexed my house a few years ago; our bedroom is down here, as well as a nice den/tv room. I couldn't have done the subdividing of the house had there not been a nice high basement where I can look out above grade at eye level. Basements can be great and I use every inch of it.
Around here we often hit rock at about three or four feet below grade - and where that is helps determine whether it makes more sense to use a slab or crawlspace. A third option I've used is to sink the first floor down that depth, so it is still able to have windows at the usual height, and can have easy access to grade. I don't know - is that still a basement?
Malcolm, are "split foyer" designs popular there? They used to be popular here, with basement floors 3-4' below grade, often with a walkout on the back. My last house had that design and it did allow for a lot of windows into the basement space, though it's a little weird to look out at ground level.
If the first floor is half below grade, does that wall need to be concrete to the floor joists to avoid a mid-wall hinge, or can a pony wall sit atop the concrete to complete the upper half of such a wall?
Tyler, my former house was built in the 1970s and had a tall concrete stem wall that stopped 6-8" above grade, and the vast majority of houses that style are built the same way. The last time I tried to get an engineer to sign off on that type of design, for the Fine Homebuilding 2016 house, they said it wasn't possible due to the hinge effect. I believe they were being a bit overly conservative; the right mix of rebar and footing design should make it allowable, similar to a retaining wall, but it would be easier to get it past code officials if the concrete extended to the first floor framing.
Agreed, there are a few other options (such as the 4ft perimeter foundation option). I chose those as I’d done calculations in the past for comparison. The post is more to get people thinking, a full basement is not necessarily the most carbon intensive when considering overall living space.
This may be bias to my part of the world (western Canada), where basements are almost always finished into living areas, but new homes are usually sold with unfinished basements. That causes many of the issues people have with basements in my opinion. The main levels are designed as a finished house, so we get duplication of purpose, or inefficient use of space in the basement, compounded by the usually small windows in odd places making them difficult to finish well.
I don’t believe it has to be a walk out basement to be good: well thought out, large windows with terraced window wells (get rid of those basic corrugated metal wells for example) can make even a deep basement look bright and inviting.
"well thought out, large windows with terraced window wells "
I've got a project going up in Oregon, 3 levels, 3 apartments. The ground level apartment is actually 42" below grade but the main bedroom (#1) has a door leading onto a small private patio, in lieu of a light well. It steps up to grade. The living room window sills are also a few inches below grade and we've provided a zone between them and final grade that's also part of the perimeter french drain system.
The reason for all of this was to keep the entrance near the same elevation as the sidewalk, for wheelchair access. The sidewalk is about 3' below the main grade of the site.
This is an interesting topic. I just did a quick study of 4 options, looking only at concrete volumes. See the attached image for the full details.
To make it an "apples to apples" comparison I've assumed a 3200 sq.ft. floor area, distributed on either 1 or 2 levels, with a flat site. I've done the foundations based on IRC2021, assuming frost protected shallow foundations where appropriate. Of course, there are a vast number of site specific factors (expansive soils, sloping sites, ...) which would overly complicate this quick analysis. So for those situations a more detailed, site specific analysis would be needed.
Option 1: single story, slab on grade. Footprint 3200 sq.ft. Concrete vol. = 1302 ft^3
Option 2: two story, slab on grade. Footprint 1600 sq.ft. Concrete vol. = 734 ft^3
Option 3: single story with half basement. Footprint 1600 sq.ft. Concrete vol. = 1069 ft^3
Option 4: single story with full basement. Footprint 1600 sq.ft. Concrete vol. = 1416 ft^3
(Clarification, "single story with basement" means 2 full levels of living space but one is partially or completely below grade.)
So, considering only concrete volumes, the single story with full basement is the worst option and the two story with slab on grade is the best.
Of course there are other factors to consider:
The cost of basements will be considerably more, on a per square foot basis, if they have to be detailed to enclose conditioned space. That money could instead go to other energy upgrades on the house. In other words, building a 1600 square foot habitable basement (options 3&4), waterproofed, insulated, window wells, ... is considerably more than building a second floor above grade (option 2). I'm not an expert on construction costs and would welcome hard data on that comparison if any of you are up to that.
A single story design (option 1) has twice the roof area of 2 story designs (options 2, 3, & 4). This has GWP as well as cost impacts.
I've shown code minimum foundation dimensions. I would never specify a 3.5" slab. I'd go to 5". But that would be the same for all 4 options. I also wouldn't do 6" footings. I'd go to 8".
On sites that slope from 3-5' front to back, or side to side, a half-basement that is "walk out" on one side might become a competitive option because of the grading issues associated with doing a slab on grade on a sloped site.
Option 2 is very efficient in terms of foundation and wall perimeter lengths, as well as roof area. This is basically the American Foursquare and there is a reason that form was so popular. It is very efficient. Of course, 100 years ago frost protected shallow foundations weren't an option since the insulation technology didn't exist. So most of the foursquares, especially in frost areas, have basements. But these days we could be returning to the foursquare model, as a slab on grade. This is probably just about the most efficient form possible.
Replacing non-structural concrete floor slabs with plywood "slabs" over rigid insulation seems like a no-brainer. Hopefully building departments will start to see that approach as normal.
Local soil conditions permitting, I would do a 40' x 40' footprint, with a perimeter grade beam and a single grade beam dividing the footprint in half, so that we have 2 20'x40' sections to reduce floor joist spans. Then I'd do a plywood "slab" on the ground level. Alternatively, slightly less efficient but allowing more flexibility in floor planning, I would do 2 interior grade beams, dividing the 40' square into 3 sections, perhaps along the line of a traditional central hall plan.
On poor soils it might be cost effective to support the grade beams on drilled piers.
My calculations were based on 8” foundation, 4” basement slab and 8” (average) slab on grade. That gives around 2000ft3 for option 1 and 1500ft3 for option 4. As you’ve shown, volumes can vary depending on standard practice in areas and also personal choice (do you go with code minimum in your area or bulk it up a bit).
The best option from a GHG (and cost perspective) seems to be removing most of the concrete from the equation. The plywood on rigid insulation is how we’re doing the basement “slab” in the addition we’re doing now. The alternative you show seems a great option to replace concrete if not digging out a basement. We are after all building relatively lightweight stick frame buildings on foundations which can hold far greater loads than we need now.
It's cool that you're able to do the plywood slab. I hope that possibility becomes more prevalent.
I used to work in California and we'd certainly bulk things up. Five inches would be a minimum slab depth and that full height basement wall would more likely have been 10" rather than 8".
I wonder about the possibility of doing a "slab on grade" with a concrete footing/foundation coming up to the floor plate level, but then doing a plywood slab inside it. I'd also consider making the top layer Warmboard to accommodate floor heat where that's an appropriate heating solution.
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