More Building a Net-Positive House
In October, 2021, Connecticut-based builder Ben Bogie of BPC Green Builders broke ground on a trailblazing project in the Hudson River Valley (climate zone 5). A collaboration with Trillium Architects, the 5000-sq.-ft. house is Japanese pagoda–inspired and promises to be lesson-rich. The overarching goal is to use building science–based best practices—products, techniques, and assemblies—to construct a high-performance, net-positive house. Keeping upfront carbon emissions to a minimum is a key objective. To that end, the project minimizes the use of virgin foam insulation, reduces concrete where feasible, is engineered to avoid the need for steel, and optimizes materials made of recycled content.
GBA will follow this build and share key construction details along the way. This is the first episode in an 11-episode video series. It looks at an insulating composite concrete form (ICCF) foundation, specifically The Perfect Block from Eco Building Systems.
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Ben Bogie is a second-generation high-performance-building obsessive working as a project manager for BPC Green Builders of Wilton, CT.
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22 Comments
This is a really interesting system. On the one hand, it is fantastic that it can be made from waste foam. On the other hand the incorporation of Portland cement in the foam adds some significant CO2 emissions. That merits critical examination in a project where minimal embodied CO2 was an explicit goal (as should be the case in any project). Was there data provided on the amount of Portland cement in these and did the team calculate the impact of that?
As a side note, "promises to be lesson-rich" sounds what I say when things are going really badly. I hope these lessons are learned less painfully than some of mine were!
Hi Charlie,
Thanks for the comment. I am the lead architect at Trillium Architects.
Yes, we are still using a concrete foundation and, as you say, there is Portland cement in these blocks. However, on this job we are using significantly less concrete than just about any home we have designed before and we are super proud of it.
1. While the footings themselves are pure concrete, the foundation walls (as Ben said) use 60% of the amount of concrete (and thus Portland cement) that a traditionally built foundation wall uses. The forms themselves are 87% recycled EPS. Due to the shaping of the blocks (more like traditional CMUs than ICFs) the amount of concrete poured between these forms is significantly less than a standard wall pour (getting us down to 60% of typical concrete use for the foundation walls).
2. There will be No Slab poured in this house. This will be our 4th or 5th Slab-less Slab house. The crawlspace below the entire house will have a 18" thick floor of Aero Aggregate (R30) which is a recycled glass product that utilizes no concrete... or cement. There will be no concrete floor slab placed on top of that. (There will be a small slab at the 2 car garage only.) So we are saving a tremendous amount of concrete by not putting a slab in the crawl space.
3. This house has a 5' crawl space and not a finished basement. So our walls are shorter, there is less space to condition and because of the ICCF Product we are using, there is no need for interior finishes or sprayed fire protection on the walls (which we would need if we used spray foam insulation). We are adding extra inches of rockwool board insulation to the outside of the foundation walls to get an even better crawlspace R Value.
All of these features add up to perhaps the most sustainably built, energy efficient crawl space or basement we have ever built. (20 years into building super sustainable homes.) So we are dang proud of the low embodied carbon of this crawl space.
(BTW when my friend Ben says 'lesson rich' he means it ~ and we LOVE it! We all geek out (over here) on learning new stuff. And while many architects and builders avoid change and avoid trying new things, we thrive on it. When Ben says this promises to be 'lesson rich' he means that in the BEST way. We are all super psyched to make this our lowest carbon per sf home to date. And if this goes like all of our previous BPC-Trillium jobs we will raise (or rather... lower...) the bar again for reducing carbon footprint. So far, so good. Stay tuned!)
And, thanks so much for your note and the opportunity to shed a bit more light on our goals for this house.
Elizabeth DiSalvo, AIA
Trillium Architects
We’ll we can stop releasing videos now that you’ve given everyone the cliff notes 😂😂😂
Lol, sorry Ben! I'll try not to do any more spoilers!
Thanks for the great additional details. That's great that it's using Aero Aggregate and skipping a slab. Avoiding both polymer and cement there will really take a big chunk out of the embodied carbon emissions. And the short foundation walls also help a lot.
My question was more about the specific choice of the blocks, and the use of Portland cement in them. It's clear that through other measures you have greatly reduced the carbon emissions of the project, but I'm wondering whether you actually know the emissions associated with the cement used in the blocks.
Here's a really rough estimate. If they are 87% recycled foam, and 13% Portland cement, that's in the same ballpark as the 10-15% Portland cement in typical concrete. I don't have numbers on the volume of the block material vs. the volume of the concrete, but just eyeballing it from the video, it might be about equal volume of concrete and block. So if the concrete volume is 60% of typical, the total volume, and thus the total quantity of Portland cement, would be 120% of a standard concrete wall the same height. That 20% increase needs to be weighed against the benefit of using the recycled foam.
For R-28, one square meter of EPS insulation would result in emissions with climate impact equal to 17 kg CO2, based on the numbers in this article: https://www.greenbuildingadvisor.com/article/choosing-low-carbon-insulation. One square meter of 12" thick concrete and/or block would have 685 kg of CO2 impact. If that's 120% of the standard foundation wall, the increase relative to the standard wall is just the 20%, or 114 kg per square meter. So that's a net of 98 kg more CO2 impact for this wall system vs. a standard concrete wall with EPS insulation. Or 114 kg more if you use recycled foam whole instead of grinding it up and gluing it back together.
I'm not confident of those numbers--there are lots of things I estimated only roughly. The point is just that this does not appear to be a slam dunk for the new material. The net impact of it, even after accounting for the savings in concrete, is more than six times the impact of using polymer insulation. There's room for considerable error in my estimates while still having this come out clearly wore than a more conventional foam-insulated concrete wall.
Even if my numbers are correct, the overall project has vastly lower CO2 emissions compared to a typical one, so please don't think I'm criticizing the project as a whole. I'm just not seeing evidence that this particular choice was beneficial in that regard.
I think I understand where you are going but I would look a bit more closely at the quantities. You are mulling over the question ' Is a simple CMU wall less overall concrete than our '13% concrete CMU plus poured concrete cores'?
If we use only hollow core CMU concrete walls we still have to pour much of that hollow wall to achieve structural requirements for point load, sheer and uplift in the house above. ... so its never an empty wall. So if you take a 100% concret hollow core CMU and fill the places you need with poured concrete (maybe 20%? of the holes? Not sure but I doubt its less than that) how much concrete is that?
These blocks are 87% NOT concrete and they have a lof of 'cross connectors int eh blocks- unlike traditional ICFs . (With traditional ICFs you pretty much pour a full concrete wall between sheets of foam board held together by very small plastic connectors. These blocks are much more like classic CMUs - there is a lot of 87% non-concrete -recycled-EPS connecting the two side walls. SO, the amount of concrete we are pouring is considerably less (hence the 60% number.)
I would wager a bet that he amount of concrete used in each wall we are talking about here is close to the same.
Anyway if we did an all concrete wall (CMU or otherwise- we then have to insulate.
What are our choices?
1. Rockwool: Relatively high embodied carbon.
2. Spray foam-big environmental issues. We are trying hard to avoid all spray foam.
3. Foam board. Basically what we are using -but in the form of 87% recycled EPS. Hard to do much better than this. We always try to use recycled foam board but its really hard to find in any sizeable amount.
We like this product because it uses REDCYCLED EPS. Great use of material.
We can then use limited amounts of Rockwool to get to our desired R value.
And Use NO spray foam.
Using this one product also means we reduce labor and shipping on so many things (foam board, travel time of different installers, adhesives, spray foam, fireproofing, etc.) Shipping and labor are all reduced or eliminated by killing 3 birds (structure, insulation adn fire proofing) with one stone - these ICCFs.
Honestly, I personally can't think of a better answer- except maybe wood foundations- but that is a big maybe. We would have to assess our structural needs and crawlspace needs to really see if this could be a fit...and still we would have to insulate them considerably to get the same R value and the only product I can think to use with wood foundations is rockwool and that has a relatively high embodied carbon value too, soo...
We're going to do the 'carbon math' on this one and will test it against other options and report back, but honestly - as we chase these things down - I cannot think of a lower carbon - and less bad for the earth over all insulated crawlspace option.
We'll keep you posted!
"We're going to do the 'carbon math' "
Please do--my math was only to show that it's not a slam dunk for this new material until somebody gets the real data. It matters, for example, if the 13% that's not EPS is mostly cement, or if it's concrete, which itself is only about 13% cement. Those details matter.
Will do Charlie. We fully agree with you in doing the numbers.
More soon,
Elizabeth
One disconnect here is the 13% number for the perfect block- that number is actually the percentage by volume of non EPS constituents (binders, portland, WATER, air space,etc) Only .96 pounds of cement are used per 10" block in order to bind the EPS. So we can look at the numbers by comparing 100/sf of 10" Perfect Block wall to 100/sf 10" cast in place-
10" cast in place has-
3.09 yards of concrete
or approx 12,300 pounds (assuming a yard is about 4000 pounds)
if 13% of the mix is cement then that equals approx 1606 pounds of cement in that 100/sf of wall
10" perfect block wall has-
.85 yards of concrete
or approx 3400 pounds
assuming the same 13% of mix means we have 442 pounds of cement in the concrete.
then there are 25 blocks (each covers 4/sf) with .96 pounds of cement per block for a total additional of 24 pounds of cement
total of 446 pounds of cement for a 100/sf wall
So that works out to a 71% reduction in cement between the two assemblies
BUT... we now need to add insulation to the 10" cast in place... even if it is 100% recycled insulation that you add to the cast in place we can all agree that there's a substantial carbon benefit to this method.
The blocks are also made in a renewably powered facility that does all of its own onsite water treatment so they generate zero waste water.
Thanks Ben--that number 0.96 lbs of cement per block is the number I was missing, and it does make it sounds much better than my first cut at a calculation!
I think a few of your other numbers are a little off, so here's what I get working it through. They make two types of 10" block. It looks like the one shown in the video is the symmetrical one, that has 6" concrete inside. That's good, because that's the one that is structurally appropriate here. That gives R26 insulation and uses 1.31 yards of concrete per 100 sf, according to the Perfect Block Technical Construction Build Manual, p. 3.1. So I'd correct your numbers to 681 lbs of cement in the concrete and 24 lbs of cement in the blocks for a total of 706 lbs of cement, or about 2330 kg of CO2 emissions.
The "PRESCRIPTIVE METHOD FOR INSULATING CONCRETE FORMS IN RESIDENTIAL CONSTRUCTION" puts 6" screen grid wall equivalent to a 5.5" thick flat ICF wall, which, adjusting your calculation for a 10" concrete wall down to 5.5 inches, yields 883 lbs of cement, or about 2910 kg of CO2 emissions.
As you say, that's just the wall, so we need to add the foam (if recycled can't be found)
16 kg CO2 per square meter of the foam comes out to 150 kg per 100 sf. Adding that to the 2910, I get 3060 kg.
So that means the ICCF does have lower emissions, if we can trust that 0.96 lb number (I don't see it in any of their documents), by about 24%.
I'd still want to see documentation of the cement content before I'd use it in a project, and maybe documentation of what else makes those blocks weigh 30 lbs if so little of it is cement, but if that's really true, combined with the other advantages listed, it seems like really good stuff. I'd also want to consider it for exterior below-grade retrofit insulation.
Elizabeth,
Great work on your low carbon foundation. Because of the nature of the forms it looked like the concrete was quite loose when they poured the walls. Are there any special specifications for the concrete mix when working with this type of forming system?
The manufacturer specifies 3/8” aggregate and a 7-8” slump. Ideally a line pump is used with a 2” hose whip but all we had available was a boom pump (partly necessary because of site conditions) and a 2 1/2” hose whip. It was manageable because of a very skilled boom operator but was a workout for the pour crew.
Very cool! Looking forward to seeing what other interesting ideas you explore with this project.
As you're using foamed glass aggregate and no concrete slab, where are you locating your vapor barrier?
Was wondering the same.
Stay tuned...
This foundation wall and “no slab” approach looks very promising. I am wondering how it would compare with a standard approach in terms of being a rodent barrier?
Those blocks have an r-value of about 1.5 an inch. If only more architects and builders knew about aerated autoclaved concrete http://www.aerconaac.com
R-26 for a 10 inch block. That's 2.6 per inch, not 1.5.
Scott,
I'm not sure it's a viable alternative for a foundation. I've never heard of using AAC below grade. It is fairly weak and doesn't take wetting very well.
https://www.greenbuildingadvisor.com/article/does-autoclaved-aerated-concrete-make-sense
What does this system cost vs a traditional foundation?
It bothered me to see the thick, black layer of spray-applied waterproofing being applied to the tops of the foundation - BEFORE the blocks were laid - such that there's no chance of a block-to-foundation bond. Yes, there will be cells filled with grout before long, and those would have a good bond to the rebar, but in proper sequence, wouldn't that waterproofing better have been applied after the first few courses of block?
David,
Any mortar to concrete bond is going to be inherently weak, and unable to resist lateral forcesby itself. It has never been relied upon for that purpose, and the only practical reason mortar is used beneath the first course, is to ensure a level first row of block. The rebar, or keyway in the footing, or basement slab has always been responsible for handling those.
If you applied it after, it would only stop water after so many ft upwards, leaving you with the potential for moisture migration into livable space, assuming a finished basement.
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