Overall Carbon Cost of Slab vs Piers
Hi hi!
My partner and I are in the early process of building a ~1000 sqft off-grid cabin in the western Catskills (CZ5 or CZ6). We are aiming to keep the embodied CO2 for the build as low as possible (as well as the ongoing electricity draw to heat the cabin during our cold winters). We’re on a tight budget, but we’ll gladly spend a little more money now to save money in ongoing costs or repairs later. We’re considering radiant flooring rather than minisplits or heat pumps, as the cabin will be airtight.
We are deciding on choice of foundation currently, based on the above rubric.
My question is: I understand that Concrete is terrible for the environment and very high in embodied CO2, but when you sum up the CO2 embodied in the complicated layers of insulation that the floor profile above Piers requires in cold climates (see here: https://www.buildingscience.com/documents/insights/bsi-031-building-in-extreme-cold), and the extra heat loss due to wind passing underneath the raised floor (and thus, more heating & electricity draw required inside living space?), is a Concrete slab still dramatically worse in total carbon cost (embodied and ongoing)?
I’m looking for something ~akin to a lifecycle analysis (LCA, cradle to grave) of these two scenarios that clearly shows that carbon differential?
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Replies
Will you be occupying the cabin full time or just for recreation? If not full time, you should consider the time it will take to bring the cabin to a comfortable temperature when you show up for the weekend- with a slab, this could take a long time.
I have a cabin built on piers, and in truly cold weather (-35c) it takes several days for the place to fully warm up. I cannot imagine trying to warm up a slab.
If the project is off grid, do you plan on using solar for the radiant floors or heat pumps? I would strongly suggest wood or propane as back-up.
You say "CO2 embodied in the complicated layers of insulation", but you aren't required to use rigid foam. That article is 12 years old. I plan to build 1,000sf studio in CZ6 on piers, and right now the design is to fill a deep floor with dense pack cellulose. I'm pretty committed to avoiding concrete, both for installation speed and the carbon footprint.
Brilliant, thank you for this Tim! I had not considered using dense pack cellulose in the floor. Have you seen any good articles about insulating a floor on piers with dense pack cellulose? And are there any other options we should be considering?
How does dense pack cellulose effect the serviceability of the plumbing?
If the floor assembly is reasonably airtight, which it should be, whether the wind blows underneath it or not does not affect heat loss. Wind makes people feel colder by evaporating moisture from our skin, but a house is (or should be) dry enough that there is no "wind chill." But when open air is colder than soil, it does increase heat loss. How much depends on total airtightness and R-values but you can use 100% cellulose in a floor, which keeps the carbon footprint very low, while under a slab you would use either foam or mineral wool, with much higher levels of embodied carbon than cellulose.
Choice of floor joists, subfloor and finished floor will also make a difference on the total carbon footprint.
The primary effect of the wind chill is on the boundary layer of air due to convection. So while evaporation may have an effect, it's definitely incorrect to say that if you reach a certain level of dryness then there's no wind chill effect.
If you look up the explanation of wind chill, evaporation isn't even mentioned:
"The rate of convection depends on both the difference in temperature between the surface and the fluid surrounding it and the velocity of that fluid with respect to the surface. As convection from a warm surface heats the air around it, an insulating boundary layer of warm air forms against the surface. Moving air disrupts this boundary layer, or epiclimate, allowing for cooler air to replace the warm air against the surface. The faster the wind speed, the more readily the surface cools." (wikipedia)
Hhff, I suggest you do the A/B analysis and let us know. And then we can all argue about it.
Also as a design consideration, consider durability in your design. I understand the drive to eliminate a CO2 footprint, and we could build uncomfortable mud huts to do that, but if they will used less, or need remediation/rebuilding more often, at what point do they cost more CO2 over the life than a 100+ year design.
Also, consider pest ingress. Last thing you want is some rats moving in, drinking all your tea and blocking your Netflix account.
Jamie