Conditioned crawlspace – how much EPS under the slab
brentwilson
| Posted in General Questions on
In a conditioned crawlspace in climate zone 6b, how many inches of EPS should go under the concrete slab (rat slab)? In addition, how thick should the slab concrete be? 2 inches? 3 inches? 4 inches?
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The first step is to find out what your building code requires.
Are you planning to use the space for storage?
Is the rest of the house built to better than code?
Most of the US has no requirement for sub-slab insulation in crawlspace foundations. The IRC calls out R15 min for crawlspace & basement walls, but has no requirement for anything under the slab. The only slab insulation called out is edge insulation on slab-on-grade foundations:
https://up.codes/viewer/new_jersey/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2
From a long term (even full lifecycle) financial perspective Table 2, p10 of BA-1005 is a good starting point:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
BA-1005 calls out R10 sub-slab for zone 6. That could be done with 2.5" of Type VIII EPS (1.25 lbs per cubic foot density). But if the other "whole assembly-R" in the zone 6 row isn't already up to those levels, it might be better to put the money else.
If using reclaimed roofing EPS it wouldn't be crazy to put 4" or even 5" down, depending on the price. In rough terms virgin stock EPS runs about 10 cents per R per square foot. If reclaimed EPS can be had for 3 cents per R per square foot or less (which is typical) it's still likely to be rational over the very long term go with twice the amount in Table 2, which is still at a substantial cost savings compared to the table values using virgin stock goods.
How would I track down reclaimed EPS? Any ideas of where to start looking?
Never mind, I found this helpful: https://www.greenbuildingadvisor.com/question/using-recycled-rigid-foam
Hi Brent.
Malcolm's right, what maters the most is what you inspector wants to see. So, that's a good place to start, though I 'd be surprised if the inspector called for any insulation. In don't believe the current IRC does (but I didn't check).
Many builders don't think any sub-slab insulation is worthwhile in basements and crawlspaces from an efficiency perspective. Slab-edge insulation may have more benefit.
In basements that may be finished or used for storage, insulating the slab does have the benefit of warming the slab and potentially preventing some condensation issue. In a crawlspace where there is nothing to be concerned about getting wet, this benefit may not be so appealing when cost is considered. You might find this helpful: Determining Sub Slab Rigid Foam Thickness
Rat slabs are usually around 2 inches thick.
We're in zone 6 & we use a minimum of 4" (R16) and at least 6" (R24) if it will be living space. You'll get one chance to do this; if you're going to skimp somewhere, do it where you can correct it later. And isolate the sides of the slab from surrounding concrete!
The stem walls will be ICFs, so the slab will be isolated from them.
>"We're in zone 6 & we use a minimum of 4" (R16) and at least 6" (R24) if it will be living space."
That's not a code requirement, and unless the rest of the house is comparably built, it's overkill (possibly to the extreme.)
The "extra" EPS thickness to go from BA-1005's starting point of 2.5" up to 4" has a significant CO2e footprint if virgin stock EPS is used. Using reclaimed EPS the CO2e footprint at any (reasonable) thickness near zero.
At the rate of global warming most zone 6 locations in the US will be in zone 5 within the lifecycle of a house. The up-front global warming impact of designing a house with a zone 6 climate over it's lifecycle is higher than that of a house designed for zone 5. The marginal operating carbon cost will be falling (perhaps to near zero) even sooner than that, making the embedded initial carbon cost of the building a much bigger fraction of the total lifecycle carbon than presumptions of a decade or more ago. (If we're still pulling fossils out of the ground and setting them on fire for space heating in 30-50 years we're pretty much cooked.)