Slab on grade insulation in CZ3
I’m going through the Title 24 energy efficiency calculations for a new construction house in California, CZ3C. I was surprised to see that insulation of the floors for slab on grade foundations is not required by code, unless there’s hydronic radiant floor heating. Since I’m not putting in radiant heating, I’m not required to insulate the floor at all.
Wdyt? Should I insulate the slab anyway? To what R value? How much extra would you spend on insulation if it was your home?
As for how to do it, I found this excellent article linked from GBA: https://www.finehomebuilding.com/project-guides/foundations-and-masonry-work/insulating-a-slab-on-grade
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Replies
I would be very cautious in selecting details, and integrate a robust termite shield. Insulation in contact with the ground, or even accessible from the ground by going through another material (stone, etc) is very tempting to termites. Once they find the insulation it is just a matter of days until they find your framing...
Fair. Termites are a real concern.
Since I haven't hired a contractor yet, I spoke to one who wants my business. They've only ever insulated slabs for hydronic heated floors, since that's the only situation where the code requires it. No one else has ever asked for it. They're happy to do it, though, for basically the cost of materials, since it's not a lot of extra labor. About $2,500 per 1,000 sq ft for 2" XPS from HomeDepot.
I'm going to just do it!
Also: the code requires only R5 for heated floors in CZ 1-15 and R10 in CZ 16.
When I plug the numbers into a Manual J calculation, I find that the heat loss from the floor in cold weather is significant. It's over 25% of the total in CZ3, and adding insulation doesn't have much impact on cooling load. There are diminishing returns for thicker insulation, but going from R10 to R15 would still reduce the load by about 500 btu.
I started out my career in California and worked there a while.
If it's a monolithic slab then I'd do perimeter insulation outside the foundation wall, R-10, with a good termite shield as others have mentioned. It would also need a protection board or 1-coat stucco treatment.
If the footing and foundation wall are poured first and the slab itself is non-structural and poured after then I'd consider doing under-slab insulation, R-10 and R-5 where the slab meets the foundation stem. This avoids exposing the insulation to the wear and tear of the outdoors. I'd still do a termite shield though.
Thanks for the recommendations!