Frost protected shallow foundation w/ attached garage (Zone 5)
I’m considering using a FPSF for a new build with an attached garage, and I’m trying to wrap my head around how the insulation should work.
-I would really like to use the (polished) concrete slab as the finish floor.
-Soils in my area tend to have a large % of clay and silt and are therefore susceptible to frost heave (if I understand correctly).
-Climate zone: 5 (NE Illinois)
-Heating degree days: 6633
-Air freezing index: 1543
I’m looking at the Builder’s Guide to Frost Protected Shallow Foundations (Sept. 2004 revision—not sure if there’s a newer one) https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf and based on that, it looks like the recommendation is for vertical insulation along the foundation for heated structures (no horizontal under slab or around the perimeter in my area—pg 7 & 33) and two options (pg 27) when an unheated attached garage is added.
So I guess my question goes beyond the foundation and to insulation of the walls and roof/attic as well… but I feel like there’s an interplay there. Does it make sense to:
-a-Continue the external rigid foundation insulation for the heated part of the house up the wall between house and garage, wrapping the whole house part in continuous insulation but leaving the garage walls (and roof?) uninsulated and then using horizontal insulation under a FPSF garage slab (unheated enclosure)?
-b-Wrap the heated part of the house in continuous insulation (including the wall between house and garage) AND fully insulate the garage? I feel like this would keep the garage MUCH more pleasant in the winter (might even be able to work in there for a longer part of the year), but then I assume you’d still treat the garage foundation like an unheated enclosure and insulate under the FPSF slab? Or is insulating the walls/roof of the garage more expensive than it’s worth?
-c-Insulate house like a but use a regular slab for the garage… but how does this affect the frost protection of the soil under the heated house?
Finally, if I don’t install radiant heat in the concrete slab/floor, am I just going to have miserably cold feet all winter, or will sufficient vertical insulation along the slab edges keep it reasonably comfortable? I would want to use area rugs strategically, but otherwise I really love the look and simplicity of the polished concrete.
Thank you for wading through my long post!
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