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Community and Q&A

Frost protected shallow foundation w/ attached garage (Zone 5)

AHen | Posted in General Questions on

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) 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?
-d-Something else.

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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  1. user-2310254 | | #1

    I'll give your post a bump. Also... Here's a quote from a Fine Homebuilding article on FPSFs.

    "So-called frost-protected shallow foundations usually consist of a monolithic (thick-edged) slab wrapped with vertical and horizontal rigid-foam insulation. Although the International Residential Code (IRC) does not require a shallow foundation to have insulation below the slab, omitting the subslab insulation is not a good idea. After all, the more insulation you have under the slab, the less heat will leak out of your house into the soil below."

    1. AHen | | #2

      Thanks, Steve! Yes, the more I read, the more I realize of course I will want the horizontal wings/under-slab insulation. Big duh. I'm left with some remaining questions, though, like...

      -So are there any guides to figuring out what the optimum amount of horizontal insulation would be, given that we're looking at exceeding the minimum design guidelines in the IRC?

      -How does the interaction with an attached unheated garage affect this? (Again, the question of how separated you keep these areas from each other, with their attendant design implications.)

      -Why does the design for FPSF under unheated buildings include a layer of "non-frost susceptible fill" between the slab and the horizontal insulation, whereas the design for heated buildings shows the slab directly above the horizontal insulation (presumably with a vapor barrier in between)?

  2. bill328 | | #3

    I am just about to break ground on this type of foundation and we are using the MonoSlab EZ Pour forms, more specifically, their Arctic Forms. I am in Climate Zone 6. You leave them in place after the slab is poured. You want to have NO insulation gaps, especially at corners. There should be continuous insulation around the slab. You might need a geotech report on the soil, especially since it's high clay content. Otherwise you might have an engineer who is reluctant to sign off on the final design. I am attaching a snip of my foundation design. In order from bottom to top: undisturbed soil, 3/4" washed gravel (washing is debatable but that's what I have available), insulation board, vapor barrier (it's reversed on the attached plans and this has been a source of debate with my architect), concrete slab. I am also going to have an exposed polished slab. Hope this helps

    1. AHen | | #4

      Bill, thank you for sharing! That product looks interesting... seems to be EPS, which is at least less environmentally unfriendly than XPS. If I can get a builder to talk to me (lolsob), I'll be able to share this! Is the brown vertical layer on your schematic (left side of the wall assembly; I put a blue arrow there) rigid foam insulation, as well? Forgive the newbie question, but why have the extra bend in the metal flashing instead of having the foam continue up directly from the place where the notch in the EZ forms is filled with a foam plug (I think that's what I'm looking at in the circled area on your diagram)?

      1. bill328 | | #5

        You have a keen eye for details. That's actually how it will end up but my architect needed to show the form lumber in place. We will be pulling that lumber out and replacing it with some additional foam insulation to run up on the wall. This creates the completely sealed slab that we are trying to achieve. The metal flashing is in place to hide and protect that small exposed portion of foam. I am tortured that I have to use yet more foam products for this build but I had to accept tradeoffs as I try to achieve a net zero, fossil fuel free, passive solar home. Some foams now use a steam foaming agent- such is the case with our SIP panels from R-Control and the 2" IX EPS Foamboard for Underslab by Insulfoam. Not sure how the EZ Pour product is manufactured but it's worth an email to [email protected].

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