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Too much insulation for Frost Protected Shallow Foundation?

Nick_Jorgenson | Posted in General Questions on

Hi all,

I’m a builder in Northern MN (Grand Marais), climate zone 7.  

I’ll be building a “Pretty Good House” this year for my family and myself.  1600 sq ft, story and a half, timber framed, double stud walls, dense packed cellulose walls and cathedral ceiling.  

I’m mostly familiar with frost wall foundations, either slabs or full basements.  
We recently moved up north where most builders are using frost protected slab on grade, with 2″ of rigid foam under slab, along slab edge, and as wing insulation.

I like the idea of FPSF up here, especially given the difficulty in excavation (mostly clay, very rocky).  That said, I’d like to reduce my impact by using less concrete in the building and would like to “pour” an earthen floor (cob) rather than a concrete slab, inside of the stem wall.
I came up with a shallow foundation detail based on what I’ve read and understand about good R-value targets and what’s in the “Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations”. 
Basic foundation design is to build a shallow, frost protected foundation wall out of ICF forms, on a footing, with high density foam underneath the footing, and frost protection.
If I’m understanding correctly, most Frost Protected Shallow Foundations designs are based on a slab on grade with a turn down footing, or thickened edge.  No foam under the turn-down footing.  From what I understand, we are relying on heat from the building to keep frost away from the building.  
I’d like to reduce heat loss through the floor slab and through the footing, at least as much as is reasonable.  I want to make sure that by adding insulation under the footing and with that much insulation at the foundation wall, we are still protected from frost-heave.  There’s really no specifications for this type of assembly in the guide, as far as I can understand.  
If, per my current design, I put 2″ of high density EPS under the footings, and have 5″ of EPS on either side of the shallow foundation stem wall, do I need to go out from the building further with the wing insulation?

Any other/general thoughts on this design?

Has anyone here designed or built something similar or have any recommendation on who might be able to consult on this?

I did contact a local engineer, who said that he can’t calculate this assembly, and that he would just follow the code book’s recommendation for FPSF (monolithic slab).

Thanks for any help/advice.

Nick Jorgenson

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  1. Expert Member
    Akos | | #1

    If you look at your foundation cross section, even without the insulation under the footing, there are no thermal bridges except maybe a slight one into the middle of the double wall. It would be simpler to skip that detail.

    With a lot of rigid under your house slab, there is no heat transfer to the soil so you have to treat the building as if it was unheated. Design it as a big unheated garage.

    This is good information for working through the required R values, wing insulation is always need in cold climate:

  2. Expert Member


    If you go back to the guide you referenced and Akos linked to, it is divided into details for heated buildings (which to some extent rely on that heat to warm the surrounding soil), and and unheated ones (which don't and are fully insulated).

    In that second section you will find the details necessary to modify the design you have sketched.

  3. user-1139192987 | | #3

    Hi Nick,
    My daughter and I are designing a small off grid timber frame + straw bale cabin to be built near Beaver Bay. We, too, are aiming for lower impact. I'd love to hear about your project with a cob/earthen floor. We'd like to do something similar. Might you be available for consulting?


    1. Nick_Jorgenson | | #5

      Feel free to reach out by email.
      [email protected]

  4. Joe_Garvilla | | #4

    I've been pouring over FPSF and went so far as to purchase the ASCE 32 standards. Two things happen when you follow ASCE32; you increase the ground warmth due to the insulation wings in a sort of "lens" effect, and that in turn changes the delta T between the floor and the soil.

    There is an optional under-slab insulation in ASCE32 which denotes extended frost wings and r-value if the floor is insulated. There is not prescriptive information about how much more insulation is needed on the exterior/frost wings. (Figure C3 in the Commentary section) Important to note that the slab is insulated between the floor slab and the stem wall, and under the slab. The Stem wall has the standard vertical and horizontal wing detail (Again, higher r-value). There is no insulation on the stem wall below the footing or on the interior side of the stem wall. *edit: There is an intentional thermal bridge as the wall sill*. I'm hesitant to post a screen grab of the detail due to licensing and I haven't the time to sketch it unfortunately. The stem wall protection relies partially on the geothermal warming.

    Lastly, I would reduce the underslab insulation to something that handles a delta T of 16 degrees (68 building temp to 52 degree soil). Albeit, the delta T is likely slightly larger in practice if there is radiant in floor heat.

    The Minnesota Department of Ag has 4 and 6" ground temperature sensors to show soil temperatures. This is meant for agriculture of course, but I find it quite interesting to look at.

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