Frost-Protected Shallow Foundation for Unheated Building
Question: Does anybody have experience using a frost-protected shallow foundation for an unheated (or semi-heated) structure? How’d it go? Apart from a builder’s guide from 2004, there’s a dearth of information online for this setup, so I’d love to hear if anybody has had success with it.
Location: Near Halifax, Nova Scotia. Winter climate is comparable to Portland, Maine (6A) so it’s safe to use that as a proxy.
Context: The building is 32’X22′ with a garage on the ground floor and an ADU above. With the exception of a small utility room, the garage portion will be unheated, with exterior walls insulated to code. The engineer and foam rep are confident in a continuous sub-slab layer of R-10 EPS, with 4′ of R-10 foam extending from the edge of the foundation. There will also be R-10 foam running vertically up the slab edge.
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Those specs seem to concur with the methods documented in this FPSF guide: https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf
In the case of EPS, one thing I would be paying attention to would be its compressive strength. Most of the readily available EPS I see would be fine under a slab, but possibly insufficient under a footing.
I'm in Maine and every time I have considered a FPSF for an unheated (or minimally-heated) building, the amount of extra foam and fill required leads us to go with frost walls. For unheated buildings, we need frost wings 5' from the building corners and 4' from the field. The guide you shared is still the best, simple explanation. If you want more details, you can buy ASCE 32-01 which is the source document for the builders' guide and the IRC code.
Thanks for the tips, Michael and Patrick. That guide was very handy. In the end we went with a 4' skirt. As Michael hinted, it was a pain to fill and properly grade. But, the end result was a nice slab with we formed up in a couple of days. The rebar was another story. Time will tell is we have any issues with heaving, but it seems unlikely given the perimeter of foam we have around the building.