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Unheated Frost-Protected Monolithic Slab for Garage in 5A?

alex909 | Posted in General Questions on

I’m planning a conditioned detached garage in zone 5A. What foundation style should I use?

I’m considering a monolithic frost-protected shallow foundation for unheated buildings per ASCE 32-01, just in case I let the heating lapse during the coldest winter days. However, I can’t find a detail of this that also includes foam insulation against the footing outside edge, or a vapor barrier.

Does anyone know of a detail for unheated-building monolithic FPSF including footing insulation and a vapor barrier?

The solution for semi-heated (build like heated, but extend footing 8″ deeper) seems like it’ll be too expensive. If I use a standard monolithic FPSF for heated buildings, and I end up not heating it, will that cause problems?

My location has a mean annual temperature of 50F and an air freezing index 1100-1400. Code does not require frost wings, but it seems like it would be a good idea since I will likely want to keep the space conditioned.

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  1. Expert Member
    1. alex909 | | #2

      I have, yes! It is quite interesting, but unfortunately it doesn't seem to offer any details for an unheated-building FPSF with vertical-wall insulation (like they do show for the case of heated buildings). Maybe that's just because it's not really needed in an unheated building? Mine is more like a semi-heated building, probably -- but the revised guide just refers to ASCE 32-01 for that, which says to build like a heated building and increase the wall/corner foundation depth by 8".

      To me, this implies that the heated building design alone is unsuitable for a semi- or unheated building. So I'm wondering if there is an alternative to the semi-heated design, which doesn't involve such deep foundations.

      I'd love to just build to the unheated building design, if that is the safest and cheapest option, and ideally add vertical foundation insulation and a vapor barrier. Have you seen any designs incorporating that?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        I'm afraid all I know about the subject came from reading the guide. I build in an area where the frost depth is 18", so it's not something I've looked into.

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