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Frost-Protected Shallow Foundation for Unheated Attached Garage

user-6991874 | Posted in General Questions on

Hello, I have built/worked on quite a few FPSF houses in Eastern Ontario, Ottawa Valley to be specific. Although I know it’s not the most efficient these always have hydronic radiant In the slabs, sometimes with wall hung boilers and frequently with outdoor wood boilers. Every one that has had an attached garage has had the garage heated and been owner occupied. Now my question; I’m building a FPSF duplex that will be a rental unit with an attached garage,  I’m wondering if I don’t heat the garage will I have issues of any kind being that a portion of the slab will be below freezing and the rest at 80-90 degrees? There is r-20 under the slab and r15 around the perimeter with 4’ R10 wing insulation 6” under grade. The lines are ran but nothing is hooked up due to the boiler being back ordered, so both heated and unheated are options at this time.  The reason I don’t really want to heat it is I’m scared of a tenant trying to save a bit of cash and turning off garage thermostat and dealing with frozen lines (prefer not to use glycol if possible). Anyhow sorry for long post just trying to pre answer any follow up questions. Thanks a lot folks.

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  1. Expert Member
    Michael Maines | | #1

    As long as you follow the ASCE 32-01 rules for FPSFs in unheated buildings, you should be fine--the insulation keeps cold air from getting to the ground, while capturing the earth's heat to keep the ground from freezing at the footing. If the ground can't freeze it won't heave with frost.

    It does not sound like your wings are wide enough for an unheated building; here in central Maine, we need 5' at building corners. What did you use for guidance when designing your foundation system? (I'm not familiar with Canadian codes.)

  2. user-6991874 | | #2

    Hi Michael thanks for the reply, pretty much mimics what I was thinking and puts my mind at ease. As for the wing insulation it was a spec on the slab drawing from my engineer. Any FPSF here over 555 square feet (don’t quote me on that number) has to be designed and stamped by an engineer. Also there won’t actually be an unheated corner as the garage is in the middle of the house with heated living area on either side. Thanks again for the help.

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