A.Bill Eich responds: You have an ideal application for a frost-protected shallow foundation. The method has been approved for use in the 2003 International Building Code. The design guide for heated structures is published right in the Code. For unheated structures, the Code references ASCE32-01, and that's where you have to look to find the design standards. At your location in Vermont, you have an air freezing index of 2,000 freezing degree days, and a mean annual temperature of 45°F. According to table A8 in ASCE32-01, your ground insulation should have an R-value of 10. It should extend 60 inches beyond the building perimeter and be at least 10 inches below finished grade (see sketch). You should place this insulation on 6 inches of non-frost-susceptible fill (sand or gravel).
By combining rigid foam insulation with a well-compacted, free-draining substrate, a frost-protected shallow foundation prevents heaving damage, even in an unheated building in the coldest climates.
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