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.
You also need to check the building loads to make sure you
don't exceed the bearing capacity of the rigid foam. This tells
you how wide the base of your thickened footing must be.
Extruded foam boards are available in a variety of compressive
strengths: 15-psi, 25-psi, 40-psi, and more. Some products
— Dow Blue Styrofoam and Pactiv GreenGuard — come
standard with 25-psi compressive strength. This gives 3,600
psf, typically strong enough unless you have a heavy point
load. Other foams, like Owens Corning Foamular, commonly come
in both 15 and 25 psi, so you need to watch what you're buying.
If you need a stronger foam, you can get it as a special order.
I usually use a factor of safety of 2.5 to 3.0 on the foam
loading to account for long-term creep potential.
To build the foundation, first prepare your subgrade pad 24
inches below the top of your finished slab. Remove vegetation
and loose dirt and either cut the high spots or use compacted
gravel fill to level the pad. Extend this prepared area 5 feet
beyond your building perimeter.
Next, place an additional 6 inches of compacted,
non-frost-susceptible gravel fill. Then lay 2 inches of rigid
foam insulation over the gravel, under the entire building
area, extending the foam board 5 feet beyond the building
Now form the slab perimeter. The Code requires a
minimum depth of 10 inches dirt cover over the foam. So
assuming you want your slab to be 6 inches above finished
grade, the thickened slab edge will be 16 inches tall. I
usually use a 14-inch- to 15-inch-tall edge form. This is a
little shorter than the required 16-inch slab edge, but it
allows for some irregularities in the subgrade.
Drive your form stakes right through the foam. Then place 12
inches of compacted gravel fill on top of the foam inside the
building and shovel out the thickened edge around the
perimeter. Install rerods, and you're ready to pour. I would
definitely use two 1/2-inch rerods in the thickened edge. The
grid in the slab provides extra insurance if the budget allows
but is not necessary if the base is properly prepared and
Pour the slab, strip the forms, backfill the outside with 10
inches of dirt, and you're done. Good luck.
Bill Eich owns Bill Eich Construction in Spirit
Lake, Iowa. He has built many homes on frost-protected
foundations, with never a callback.