by Lee McGinley
I first discovered shallow-grade frost- protected foundations
in 1976, while designing a home for the FmHA market. The lead
carpenter working with me on the project suggested that I try
this "new Scandinavian system," which he had used twice with
success. I was further encouraged when the FmHA county
administrator agreed to approve a mortgage for construction
using the foundation system. The house sold quickly, and I have
been a proponent of the system ever since.
With a , it isn't necessary to place footings below the
frostline. Instead, high-density rigid foam insulation placed
around the perimeter of the slab holds the heat of the earth
and prevents the foundation from heaving (see "Frost-Protected
Shallow Foundations," 9/96). A bed of compacted gravel below
the foam provides a free-draining base, further protecting
against frost action.
Over the years, I've experimented with various methods to form
up the slab, secure the foam to the concrete, and protect the
foam from harmful UV rays. I was looking for techniques that
could be accomplished quickly in the field, and were relatively
foolproof. This article describes my latest refinements. The
details described here are for my climatic zone in the
Northeast. Before designing an FPS foundation, get a copy of
the Design Guide for Frost-Protected Foundations ($29 from the
NAHB Research Center, 400 Prince Georges Blvd., Upper Marlboro,
MD 20774; 800/638-8556).
Getting to Work
First, the excavator scrapes the topsoil and digs the perimeter
trench, sculpting the sides to form a thickened edge and
allowing for a 12-inch base of compacted stone or sand.
Next, we form the slab with 2x12s, using wooden stakes and
braces to hold the forms in place and keep them steady. In our
zone, we attach 1-1/2-inch-thick extruded polystyrene
insulation to the inside of the 2x12s, securing it to the forms
with roofing tins and 8d nails or 2-1/2-inch Plasti-Caps. We
nail every 12 inches or so, keeping the nails within a few
inches of the top surface of the foam so that we can cut them
off later with a recip saw. We use the full 2-foot width of the
rigid foam, because this allows us to maintain the proper
elevation above grade and gives us enough material below grade
against which to backfill.
Insulated formwork.Two-by-twelve form boards
are lined with 1-1/2-inch-thick extruded polystyrene insulation
board. The author digs the footing trench a full 24 inches deep
(right) to make use of the full width of the foam board
insulation. Be sure when you order foam for an FPS slab
that you specify a high-density board suitable for use under a
slab. We use Dow Styrofoam brand, but several other brands are
available.