Making certain that the foam board fits snugly against the
slab and remains in place has always been a problem. If this
isn't done properly, soil can get between the foam and the
concrete, which reduces the insulation's effectiveness.
Recently, we've been accomplishing this by making "pilasters"
from 2-inch-thick rigid foam.
High-density foam lines the forms and covers the subgrade of
this slab-on-grade. Note the foam "pilasters," which help
stabilize the rigid foam against the 2x12 form
We glue these to the form work every 4 feet on-center with a
foam-compatible construction adhesive. After the concrete is
placed, these help lock the perimeter foam in place. Once the
insulation board is snugly in place, we backfill against the
Extra Foam for Extra Energy
Although the design guidelines don't require it, I prefer to
place 1-inch-thick rigid foam under the entire slab to save
future energy costs.
Ready to pour.After the sand base is
compacted, 1-inch-thick foam board is laid over the entire
subgrade and in the footing trenches. The forms have been
backfilled to prevent movement during the pour. Note the
pressure-treated 2x6 screed attached to the house.
Once this foam is in place, we are ready to pour the slab.
While the concrete is firming up, we set anchor bolts around
the perimeter every 4 feet on-center, starting 1 foot in from
each outside corner.
After the slab has set, I take a reciprocating saw with metal
blade, slip it between the perimeter foam and the 2x12 form
boards, and cut the nails. The forms fall away and we're ready
Just before the final grading, I coat the exposed foam board
with a cementitious coating to protect it against UV
degradation and physical damage. We use a product called Retro
Flex, manufactured by Retro Tek (P.O. Box 220, Waunakee, WI
53597; 800/225-9001). A two-gallon pail was enough to do the
slab pictured here; it cost $37, including shipping.
The addition shown here, which my company built in 1995, has a
450-square-foot footprint. Two men were able to set the forms,
place the rigid foam, and backfill by hand in less than a day.
The savings over a conventional 4-foot concrete wall was about
Certified Remodeler Lee McGinley builds and remodels in the
Casco, Maine, area.