A.Stephen Smulski responds: Pest-control
professionals are right not to make promises when faced with an
insulated foundation, because it’s impossible to inspect
inside and behind the insulation.
A 1/32-inch gap is all termites need to sneak into a house.
To eliminate these gaps, begin by choosing a foundation type
and materials that present the fewest possible entryways.
Of all foundation types, slabs-on-grade are the most
vulnerable. In your design, for example, termites can slip
through the foundation/slab joint, inside the hollow concrete
blocks, or between the insulation and foundation (Illustration
A). Monolithic slabs, where grade beam and slab are cast in one
pour, lack joints and therefore present fewer entry routes than
slabs supported on foundation walls. Likewise, a cast concrete
stem wall, with its solid center and reinforcement to minimize
cracking, is better than a hollow block wall. However, block
walls can be made more termite-resistant by capping them with
reinforced concrete or solid blocks, or by plugging the hollows
in the top course with mortar.
Making any foundation termite-resistant requires
treating the soil under the slab and surrounding the foundation
with a termiticide.
Treating the soil under the slab, of course, is best done
before the slab is cast, and treating the surrounding soil
should be done during finish grading.
Termites prefer wetter soils, so make sure water is directed
away from the foundation by properly sloping finish grades, and
by using gutters and downspouts. Also, keep the below-slab
drainage pad higher than the outside soil by elevating the slab
surface at least 8 inches above the finish grade. Reinforce the
slab to minimize cracking, and design utilities so that the
slab penetrations are minimized or eliminated. All penetrations
and joints should be sealed with roofing-grade coal-tar pitch.
Also, make sure that all wood (read "termite food"), such as
stumps, grade stakes, formwork, and scraps, is removed and
disposed of off site, and not buried during backfilling.
When detailing the insulation, you may have to sacrifice
some energy efficiency in return for being able to properly
inspect for termites. The details in Illustrations B and C show
two ways to leave the foundation exposed for inspection and
also reduce the number of possible entries. The metal shields
shown in these details should be thought of as only one
part of a home’s anti-termite defenses. They’re
seldom fabricated and installed as tightly and as carefully as
is really necessary.
Dr. Stephen Smulski is a wood scientist and president of
Wood Science Specialists in Shutesbury, Mass. His article "A
Builder’s Guide to Wood-Destroying Insects" appeared in