A. Bruce Nelson responds: We have
found that many different materials can work, but that some
require more expensive installation than others to give equal
performance. There’s still much we don’t know, but
our investigation of 59 foundations in Minnesota and
discussions with others in the field suggest the following:
- Extruded polystyrene (Amofoam, Certifoam, Foamular, and
Styrofoam) always works well.
- Although only two specimens of
polystyrene (EPS) were observed in our
study, we feel EPS can be used successfully. However, use
only the higher density products below grade, not the
typical one-pound-per-cubic-foot variety found in most
- If you use polyisocyanurate or spray urethane, include
a protective coating below grade that is long-lasting and
strong enough to avoid puncture from backfill. In some
cases we observed higher water absorption, which may have
been caused by damage to the below-grade protective
Also, foam plastic is not your only
option. Consider high-density fiberglass board for sites where
foundation-wall drainage is a must. This material is available
as a commercial roof insulation from Owens Corning. It is only
available for residential use in Canada, as BaseClad (Fiberglas
Canada, 4100 Yonge St., Suite 600, Willowdale, ONT, Canada;
416/733-1600). This product must always be installed with
exterior perimeter drains.
No matter what type of material you use, two elements are
essential to prevent moisture absorption and deterioration.
First, insulate to the top of the foundation wall, making sure
to leave no gap where the insulation meets the siding. Use
mechanical fasteners or adhesive to prevent the insulation from
slipping down the wall.
Second, protect the insulation above grade from physical
abuse and sunlight. Since many of the installations we have
seen have missing or damaged coatings, we recommend an
above-grade covering material at least as durable as the
siding. Examples include 1/2-inch pressure-treated plywood,
high-quality stucco, and fiberglass panels.
Bruce Nelson is a senior
engineer for the Minnesota Department of Public