A.Bill Rose,
architect and building researcher at the University
of Illinois Urbana-Champaign, responds:
Ideally, insulation should be installed on the
exterior of your basement wall, but it is probably
too late for that. If the soil at your site is very
well drained and dry, then it is a fair thermal
insulator, and (energy codes aside) you might
consider omitting the drywall, and simply painting
the interior walls with epoxy paint.
Assuming the client insists on interior
insulation, there is no clear answer on the
question of where to put a vapor barrier.
Condensation usually occurs when moist air
encounters a cold surface. You could argue for a
vapor barrier on the inside of the insulation
(assuming the foundation wall is cold), or on the
foundation side of the insulation (assuming the
foundation wall is wet). Chances are, the
foundation is cold at the top and wet at the
bottom. This situation leads to passionately held
opinions on both sides of the issue.
Since interior basement insulation is
susceptible to getting wet, the most important
factor in choosing an insulation system is not
where to put the vapor barrier, but how well the
components can dry out. Although you claim your
basement is high and dry, I think it would be wise
to assume the possibility of a "water event" some
time in the life of the house, and design
accordingly. Recent research performed at the
Canada Mortgage and Housing Corporation compared
the drying ability of different types of basement
insulation systems. The interior basement
insulation system that performed best was spray
polyurethane insulation covered by Dens-glass
sheathing, closely followed by Owens
Corning’s Basement Wall Finishing System
(consisting of removable panels made of rigid
fiberglass insulation in a vinyl frame, covered
with polyester fabric). The worst performing
system, in terms of ability to dry out after
wetting, consisted of wood studs, fiberglass batts,
polyethylene, and drywall.
Of the two best-performing insulation systems,
the one from Owens Corning is the easiest to
install. These panels can be removed periodically
to inspect for moisture problems, which is not
possible with a conventional stud wall.