Q&A: Finishing a Basement Wall

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.