Expert Answers: Foundation Wall Insulation

Adding insulation to your basement but don't want to risk trapping moisture? HANDY has the answer.

Q: I plan to insulate my poured-concrete basement from joist to floor level with R-12 rolled fiberglass. A contractor friend advised me to leave a 4-in. gap at the floor when insulating basement walls with fiberglass rolls. He claims this will help to prevent the insulation from getting damp. Is this an accepted practice? - Allan Mills, Mississauga, Ontario, Canada

A: Doing what your friend suggests won’t mean significant heat loss because there is very little value in insulating walls much below the frost line. A better approach is to cover the lower portion of the wall with 4-mil polyethylene vapor retardant that extends out onto the floor before you build stud walls and insulate. Do not, however, cover the entire wall. Doing so can drive moisture up into the wall of the floor above.

The best way to keep dampness from coming through a foundation and being absorbed into your insulation is to stop the moisture on the outside. Direct water away from the house with proper grading and downspout extensions, and apply a damp-proofing material, such as UGL’s Drylok, to the interior of the foundation walls. In “real life” situations, however, those efforts occasionally fail, hence your friend’s suggestion (which is not standard practice, by the way). - HANDY