Common sense, in this case, is bang on. A vapour barrier is needed with conventional insulation and must go on on the warm side; otherwise, you'll invite condensation where the warmer air meets a cooler surface. We talked to David Graves, of Davidson Home & Cottage Works in Parry Sound, Ont., and also to Don Ferguson, of Do It All construction in Emsdale, Ont., about the installation process. Both men agree that it's a tricky job but can be done, albeit with a lot of plastic and patience. The challenge is to wrap the plastic sheeting up under floorboards and around joists and beams, keeping the sheeting in one piece insofar as it's possible, cutting where necessary, and sealing all cut edges with acoustical caulking where needed. What you want, of course, is to end up with a one-piece moisture shield.

Both pros suggest that before going this route, you look at the top-of-the-line solution: blown-in polyurethane foam insulation. It's a job for a licensed contractor. But the foam acts as vapour barrier and insulation in one, besides being unaffected by moisture and unattractive to burrowing critters. Don Barrick, of Don Barrick Insulation in Bracebridge, Ont., says 2"-3" of foam (each inch of thickness gives an R-value of 6) should be enough, and should cost $2-$3 per square foot.

For an operator in your area, contact the Canadian Urethane Foam Contractors Association at www.cufca.ca.