Basement Insulation Strategy

Q: How should I insulate the basement walls in my 7-year-old home? The builder installed pink fiberglass insulation blankets on the top half of the walls only, with a vapour barrier on the inside surface. There are not studs. But in summer, enough condensation forms on the inside of the vapour barrier to thoroughly soak the fiberglass. I removed all this insulation and left the walls bare for a year. No moisture was visible anywhere. I then had a contractor frame the basement, install tarpaper against the walls, put in new fiberglass insulation, then apply a taped and sealed vapour barrier. Condensation appeared again the following summer on the inside of the plastic. Now I’ve been told that spray-on insulation is the way to go. Someone else suggested rigid sheets of foam. What’s the truth? I’m tremendously disappointed in the lack of building standards here. No insulation contractor seems to really understand my basement problem and the best way to address it. Can you help?

A: Yes, I can give you a definitive solution. You're right in your observation that few building professionals understand the unique approach required to insulate a basement well. Even building codes fail to prevent big mistakes here, leading to widespread problems exactly like you're experiencing. The first place to start is with moisture control. Even apparently dry basements can let in significant amounts of moisture vapour through masonry walls, not to mention the ever-present danger of a flood. This is why I always steer people away from using fiberglass insulation in basements whenever possible. Even small amounts of moisture leads to mold and rot and reduced thermal performance. If your basement has absolutely no water leaks, and appears dry year-round, then finishing is an option. Begin with a coat of waterproof paint on the inside of masonry walls. According to independent consumer testing Drylok is the best. Next comes insulation. You'll get excellent performance using extruded polystyrene foam. This is a closed cell, rigid foam that not only tolerates some moisture, but acts as its own vapour barrier. A product called Wallmate is a specific version of this foam that’s ideally configured for application on basement walls. Wallmate is grooved so it can be held in place with wood battens that also create a nailing surface for wallboard. As you install this foam, apply beads of expanding polyurethane foam to the joints, to seal them and prevent warm, moist indoor air from causing condensation behind the rigid foam during winter. You should also apply beads of this foam along the top and bottom of each sheet, to seal these areas, too. The combination of rigid and expanding foam is also ideal for insulating the space between floor joists. Since you already have wall frames in place, I'd go ahead and spray icynene or poly foam against the walls, between the studs. It’s more expensive than rigid sheets but works well.