Basement Vapor Barriers

Pierre writes: My basement wall is insulated with a 2 inches of Styrofoam and I built a 2×4 wall and added Roxul insulation. I planned to install vapor barrier on top. Is it OK to add this vapor barrier? It is for a bathroom with a tiled shower. I live in central Ontario, Canada.

Steve Bliss, of, writes: The short answer is: Don’t use a vapor barrier in a finished basement. This is asking for trouble, especially with a full bathroom, which generates a lot of humid air. Rigid foam board on the interior, like you have, is the ideal insulation. Unfaced fiberglass or Roxul (rock wool  is also acceptable if you already have 2 in. of foam against the concrete. Or you might consider spray foam between the studs. Avoid foam insulation that is faced with foil or plastic, to allow for some drying to occur through the foam.

The problem is that moisture tends to condense on the cool concrete walls. The water vapor can come through the concrete from the soil or from indoor air that comes in contact with the concrete. With fiber insulation and a vapor barrier, moisture can get trapped in the insulation leading to moldy insulation and decay in the wood framing. With 2 in. of foam against the wall, this should not be a problem in your climate.

The foam layer should be continuous and sealed at joints to keep indoor air away from the concrete. Also pay attention to the band joist are in the floor framing to avoid condensation and moisture build-up here during cold weather. Insulate the band joist with 2 in. or thicker foam blocks, sealed tightly in place with spray foam. Again, avoid foam faced with foil or plastic sheeting.

It gets a little more complicated since you are adding a tiled shower, where the recommended method is to vapor barrier behind the backerboard and a waterproofing membrane between the backerboard and tile (thinset is applied over the membrane). This detail should be fine here since the waterproofing membrane and tile have already created an effective vapor barrier.  However, I’d avoid a vapor barrier in the non-tiled areas.

I would also recommend an airtight air/vapor barrier on the bathroom ceiling to keep any moisture out of the floor system above. Also install a good bath fan and use it when showering.

All of this assumes that you have a dry basement with good drainage and dampproofing on the exterior. If you have existing basement moisture problems or leakage, make sure you solve the problems before insulating and finishing.

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