What is a vapor barrier?

A vapor barrier is a layer of material installed in walls to stop water vapor from traveling through cavities and reaching the insulation, framing, and sheathing behind the drywall. In bathrooms, where steam and humidity are constant, this layer protects against moisture damage that can cause rot, mold, and structural failure over time.

Most vapor barriers are made from polyethylene sheeting (typically 4-6 mil thickness) or kraft paper facing, installed on the warm side of the wall cavity before drywall goes up. The barrier slows the movement of moist air into spaces where it can condense and cause problems. This differs from a waterproofing membrane, which is applied to surfaces exposed to direct water contact, like shower walls or tile backers. Waterproofing membranes block liquid water, while vapor barriers block water vapor in the air.

Building codes in Connecticut typically require vapor barriers in bathrooms and kitchens because of the high moisture environment these rooms create. Proper installation means the barrier must be continuous, with seams sealed and edges carefully placed. If installed incorrectly or if moisture still builds up due to poor ventilation, the barrier can trap water and accelerate decay. Most bathroom remodelers who work on full bathroom renovations include vapor barrier installation as a standard part of wall assembly to ensure the work lasts.