My father-in-law swore by plastic sheeting behind showerboard, I thought he was nuts

My father in law is this old school carpenter who did tons of bathroom work back in the 80s. When I was gutting my guest bath last fall he kept telling me to put up 6 mil poly sheeting behind the cement board, even over the vapor barrier in the insulation. I figured it was overkill and just taped the seams on the insulation like I read online. Well 4 months later I started noticing a musty smell and a little dark spot near the base of the shower. Pulled the bottom row of tile off and the cement board was damp on the back side, no mold yet but it was heading there. So I ripped it all out and did it his way with the poly sheet stapled right over the studs before the board went up. That was 8 months ago and it's bone dry in there now. Anyone else run into that debate about double vapor barriers in a bathroom?

3 comments

3 Comments

charles8368d agoMost Upvoted

Keep it simple. Old school works better than internet advice.

anthonynelson8d ago

Yeah man, that's rough. I've seen the same thing happen to a buddy of mine, he thought he was doing it right and ended up with a mess. Hoping the vapor barrier issue works itself out for you.

theas288d ago

Did you ever check if your insulation had a vapor barrier built into the facing? A lot of modern fiberglass batts already come with kraft paper or foil on one side, and slapping poly over that plus another layer behind the board can trap moisture between the two if there's even a tiny gap. I've seen basements with double vapor barriers that ended up growing black mold because the air couldn't circulate at all. Your father in law's method might work in a dry climate, but in a humid area it could backfire badly.