Just found out my 'vapor barrier' was actually trapping moisture in my attic

I was up in my attic last weekend checking for leaks after that big rain we had. I started noticing black spots on the plywood and pulled back the insulation. Turns out my vapor barrier was facing the wrong direction and it was holding all the winter moisture against the roof deck. I found a quick read from Building Science Corp that says this mistake causes 80 percent of attic rot cases. I spent 3 days stripping it all out and redoing it right with the permeable side toward the house. Has anyone else had to fix this basic mistake that the previous homeowner left behind?

2 comments

2 Comments

charles_coleman1mo ago

Actually the vapor barrier should face the warm side of the house, so for most of the northern US that means toward the living space below. But if you live in a hot humid climate like the deep south, the orientation flips and you want it facing outward. I had the same problem in my place though, the previous owner put the wrong facing up and it was trapping sweat against the roof deck during summer. The Building Science Corp article is good but just remember that the 80 percent number comes from cold climate studies specifically, so its not universal. You probably did the right thing for your area but its worth double checking your climate zone.

lily3601mo ago

Oh wait, what about folks with flat roofs though?