Moisture Control in Unvented Attic
I have moisture and mold in my attics. I’ve tested some solutions and they made the problem worse. I’m perplexed and would welcome some good advice.
Situation:
1941 home
Portland Oregon
1.5 stories with full unfinished basement
3 unvented side attics adjacent to the upstairs bedrooms
They have fiber glass between the 2×4” rafters that is covered with a sheet of vinyl
There is fiberglass in the knee wall and wood chips in the floor
I found mold on the insulation side of the vinyl and some on the OSB roof deck
I wanted to keep the space unvented if possible. Since I wasn’t confident in my approach, I tested a few options. All failed. The test raised more questions.
Testing:
I replaced the fiberglass insulation in the rafters. In one area I covered that with Tyvek and sealed the edges well. In the other area I left it uncovered. I left the attic door open to allow circulation of bedroom air into the attic. I sealed along the soffit to prevent entry of outside air. I also sealed around the can lights and exhaust duct in the attic floor.
Results:
High moisture near the OSB.
Measured ~85-95%RH
Corrosion of nail tips
Mold growth
The attic air temp was ~57-65F with RH of ~45-55% (see attached for details)
Notes:
I have been aggressive at addressing sources of moisture in the house – new exhaust fans in bathrooms/kitchen, sealed ducts, no house plants, no seepage in basement, just two occupants. The basement is typically 65F/50% RH. The living area is typically 68F/45% but is allowed to drop to 60F at nights.
Portland weather is pretty damp. During the period of this test (Jan 2021) the avg temp/RH was 43F/86%.
My questions:
- Is the RH inside my house reasonable considering Portland weather?
- Is there a DIY approach for insulating the side attics and keeping them unvented?
- If I had someone spray in open cell foam, wouldn’t it have similar moisture issues as my test – high RH at the OSB? My understanding is that open cell foam and Tyvek are both air barriers and not vapor barriers.
Thanks for your help.
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Replies
Why don't you want to vent the attic? If venting is an option, that's probably the easiest way to solve the problem -- then just insulate the floor of the attic instead of the underside of the roof.
If you want to keep the attic as conditioned space, then close cell spray foam against the underside of the roof sheathing is likely your best option. You don't want vapor open insulation here, as moisture will tend to migrate through it and condense on the underside of the cold sheathing, which is where that mold you found would have come from.
Air sealing will help by limiting how much mositure gets into the attic, but without venting, you'll still see moisture levels gradually climb.
Bill
You confirmed my suspicion about moisture build up. I'll explore closed cell foam a bit more before going down the venting path.
I had wanted an unvented solution because:
1) Easier to do. Venting will require quite a bit of sealing to be done on the knee wall.
2) Cooler attic temps for storage.
3) I was trying to avoid ugly roof vents on the street-facing side of the roof. (The eves aren't big enough for soffit vents and I can't connect to the ridge vents because the side attics are not connected to the top attic. So I'll need 4 roof vents front and center.).