Membrain or vapor retarding primer in attic?
I have a 1911 brick farmhouse in Racine, WI (20 miles south of Milwaukee) in which I insulated the attic with 6″ of Icynene open cell spray foam, sprayed right to the underside of the roof deck (no air vents). Most of the perimeter of the roof is sloped approximately 45 degrees, but the center section of the roof is flat. Because of varying differences in roof joist thickness and overall inadequate depth for the foam, I decided to “sister” 2x4s to the sides of the roof joists to increase the cavity depth to a uniform 6″. An added benefit to this is that it created a thermal break.
I then proceed to start to cover everything with 4 mil polyethylene, stapling and carefully taping all the seams. I had about 80 percent of the ceiling covered. My plan was to then cover over the poly with drywall, but I never got to that step.
We also had a central air conditioner installed in the now conditioned attic space. The first day in summer that it got hot enough to run the AC, I went up to the attic to find that water was pooling on top of the polyethylene (between the spray foam and the poly), with no way to escape. I figured the trapped water could eventually escape back outward through the foam, but the next day was hot again and running the AC caused even more water to pool. I thought this couldn’t stay this way all summer, so I took a utility knife and slashed all the poly, and could not believe the amount of water that gushed out (I’d guess a gallon or 2).
Luckily I hadn’t yet put up the drywall, or I wouldn’t have seen all the trapped water.
So now my question is what to do in our cold weather climate where we also have a month or 2 in which we need to run the AC?
Our local code says that we need to install a vapor barrier (retarder?) to the inside of the wall. Would CertainTeed Membrain be the answer? Or a vapor primer under latex paint?
I’d also like to install more insulation to the roof. I”m thinking 2″ of rigid foam to the interior of the sloped perimeter areas (so I wouldn’t need to reshingle or increase the facias), and 2 inches of rigid foam to the exterior of the flat areas (so I wouldn’t lose the interior ceiling height). Would installing this additional rigid foam cause any change to my vapor retarder issue?
Thanks for any responses.
Darin Weisensel
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Replies
Darin,
You describe a classic example of inward solar vapor drive, a phenomenon that can drive moisture through asphalt shingles into a roof cavity. During the summer, moisture condenses on the first cold surface -- in this case, the exterior side of the poly.
The fact that you could see the water saved you. For an energy nerd like me, it's an exciting example that vivdly illustrates a textbook phenomenon.
Obviously, all of the poly needs to be removed. Until recently, I would have advised spraying the cured foam with vapor-retarder paint. Recently, however, Dr. Joe Lstiburek reported that lab tests reveal that when vapor-retarder paint is sprayed on cured foam, it doesn't acheive the permeance listed by the paint manufacturer.
So your best choices are either to install the MemBrain or to install drywall and then paint the drywall with vapor-retarder paint.
Adding additional rigid foam to your roof and ceiling shouldn't cause any problems.