Is a vapor open membrane needed below a ventilated attic?
I am building a passive house in NH climate zone 6A with thick EPS exterior insulation, where the WRB/Air barrier will be between the foam and the sheathing. The WRB will likely be a vapor closed peel and stick membrane. I have read that as long as the sheathing can dry to the interior of the home, I am safe.
The attic of the home will be a traditional ventilated attic with an 8 pitch roof and blown in cellulose insulation. This got me wondering is there any benefit to using a smart vapor open membrane such as Siga or pro clima between the drywall and the attic insulation for an air barrier? Or would less costly vapor closed options be just as fine?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The membrane there is a good idea as your secondary air barrier. It doesn't need to be anything fancy, if you are in a heating climate standard 6mil ploy works just well.
This way if there are the inevitable cracks in your drywall down the road, there is a backup behind it.
Try to bring your peel and stick on the walls over your top plates and connect it to this secondary air barrier. You can do this by leaving a wide strip of it hanging on the inside without the backing removed which you can connect after the rafters/trusses go up.
"Sheathing can dry to one side" alone does not assure a safe wall. But a "PERSIST" wall design is fine.