Cathedral ceiling vapor barrier
DonaldWardlaw
| Posted in Construction Techniques on
I’d like to hear points of view on this situation.
We have a cathedral ceiling. Space between finish ceiling material (wood) and plywood roof deck is filled with 3 1/2″ poly iso. No air space.
Our climate is San Francisco Bay Area.
Some rooms will generate humidity (bathrooms).
I’m thinking I should try to keep vapor out of the insulated cavity. The cavity is pretty tightly filled, but there will be an occasional small crevice. I’d like to know what people thing about these things:
1. A vapor barrier is needed?
2. If so, bathrooms only?
3. If needed, what material is best and how to install?
4. If installed is sealing of edges critical?
Many Thanks,
Donald
Replies
Up here in Toronto,Can. the vapour barrier is a critical element of the building envelope. We use 6 mil. poly. stapled and taped, accoustic caulk at termination points. Further, in our building code, poly-iso is not considered a V.B., being an open cell foam. Poly-urethane, being a closed cell foam does qualify as a V.B. I would poly. the entire cieling and any where the foam is. Small voids are O.K., but larger voids should be filled with fibre glass to prevent any free air. You have what we call a "sandwich roof" meaning, basically, solid construction, no voids, no chance for the big problem, condensation.