I have a cathedral ceiling with 2 by 6 rafters (not exposed ) that I want to insulate.The plan is to use R20 six inch fiberglass batts between the rafters with baffles under the roof boards to give ventilation from soffit to ridge .Then cover the bottom face of the rafters with 2 in Styro SM then a six mil vapour barrier and 1/2 inch drywall. Am I creating a problem of double vapour barrier with the SM and six mil poly? The property is in Southern Ontario, Canada.I would really appreciate any advice on this, thankyou Jako
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Why not add 2x2's to the u/side of the rafters...that'll give you R32 and eliminate the 'double VB'. And with SM at 25 bucks a sheet, a lot cheaper. Of course, the more insulation over East, the better.
All the best...
To those who know - this may be obvious. To those who don't - I hope I've helped.
I built a similar structure for an outbuilding, and in hindsight I wished I had gone with deeper rafters so I could eliminate the fiberglass/foam combo. Oh well; you're probably in the same boat.
Anyway, I went with FG between rafters, with drip chutes as you mentioned, and then foil-faced polyisocyanurate (sp?) board screwed to rafters from below. It provides r-value and vapour barrier all at once and is easy to cut and caulk where your rafters and walls meet.
Check with your inspector about total r-value required which will determine the thickness of the iso board.
Scott.
Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”
Is the venting required?
If not, dense pack it with foam and you're done, or ad the foam too.
The foam is a VB, dense pack doesn't allow the air flow so no moisture flow there either.
Joe H
I cosidered dense pack but the roof is old (115years) with 12" -18 "wide boards and large gaps I feel the foam will push up the shingles. But thanks for the suggestion.