Attic insulation
Climate zone 7, Canadian prairies, Cape Cod House
Unvented, unconditioned attic
The sloped part of the roof was spray foamed. Now I want to add roxul mineral wool to the flat attic.
I’m including a picture to show you how the spray foam tapers off at the intersection of the rafters and collar ties.
Questions:
When I add the roxul (1st layer runs between the collar ties, additional layers run perpendicular) is it correct that it should not touch the underside of the roof sheathing?
If this is true, it seems odd that there will be an uninsulated gap.
The total assembly goes like so: attic side kneewall has 3″ EPS type II, roxul in the kneewall stud bays, closed cell spray foam in sloped part of roof, and roxul batts in flat attic. Is it ok to run smart vapor over the entire thing?
Thanks.
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Replies
Mill,
Q. "When I add the Roxul (1st layer runs between the collar ties, additional layers run perpendicular) is it correct that it should not touch the underside of the roof sheathing?"
A. In general, yes -- which is why it would have been better if your spray foam installer had continued the spray foam up into the attic a little farther than was done.
In your case, the danger is air leakage through the ceiling (which allows indoor humidity to reach the cold sheathing boards). If you pay scrupulous attention to air sealing, your sheathing boards will be OK, even if the mineral wool batts touch the sheathing boards.
The nuances are not working in my favor.
The reason the spray foam stopped is because I was concerned after reading an article on GBA, and subsequent Q+As on the site, regarding condensation issues with not having a thick enough layer of spray foam on the roof sheathing and then butting insulation up against it.
Regarding the other question I had:
The total assembly goes like so: attic side kneewall has 3″ EPS type II, roxul in the kneewall stud bays, closed cell spray foam in sloped part of roof, and roxul batts in flat attic. Is it ok to run smart vapor over the entire thing? If yes, how can I air seal in between each of the rafters (ie between the collar ties)? Will acoustical sealant bond to the spray foam?
Thank you.
Martin,
I have a similar situation. I live in climate zone 6. I have a trussed house that has 10" of open celled foam sprayed on the roof deck and the gable ends are also sprayed.. I was going to install rockwool insulation against the open cell to boost the r factor. Would there be an issue with trapping moisture. I have no vapor retarder on the open cell.
Thanks.