Cathedral ceiling retrofit?
Hello,
The house we live in has a vaulted cathedral ceiling. We live in climate zone 6a. The roof has a lot of ice in the winter. The interior has about 8 inches of closed cell spray foam, with an interior vent space above the foam, then plywood sheathing, then grace ice&water then a metal roof.
Question: If I want to add exterior foam insulation (and add enough so as to prevent condensation) can I keep the GI&W, even though it is not vapor permeable because there is a vent space already existing on the interior insulation? Followup for my understanding — if this was an unvented interior space could an exterior vapor impermeable membrane be used with adequate exterior rigid foam if the interior insulation was more vapor permeable, like mineral wool?
Thanks for time and expertise!
Trevor
Ref: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
https://www.greenbuildingadvisor.com/article/are-dew-point-calculations-really-necessary
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
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Replies
I'm not sure what your rafters are but even with 2x lumber, that roof should not ice up a lot. I have a low slope metal roof with similar specs (slightly higher R value and no venting) in Zone 5 and have never had issues with ice.
Metal roofs do shed snow more quickly and sometimes you can get a thick layer of snow piled up above the gutters. The thick snow is a good insulator and might be what is causing the issues.
It could be that a bit of snow retention on the roof might do the trick. Much cheaper than re-roofing with a new layer of rigid insulation.
A couple of pictures of where the problem is and the roof layout might help.