Replies

1. Expert Member
   BILL WICHERS | Sep 23, 2020 11:50am | #1

   Can lights are bad, very bad, in an unvented roof assembly. I would seriously consider replacing them with something sealed. In my own home, I have two can lights in a cathedral ceiling, a vented one, and in the winter you can clearly see the snow melt above them running up the roof. That was the same area that needed some sheathing replaced when I put on a new roof several years ago. I have an upcoming project to get rid of those can lights.

   You can put roof vents in for vent channels that stop before they get to the ridge vent, or that start after a penetration like a skylight or chimney. You should put one in for every rafter cavity to ensure proper ventilation for these broken cavities.

   Spray foam generally needs to be applied directly to the underside of the sheathing to make sure moisture doesn’t condense directly on the underside of the sheathing. I see no issues with your vented assembly, although I don’t think you gain anything with spray foam there if you already have that 1” polyiso layer taped properly, but for your unvented assembly I would use entirely closed cell spray foam directly against the underside of the roof sheathing. You need enough spray foam to get the proper ratio for your climate zone (lots of info on this here), once you’ve done that you can use whichever type of insulation you choose to fill the rest of the space.

   Bill

2. GBA Editor
   Kiley Jacques | Sep 23, 2020 12:33pm | #2

   There are a number of roof assemblies discussed in this article by Joseph Lstiburek that could help inform your approach: BSI-100: Hybrid Assemblies.

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   Deleted | Sep 23, 2020 01:36pm | #3

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   Deleted | Sep 23, 2020 05:30pm | #4

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