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Rockwool in an enclosed rafter assembly per R806.5.2.2

DougSurber | Posted in General Questions on

I live in a 1980 MCM home in Sacramento, CA. Climate zone 3B. My house has a low pitch roof with the ceiling drywall attached to the bottom of the 2×8 rafters. For a number of reasons I am replacing all the insulation in my roof as part of a reroof project. The roof is currently a vented rafter assembly, at least in theory. I want to rebuild it as an unvented rafter assembly per R806.5.2.2. My plan was to use R30 Rockwool Comfortbatt in the rafter cavity and 50 mm Rockwool Multifix above the deck with a PVC membrane. This meets the above deck insulation requirements of the code.

This is the first time my roofer has seen this construction. They contacted Rockwool and Rockwool said they would recommend 5 1/2″ Comfortbatt topped with 2″ of spray foam to form an impermeable barrier. The residential code clearly doesn’t require this. Innumerable reputable sources, including on this site, have said that the impermeable layer isn’t needed in an enclosed rafter assembly built as described in R806.5.

Any thoughts on why Rockwool would say this? Any technical reason not to go ahead and build the roof with R30 Comfortbatt (or similar) and no spray foam?

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  1. Expert Member
    Michael Maines | | #1

    Vapor diffusion ports are a fairly new idea, and Rockwool's technical department representative may be behind the times. Rockwool may not endorse the approach but I would be surprised if that's the case. I would try again with a different representative, or send them supporting information from GBA or

    1. DougSurber | | #2

      Thanks, Mike. That is what I was thinking as well. It's hard for me to imagine that Rockwool would be so behind as to send me off to use spray foam. I gave my roofer all the materials from both sources you mentioned and others that I could find. I know he sent some to Rockwool. I guess a tech rep that far behind wouldn't take the trouble to read the material and check with their colleagues.

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