Install fiberglass batts under foam in cathedral
user-6259794
| Posted in Energy Efficiency and Durability on
I am building a new in zone 5. We have cathedral ceilings that were sprayed with 2″ of closed cell, followed by 5″ of open cell foam(approx R32). The insulation contractor seems to think this will be plenty but I wondered if we could or should add fiberglass batts, pressed up against the open cell foam?
Thanks,
Michael
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Replies
Michael,
In Zone 5, building codes require a minimum of R-49 for roof assemblies, so R-32 is not enough. You can definitely add fluffy insulation like fiberglass on the interior side of the spray foam, as long as you follow the ratio rules outlined in this article: Combining Exterior Rigid Foam With Fluffy Insulation.
In Zone 5, at least 41% of the total R-value of the roof needs to come from the spray foam. In your case, that means that you can install up to R-46 of fiberglass, and you probably won't be installing that much.
-- Martin Holladay