Closed-cell spray foam under roof deck in Zone 5
Hi,
What is the suggested minimum of closed-cell that should be sprayed under a roof deck in Zone 5?
My intent is to spray open cell over the closed to a total R-40. Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Consor,
Here is a link to an article that will answer your questions:How to Build an Insulated Cathedral Ceiling.
Briefly:
1. According to most building codes, the minimum R-value of an air-impermeable insulation material installed on the underside of the roof sheathing (in your case, closed-cell spray foam) in Climate Zone 5 is R-20. That would require a little more than 3 inches of closed-cell spray foam.
2. Once the spray foam has cured, you can install almost any type of insulation on the interior side of the closed-cell spray foam to make up the total R-value you seek. (R-40 may be allowed by your local building department, but R-49 or more would be better.) The insulation installed on the interior side of the closed-cell spray foam should be in direct contact with the closed-cell foam.