Replies

1. GBA Editor
   Martin Holladay | Jan 29, 2017 05:46am | #1

   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.

2. 
   Cher89 | Oct 16, 2023 10:16am | #2

   I’m having an art studio with small bedroom and small bath built in zone 5 upstate NY. I plan to use 3 inches of closed cell spray foam under roof sheathing /roof deck. There are premade roof trusses for the roof. The lower joists of the trusses are the ceiling to the studio. I will put R- 49 or R- 28 fiber glass batts or rock wool batts just above ceiling sheet rock. There will be an empty space between the Close cell spray foam under the roof boards and the insulation just above the ceiling. The carpenters already sealed up the top of the roof with shingles. There is no vent because we were using close cell spray foam. Will there be any issue with moisture in this empty space in the trusses between the Insulation batts above the ceiling and the spray foam under the roof deck? Building inspector says this is ok and will meet code.