Dr. Joe Lstiburek Discusses Problems With Closed Cell Spray Foam in the Attic
rockies63
| Posted in General Questions on
In this Building Science Corp article (Oct 2023) from the father of Forensic Building Science Dr. Joe Lstiburek, he discusses several major problems that can occur when using closed cell spray foam that has been applied to the underside of roof sheathing.
On page three he describes how to correct these problems, and the solutions are a lot more work than you might have thought.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Very briefly, he notes that for a variety of reasons a roof can move, and if it's insulated with closed cell spray foam the movement can cause the foam to separate from the sheathing, which then leaves the sheathing vulnerable to condensation damage.
He proposes two solutions. One is to put a layer of sheet foam insulation between the sheathing and the spray foam, which gives greater resistance to movement. Kind of like cut-and-cobble.
The other solution is a layer of foam board insulation above the decking. It seems to me if you're going to do that you may as well skip the spray foam altogether and use batts below the decking.