I’ve heard/read in several places that open-cell foam insulation would be preferable in rafter bays of an unvented attic application (NOT a cathedral ceiling) to allow any potential leaks to drain through (and be discovered).
Using closed-cell foam is said to create an impermeable barrier, causing any intruding water to remain trapped under the decking, causing rot. However, I question whether the sources of this “wisdom” are biased due to their reliance on open-cell foam for their livelihood.
Closed-cell foam seems superior in other applications (walls, etc.), providing more structural integrity, moisture barrier and better R-value performance. I would lean toward using closed-cell foam everywhere, but I don’t want to steer my client wrong. Would using Grace Ice and Water Shield on the exterior deck provide some insurance, or is that overkill? For climate reference, I am in Dallas, TX.
Any experiences or advice, pros/cons on either option?
Thanks in advance for your $.02.
LB
Replies
I'm by no means an expert but I did just order $700 worth of closed cell spray foam for my attic this morning... so I have a lot riding on this answer too!
Wouldn't ANY properly installed vapor barrier (like plastic sheeting) cause the same situation? I thought it would be an advantage to prevent any water from soaking down into the ceiling, but I see your point about leak detection.
-brad
I used closed cell on my roof deck. That undetected leak situation lurks in my brain all the time. The performance of the system makes me try not to think about it too much.
I spray foamed my roof with closed cell, and I've a persistent leak (that has been for many years, before I bought it and foamed it). It still leaks even with the foam, since you can't get a 100% percent seal. The good thing about closed cell is that it won't absorb any water at all. I'd be more worried about an open celled roof that has a leak and begins to mold. Even if they say it won't, it's not really something I want to find out. I think the fear is unfounded. They replaced the entire roof and never fixed or found the leak, so what's the difference? They had a clear shot at the underside of the roof and still couldn't fix it.
Z