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
If the roof leaks, you're going to have problems regardless of whether or not you can tell it's leaking. I have installed closed cell foam to the underside of my roof, and I still had a persistent leak, one that existed long before I got the house as evidenced from the hole in the floor below it. When I replaced the roof two weeks ago, the leak was coming from over 30 ft away, running down the old metal roof, under the newer mod-bit until it hit a spot where the metal roof had rusted through. The water ran down a rafter (actually through a crack in it I believe) and dripped on the floor the way it had been for 15 years already. The closed cell foam had no effect on that. I found multiple other places where persistent leaks had gone undetected even though there was originally only FG batts. So in my mind, you can insulate it normally, have leaks, and still never know/see them. And just because you have a leak, doesn't mean that you've any idea where it's coming from, so open celled foam is going to be of no help there.
Christ, I had several windows that would have visible water coming through them during hard rains, and the POs never fixed them. With the amount of rot I found when I tore them out, it appears that it was that way for a looong time. So even visible leaks don't get fixed ;)
Shoot it with closed cell. I think if you want open cell, it'd be cheaper to do cellulose with about the same R-value.
Z