Trade offs of closed-cell foam between roof rafters
Hi All,
I’ve been perusing the literature for a while now regarding insulating a 1.5 story cape on Long Island, NY (zone 4a). The attic will be a conditioned living space, and I’ve been leaning towards insulating sans ventilation. Insulating above the roof deck is not an option due primarily to cost. Instead, I’m looking at foam between the rafters. Now, the question comes to open- or closed-cell. I read and understand Holladay’s article regarding the potential costs of using open cell, but I’ve also had a few contractors encourage me to use open cell. Their justification is that if the roof leaks, closed-cell foam will not let that water through, thus signaling a problem. It seems like both methods have drawbacks. My question boils down to this: which method is likely to do the least harm? Is a leaky roof more likely than condensing water vapor near the rook deck?
Any insight is greatly appreciated! This site really is a wealth of knowledge and I feel fortunate to be using it.
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Replies
William,
You'll see many opinions expressed on the question of whether different types of spray foam affect the detectability of roof leaks. As a former roofer, I tend to side with Joe Lstiburek on this question. Here's what Lstiburek has to say:
"There has been much discussion about open-cell being better under a roof deck than closed-cell because if there is a leak the leak either dries faster or you can see it sooner. Not true. There is no compelling evidence one way or the other. None. I started all this SPF stuff under roof decks in the early 1980’s. I have been looking at this roof leak issue since the very beginning. I have played with mock-ups and wetted them and watched them. I have taken apart existing leaking roofs all over the place, open-cell, closed-cell, SIP’s, compact, etc. With the mock-ups and the field experience I have not noticed any difference between open-cell and closed-cell in terms of detectability and damage with respect to roof leaks. Did I mention the fact that there is no compelling evidence one way or the other regarding roof leaks and open-cell vs. closed-cell?
"Ready for this…here it comes…roofs leak. They always have. They always will. Sometime you see the leak, sometimes you don’t. This is true regardless of what system you use. We build big box stores and office buildings and schools and pretty much everything else with multiple layers of insulation and membranes and decking and somehow we manage to deal with roof leaks and it is not the end of the world. Ready for this? Big leaks always leak enough to be noticed. Small leaks don’t always leak enough to be noticed.
"Ready for something else? Here it comes….sloping roofs are less of a risk than flat roofs. Wow. This gravity thing is a big deal. With a sloping roof you have less hydrostatic pressure and if you do have a leak it is more likely to be seen inside regardless of roof type."
I'll add two points to Lstiburek's observations:
1. If you are concerned about detecting roof leaks, the best approach by far is to design your house to have a capacious ventilated unconditioned attic -- so capacious that it is easy to enter and stand up in. This advice doesn't apply to you, since you are discussing an existing home.
2. If we're discussing insulated sloped roof assemblies that are unvented -- sloped roof assemblies insulated with spray foam -- then it is safe to say that roofs with closed-cell spray foam have fewer problems with damp sheathing than roofs with open-cell spray foam.
Martin,
Thanks so much, if that wasn't a definitive answer then I don't know what is. Your insight is greatly appreciated!