Open Cell for Attic Roof
daveh4
| Posted in General Questions on
I hate asking this as I know it’s been done to death. I also feel like I know the answer.
I’m planning to spray foam the interior of my roof deck in upstate NY (zone 5). A very reputable insulator recommends going with 10” of open cell. I raised the concern of roof rot in cold climates. He’s claiming that at 10” it has the same vapor barrier properties as closed cell. I’m very hesitant to go with open cell in my climate and just double checking that I’m not missing something. Thanks.
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Replies
The contactor is wrong. There is no grey area here, building scientist have look at it and all open cell foam hot roof is a risky assembly in Zone 5.
You can make open cell work if you install a vapor (ie Membrain) but it is still a risky assembly.
The better option is to go with enough closed cell foam for condensation control (R20 closed cell for an R49 roof) and the rest open cell or batt insulation.
You can read more about the proper assembly here:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-116-interior-spray-foam
Thanks! That’s all I needed. I pitched the flash and batt and they told me THAT was the risky assembly. I’m sure they’re thinking. About condensation on the foam if it’s not thick enough but I even pitched R30 foam and then batt and they still said it was a bad idea.
I know it's extremely hard to find a competent contractor sometimes, but personally, I'd recommend avoiding this particular company. At this point they've shown you what you need to know. Even if you can convince them to do what you want (i.e., closed cell), they are going to be doing it reluctantly and grudgingly, and if you have ANY issues down the road, they are going to blame it on you. If at all possible, I'd try to find a contractor who proposes a good solution to begin with. Or, at a minimum, openly embraces a good solution when it is proposed to them. These guys seem to be embracing the bad solution and fighting against the good one.