Spray foam roof deck (interior) in Climate Zone 4
Hello – I live in Zone 4 (Long Island), but not the far from Zone 5. My HVAC equipment is located in the attic (I know, terrible place for it), so I’m going to create a conditioned attic. My questions are as follows:
1) From what I read, open cell would be acceptable in zone 4, however, would closed cell be preferable?
2) If I use closed cell, how many inches would be necessary for dew point control? Contractor is recommending 4.25 inches of closed cell.
3) To address thermal bridging, can I spray open cell over the closed cell and cover the rafters? I have 2×8 rafters. The bottom slope of some of the rafters would be difficult to reach if I wanted to put batts.
4) My existing attic floor insulation is pitiful. I can see the drywall ceiling in some spots. Should I remove all of the floor insulation? The contractor said it wouldn’t be necessary to remove all of it, but from what I’ve read that doesn’t seem to be true. I have pull down access to attic so that would be one area for air exchange.
Any advice would be greatly appreciated.
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You can spray closed cell foam up against the bottom of the roof deck for an unvented roof assembly. If you are close to zone 5, you'll want to hit R-49, I believe. I don't think it is best practice to spray over the rafters, but you can easily increase their depth in a number of ways, or you can fill the rafter bays and add some rigid foam to the bottom side of the rafters which would help with thermal bridging. I don't see why you would need to remove the insulation on the attic floor, but it won't be needed once you bring the attic into the building envelop.
I'm sure you'll get some more insight on this soon. In the meantime, here are some articles to read that may be helpful: