Does OC spray foam cover the underside of attic Vapor Diffusion Port?
2screws
| Posted in General Questions on
I’m planning a new house in CZ3 on a slab with a sealed conditioned attic. I’ll make a vapor diffusion port along the ridge of the hip roof. Contractor suggests OC spray foam on underside of roof deck and exterior 2×4 walls, since mineral wool won’t give attic required R value thickness with thin roof trusses.
My question is: on the underside of the Vapor Diff. Port, is the OC spray foam sprayed there, or is that area blocked out to not have foam?
Thanks for your advice, I’m a beginner!
Chris
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Replies
> Contractor suggests OC spray foam on underside of roof deck and exterior 2×4 walls, since mineral wool won’t give attic required R value thickness with thin roof trusses.
Open cell foam and mineral wool have very similar R-value, so it's somewhat of a moot point.
Your designer should not be specifying roof framing that isn't deep enough to accommodate code minimum insulation levels unless the insulation is actually intended to go somewhere else (like on top of the ceiling).
And theoretically, the design shouldn't pass plan review without a code minimum amount of roof insulation specified.
I just answered your same question in another thread...
Vapor diffusion ports are allowed in the 2018 IRC (R806). I happen not to have jumped into the VDP’s wagon. My main reason is labor quality here in TX (CZ3), especially when there is so much at risk. For shingled roofs, I specify 2” R13 min. ccSPF against the roof decking plus 7” ocSPF R25 min., for a full R38 min. For metal, tile or flat (TPO) roofs, I specify 1.5” R10 min. rigid foam on top of the rood decking and 8” R28 min. ocSPF under the roof decking.
Many insulator contractors con builders and owners into spraying 5.5” R20 min. ocSPF under the roof decking, because under the performance and AU codes, you are allowed to reduce the insulation amount, but I believe the “laws of physics and chemistry” do not care what codes you are applying. That’s something I learned from Dr. Lstiburek 25 years ago, and I believe that is very risky. I won’t do that to my clients.
Thanks very much for replies.
Patrick- contractor was against Rockwool because of anticipated difficulty getting batts to stay in place since they'd have to be thicker than trusses.
Armando- Yes, contractors and labor here in central NC don't seem familiar with VDPs at all. Shingled roof planned.
Chris
> Patrick- contractor was against Rockwool because of anticipated difficulty getting batts to stay in place since they'd have to be thicker than trusses.
What is their basis for saying that open cell foam solves the problem? The R-value will not be greater...