Smart vapor retarder VS Kraft paper
Akos
| Posted in Green Products and Materials on
For some reason any of the smart vapor barriers up here in the great white north cost silly money (I haven’t found a cheap source).
I’m trying to see if there is any reason to use it over regular kraft paper vapor barrier or faced batts.
As far as I can tell, the materials have similar variable performance. See Table 1 in https://www.aceee.org/files/proceedings/2008/data/papers/1_8.pdf
The only thing I can think of is that a plastic film can be detailed as an air barrier whereas the paper rolls would be pretty hard. That doesn’t seem like a big issue for me as the main air barrier is the sheathing anyways.
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In the case they studied, they say the difference is between "adequate/good" and "excellent". Also note that they discuss why an *interior* side air barrier is important. If you don't use AC much, poly is a low cost option. If you don't use exterior insulation, keep exterior perms high.
Fixed link:
https://www.aceee.org/files/proceedings/2008/data/papers/1_8.pdf
Thanks for the catch. I fixed the initial link as well.
I'm in cold climate and this would be for an unveted roof with exterior rigid insulation. Since it will have a vapor barrier right under the foam, the only drying is towards the interior, thus the variable perm VB. Although poly is cheap, it won't work in this case.
I'm just having to justify spending the equivalent of about R7 of rigid insulation on this.
Reading through the study, there is no real benefit of this over kraft paper except for the fact that it works as an air barrier.
It's 3+x more drying. But this may not be needed.
"Wet cup: 12.2 Perms" vs "Wet cup: 0.6-4.2 Perms".
See Fig 10, here. Low perm paint might be as good as kraft.
I think you are right. Drying capability isn't quite as good, but should be good enough. I would detail your drywall as an additional air barrier, in addition to the sheathing.