Vapor Retarder for Attic Knee Wall
tklinr
| Posted in General Questions on
Hello, I need some advice. I’m planning to insulate a detached garage attic space with fiberglass or mineral wool batts. At the knee walls I want to install rigid insulation as sheathing to keep the 2×4 batts from falling out. Edges and seams of the rigid will be foamed/taped. I’m also planning to use a “smart vb” on the interior but am now concerned I’m creating a double vb. I’m in zone 6. Since this is not habitable space I’m just trying to make it comfortable but don’t want to create a problem. Any advice? Space will be intermittently conditioned only when in use – mainly on weekends.
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Replies
Tkliner,
I don't think you have much to worry about especially if this area doesn't have showers/stovetops/ hot tub/ lots of household plants. Vapor transmission through walls might be the least of my concerns for a building like this.
If it really worries you, get rid of the vapor retarder or keep it as your air barrier and use EPS foam as your rigid foam. It has a meaningfully high perm rating within 2-3 inches or so.
In zone #6 you'd need the foam to be at least R7.5 for dew point control inside the R13 kneewalls.
A "smart" vapor retarder is considered smart because it simply CAN'T become "...a double vb..." problem. When the moisture levels in the fiber insulation are high enough to support mold the smart vapor retarder becomes sufficiently vapor open to dry quickly.
When the moisture levels are low a smart vapor retarder becomes much more vapor tight, limiting the rate of moisture accumulation. When it's cold enough for the foam board to begin condensing, the very act of condensing moisture out of the entrained air inside the wall cavity the moisture level in that air (of course) drops, at which point the vapor retarder becomes much more vapor tight, lowering the rate of moisture diffusion from the conditioned space indoor air into the wall cavity.