Vapor barrier over interior plywood sheathing
I’m remodeling my basement, in a 1971 house in climate 4c (seattle) and trying to make sure I get the vapor barrier right.
As part of the remodel I did an earthquake retrofit, which ended up including 1/2″ cdx plywood on the _interior_ of the outer walls. (the engineer specified this on the retrofit because the house was built without exterior sheathing)
The wall assembly from the outside-in is currently:
– shiplap cedar siding
– 50 year old tar paper
– 2×4 studs stacked for 7″ total thickness (this was as-built)
– rockwool r-23 between the studs
– 1/2″ “exterior” grade cdx ply as mentioned above
I’m planning to add drywall to that, and am considering whether to do a layer of membrain or other vapor barrier between the plywood and the drywall.
I’m also curious whether the plywood (vapor semi-permeable?) is already creating a vapor barrier (and a non-smart one), and whether that could be a bad thing regardless of whether I add membrain.
Thanks,
Theo
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Replies
Theo,
The plywood is already a smart vapor retarder. Plywood has variable vapor permeance. No need to add another layer.
Agreed. Wood is one of the original smart vapor retarders. You do still want to seal against air leaks through the plywood layer as much as possible.
Thanks! I'll skip the additional vapor barrier layer, and caulk/foam any penetrations in the plywood.
Hi Theo -
The relationship between vapor permeability and moisture content of materials is graphed as an adsorption isotherm. If you go here you can see one uploaded by Sam Glass, a topnotch wood scientist from the Forest Products Lab. Great organization, great guy.
https://www.researchgate.net/publication/273693403_Moisture_Meter_Calibration_for_Untreated_and_ACQ-Treated_Southern_Yellow_Pine_Plywood/figures?lo=1