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.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part