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Community and Q&A

Vapor barrier over interior plywood sheathing

tayom | Posted in Energy Efficiency and Durability on

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.


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  1. GBA Editor
    Martin Holladay | | #1

    The plywood is already a smart vapor retarder. Plywood has variable vapor permeance. No need to add another layer.

  2. Expert Member
    Peter Engle | | #2

    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.

  3. tayom | | #3

    Thanks! I'll skip the additional vapor barrier layer, and caulk/foam any penetrations in the plywood.

  4. Expert Member
    Peter Yost | | #4

    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.

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