Vapor-permeable air sealing for cripple walls
In a 1930 stucco home in earthquake country, some seismic strengthening measures will go in in this winter, including plywood shear walls to cover some cripple walls in a 3-foot crawlspace. First I want to air seal the cripple walls that will be behind the plywood. The air seal must be vapor permeable. I’m leaning toward use of either kraft paper or a smart vapor retarder. I’m inclinded to lay the air seal against the outer wall, then bolts/holddowns, then mineral wool batts in the cavities, and then the plywood. The cavities might be inaccessible for decades.
Which vapor retarder? Paper (such as kraft paper or DB+) vs polymer vapor retarder (such as Intello Plus or MemBrain)? Or if someone thinks I ought to ditch the idea of continuous seal and seal wood joints only instead w/caulk or limited foam bead (edit: or tape, see below), maybe they can say why.
One thing that bugs me about vapor retarder is manufacturer remarks about “summer” humidity. In my climate zone humidity is a year round issue. Are these products going to do their job when it’s 90% humidity at 45 degrees just about sunrise in the winter?
Climate zone 3C, stucco exterior, paper behind stucco from 1930 is fine but it’s paper, yes wind and sun both drive exterior moisture into walls in any season, yes there are interior sources of humidity in any season, no there is no capillary break in the footing. No inspector, contractor, or anyone else is going to be OK with impermeable anything in this location. (Edit: I mean not behind a shear wall. Foam’s great where appropriate. Also moisture is probably piggy backing on air leaks. Also there are flashing defects typical of the period. Despite almost a century of this the engineer seems to have found no need for remediating framing and it’s good to go for voluntary strengthening.)
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