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Vapor-permeable air sealing for cripple walls

DPiranesi | Posted in Green Products and Materials on

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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  1. Expert Member

    Why not tape the plywood and use it as your air-barrier?

  2. DPiranesi | | #2

    There will be holes in the plywood for termite inspection, maybe other inspections. I myself have used a borescope through similar holes to look at sill bolts. I thought of doing it somewhat like you describe but making the membrane removable. Seems a little iffy to do that plus it will look unfamilliar to building inspector.

    I also have a vague idea that sealing outside air out at the first opportunity to keep it out (interior side of the the house sheathing 1x6s) is better than letting it seep into the cripple wall cavity.

    I also might cover the finished shear wall with easy-to-replace kraft paper later, in addition to but not instead of sealing the sheathing. At least for now I still think the question is how to seal the interior side of the sheathing, not whether.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      Craft-paper is a terrible air-barrier - and isn't accepted as such under my code I know of.

  3. GBA Editor
    Brian Pontolilo | | #4

    Hi David.

    It sounds like you are not actually looking for a vapor retarder, but just an air barrier. Is that correct?

    If so, the first thing that comes to mind is a vapor open housewrap. They are typically known to be difficult to detail as an air barrier, but that is because of all the seams and fasteners. The materials themself do block air flow. You'd be able to use a continuous piece, will only need to determine how to seal the perimeter (probably an air sealing tape) and you won't need to punch a bunch of holes in it.

  4. DPiranesi | | #5

    Brian - yes, an air barrier, exactly. It's just that I'd never heard of vapor-open material before, sounds exactly right. I wrongly guessed that vapor retarder was as permeable as it gets. If I don't have a magic search word for Google I can miss a lot. But bearing in mind GBA has written that house wrap isn't usually an air barrier. I haven't untangled that.

    Now I see that even Tyvek is considered vapor-open (I had no idea) and there are plenty of other choices. Also now I see Home Depot is advertising dirt-cheap asphalt-impregnated kraft D paper for precisely my application. Need to pick a reasonably economical one that can tolerate occasional damp at bottom (no capilary break, and at least for now no stucco flashing at grade though I keep hoping some skilled person needs a small job sometimes). Reduced environmental impact of the material itself is potentially a factor. I'm not expecting any need for bulk water to drain so that's not a factor.

    As for details my gut is it's the same level of effort or less compared to caulk or foam bead or tape at every wood joint, though I'll find out, won't I. Considering questions about whether house wrap can air-seal, maybe I still have to look at tape and like that. Maybe some materials would be harder to detail than others. Any further suggestions welcome.

    1. GBA Editor
      Brian Pontolilo | | #6

      You got it David. Tyvek is one of the most vapor open building materials. And as a material, Tyvek and other housewraps do not allow air to pass through them. It is the installation on a large and complex wall that makes them poor air barriers. Think of all the seams, transitions, fasteners, and other details that compromise air tightness. In your situation, if you can install one length and use some good quality air sealing tape around the edges, and not penetrate the material, or detail a few penetrations well, it should work pretty well, I think.

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