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Slab vapor barrier sill sealing question

vashonz | Posted in Green Building Techniques on

What’s the best way to integrate sub slab vapor barrier into the exterior air barrier?

I’m getting ready to flash openings and install my WRB on my house this weekend.
-VP100 WRB
-Prosoco Joint& Seam Filler and Prosoco Fast Flash for seams and window openings.
-OSB Sheathing
-6mm Sub Slab Vapor Barrier, left long under sill sealer.

Should I cut fold down the vapor barrier, bridge the sill plate with a butyl tape, then lap over that with the VP100?

Use Joint and seam filler along the sill plate, cutting the vapor barrier short, then lap over that with the VP100?

Fold vapor barrier UP and cover it with VP100?

I’ve found a number of different detail drawings, but am not sure of the downsides to different methods.

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  1. Expert Member


    Whatever method you choose, don't lap the seal too far down onto the concrete. You then have to drop the sheathing and siding too, and there is nothing for fasteners to attach to. You can see this in the first section you posted. The bottom nail is fancifully floating with just the head in the plywood.

  2. Expert Member
    Michael Maines | | #2

    Chris, Siga Fentrim is my favorite tape for sealing airtight sheathing to concrete. You only need to lap onto the concrete by 1/2" or so. It's incredibly sticky. Pro Clima Solida Exo is apparently similar in composition and tenacity; I just don't have first hand experience installing it.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Chris,

    Many builders run the poly under the sill plate and up onto the sheathing where they tape it with air sealing tape, as you are considering. But I can't remember seeing this detail with anything but mechanically fastened WRBs that then lap over the poly. I'm not sure if this is appropriate with a fully adhered WRB, which is meant to be adhered to the sheathing.

  4. vashonz | | #4

    Siga Fentrim Looks like it would do great. Gonna call Small Planet Supply today, talk to them. I'll likely need either more Joint& Seam Filler or order the Fentrim.

    I found a video by Matt Risinger last night showing the detail published by Hammer and Hand.

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