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Vapor Retarder only halfway up exterior wall?

JayMart | Posted in General Questions on

Quick question for a new build in Climate Zone 5, standard 2×6 construction, no exterior insulation and rockwool batts.

I have exterior brick approximately one quarter up my exterior walls, then a transition to traditional siding. I was going to opt against a smart vapor retarder and go with the Class III approach, but was reading how its recommended to protect myself in the summer due to the moisture collecting brick cladding.

Does the MemBrain need to run up the entire wall in this case or only as far up as the brick goes?


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  1. Expert Member
    BILL WICHERS | | #1

    You want to run the vapor retarder all the way up on the interior side of the wall. If you were to only run it half way, you might be a little better off than nothing at all, but mostly moisture will just get in as before and diffuse around in the wall over time.

    Personally, I'd go the extra step of putting a bead of sealant around the perimeter of the wall prior to putting up the membrane so that the membrane can also act as an air barrier (you also have to detail any penetrations to make this work). That's my usual way to go, membrane detailed as an air barrier and then airtight drywall over that. Belt and suspenders.

    BTW, I really like the little pneumatic staple gun at the link below when putting up membranes like this. This staple gun shoots "regular" staple gun staples (not the beefier staples used in most power staple guns), so it makes it a lot easier to tack up things like vapor retarders. I've never been a fan of the slap-type staplers, and the manual ones just kill your hand after a lot of use.


    1. JayMart | | #2

      Thanks Bill - Makes sense. On the sealant, do you mean a bead running against the top and bottom plate of the exterior walls?

      If I do this, will I only need to worry about my electrical boxes? I have the draft tight ones, but the back of the boxes are not caulked. I would imagine caulk there and the Membrain installed as an air barrier should really give me a robust assembly (my plywood sheathing is already taped).

      Great find on that staple gun. Thanks

      1. Expert Member
        BILL WICHERS | | #3

        Yes, I run a bead of sealant around the interior perimeter of the wall -- the edges of the top and bottom plates, and the stud at each side too, sealing the wall as a bit plate.

        I don't really trust any of the "tight" electric boxes, so I tape the vapor retarder to the sides of the box around the opening, and I also seal up the back of the box and any holes with caulk, tape, and/or canned foam, taking care to not let any sealants ooze into the interior of the box very much. I usually just use the white fiberglass "hard boxes", which don't have any holes -- they have "knockouts" that are thin spots you can punch through. These boxes are easier to air seal, since they don't have any holes in them besides the ones you make for the cables that you need.

        My basic goal is to try to seal everything up as much as possible, so that there is less potential for future leaks. A little extra effort here can only help you in the long run.


  2. AC200 | | #4

    Does you building code require a vapour barrier? I could never get away with running it halfway up the wall or leaving it a Class 3.

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