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Community and Q&A

Do I need a vapor barrier behind brick veneer?

whitenack | Posted in Green Building Techniques on

In Joseph Lstiburek’s “Top Ten List of Dumb Things To Do In The South”, he says that all brick should have a vapor barrier drainage plane.

I am building in Central KY (Zone 4), and planning brick veneer over tyvek, then 2 layers of 1-1/2″ fiber faced polyiso, then osb, 2×6 stud with blown cellulose, then drywall. I understand that tyvek is not a vapor barrier and polyiso is just a class II vapor retarder, not a vaper barrier.

Have I missed an important component? Do I need to add a vapor barrier somewhere in the wall? If so, what and where? I don’t see any conversation about vapor barriers in exterior rigid foam construction but maybe it is because most folks side with things other than brick?

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  1. GBA Editor
    Martin Holladay | | #1

    The problem that Joe Lstiburek wants to prevent is inward solar vapor drive. A good ventilated rainscreen gap (one that is not clogged by mortar droppings) goes a long way towards reducing problems associated with inward solar vapor drive. Since your wall includes 3 inches of polyiso on the exterior side of your OSB, you're fine. You won't have a significant amount of vapor being driven inward through your polyiso.

    The worst sheathing behind brick veneer is fiberboard. All of the wall disasters associated with inward solar vapor drive had a very vapor-permeable sheathing like fiberboard. You're fine.

    Here's my basic advice: a good brick veneer wall has rigid foam sheathing and a ventilated rainscreen gap that isn't clogged by mortar droppings. You've got that (I hope), so you can sleep at night.

  2. whitenack | | #2

    Thanks, as always, Martin.

  3. iLikeDirt | | #3

    In your climate, a radiant barrier facing the brick veneer using the drainage gap as an airspace can be effective, especially with thermal mass (your brick veneer) on the outside but not the inside. If you get foil-faced polyiso instead of fiber-faced polyiso and put the Tyvek under rather than over the foam (or sandwiched between layers) then you'll get this radiant barrier for free.

  4. whitenack | | #4

    Thanks for the additional comments Nate. I have already pulled the trigger on the fiber faced ISO. I got a great deal compared to the XPS I was originally going to use (saved $10 per sheet multiplied by ~350 sheets). Plus, I'm going with "outie" windows, so the tyvek needs to be outside the foam.

  5. Expert Member
    Dana Dorsett | | #5

    Most fiber faced polyiso has a vapor retardency less than 1 perm. Dry half inch OSB is also less than 1 perm. In combination you'll be a the very tight end of class-II vapor retardency, which means you won't have any issues with the moisture drive of the brick creating condensation on the wallboard side of the assembly during the cooling season.

    With 3" of polyiso you also have HUGE dew point margin at the OSB layer from wintertime moisture drives.

    Asphalted fiberboard was popular behind brick veneers during the 1940s-1950s due to it's very high moisture tolerance- it wouldn't mold or rot like plywood or milled lumber often did. It worked very well in uninsulated houses that were not air conditioned or only window AC.

    It didn't become a disaster until people A: insulated the wall cavities and B: air conditioned the place to a lower indoor temp 24 hours/day with central air. The combination of insulation and constant cooling brings the average temperature of the finish wall side lower than the outdoor air's dew point at times in the southeastern states, and much lower than the dew point of the masonry cavity air when the sun is driving moisture out of the brick. But people also figured out how to make it even worse by using vinyl & foil wallpaper, making it impossible for the AC to dry out the wall assembly.

    Asphalted fiberboard is still a useful building material, just not behind brick in well insulated air conditioned houses. Under rainscreened (non-masonry) siding in cold climates it offers superior wintertime drying capacity, and it isn't bothered by bulk water incursions- it's not even damaged if left uncovered out in the rain on construction sites, and dries quickly.

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