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Vapor retarder vs Airtight Drywall…or both? …or neither?

Andy_ | Posted in General Questions on

Remodeling a couple rooms of a 1947 cottage in climate zone4 (Seattle). From outside to in it is currently horizontal cedar siding, tar paper, board sheathing, 2×4, plaster and lath. No insulation in the walls (though the crawl and attic are insulated). The plaster and lath is already down and I’m installing rockwool insulation. 
The question is what type of vapor retarder for the interior? I know that poly is a no-no, so looking at Membrain but wondering how necessary it is. Would an airtight drywall approach make the Membrain redundant? Would Membrain be better than bothering with airtight drywall? Is a vapor retarder even needed in zone 4? Is Rockwool ok without a vapor retarder? Is the fact that it’s board sheathing on the outside wall make all this pointless? Aaaaaaaagh!!!!
Too many variables and too little definitive information have left me second guessing everything. 
Thanks in advance for any help,
-Andy S

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  1. Jon_R | | #1

    Search for "Zone 4" below (and review local code). Do air seal.

    1. Andy_ | | #2

      "2. Zone 4 (marine) requires a Class II (or lower) vapor retarder on the interior surface of insulation in insulated wall and floor assemblies where the permeance of the exterior sheathing/cladding assembly is less than or equal to 1.0 perm and greater than 0.1 perm as tested by Test Method B (the “wet cup” method) of ASTM E-96).

      3. Zone 4 (marine) requires a Class III (or lower) vapor retarder on the interior surface of insulation in insulated wall and floor assemblies where the permeance of the exterior sheathing is 0.1 perm or less as tested by Test Method B (the “wet cup” method) of ASTM E-96) and the interior surface of the exterior sheathing shall be maintained above the dew point temperature of the interior air. Under this design approach assume steady state heat transfer, interior air at a temperature of 70 degrees F (21 degrees C), at a relative humidity specified in Table 1 and exterior air at a temperature that is equal to the average outdoor temperature for the location during the coldest three months of the year (e.g. December, January and February)."

      "Latex painted gypsum board (one coat of latex paint) is a Class III vapor retarder."

      What does this really mean? Just put up drywall and paint it? ...or do I have to hire an engineer to test my wall assembly and then convene a panel of scientists to determine the actual permeance level and then calculate the impact of climate change possibilities before I can proceed?
      I'm glad that building scientists are out there doing the research and setting the benchmarks so that we can build better homes. But man is some of this stuff dense! Is there a deciphered version of this somewhere with a best practices manual that a lowly builder or homeowner can understand and put to proper use?

  2. GBA Editor
    Martin Holladay | | #3

    The most important point is that airtight details are essential. Pay attention to air sealing -- before you install the insulation in the stud bays, and as you install the drywall (especially at electrical boxes).

    Vapor diffusion is unlikely to cause problems in your climate zone, but vapor-retarder paint is probably required by code and is easy to install. I don't see any reason why you have to worry about your code-enforcement official, but if you have any doubts, call up the local building department and ask.

    1. Andy_ | | #6

      Thanks Martin, I can air seal the interface between the drywall and the cavities, but the outside of the house is board sheathing, 75 year old tar paper (and who knows how continuous or intact it really is) and then lap siding. Thankfully, the builder of this house realized it was a wet climate and built generous roof overhangs that keep the rain off the siding 95%+ of the time. I realize that outside of catastrophic roof leaks, any moisture in the walls will most likely come from condensation via air from the interior.
      If the main leak points like outlet boxes, window trim, top plate, bottom plate are addressed with a sealant, could a vapor retarder such as Membrain be omitted with low risk? Or is the Membrain a better way to achieve the needed air seal?
      The walls will be painted regardless of which air seal is used.

  3. Expert Member
    Peter Yost | | #4

    Hi Andy -

    Interior vapor retarders restrict the movement of interior humidity, during the winter, into your building enclosure, where that moisture can condense on a cold surface in your building enclosure. How much of an issue this is depends on two things: how cold that surface gets and how high your interior relative humidity is in the winter.

    You can warm that cold surface, you can manage your wintertime interior humidity, or you can retard its movement into your enclosure.

    Most building enclosures get wet from bulk water leaks, or air leaks, not vapor diffusion. Slap on that vapor retarder paint and focus on bulk water management and your air barrier.


    1. Jon_R | | #5

      > How much of an issue this is depends on two things:

      I'd expand this, adding: outward drying capability (see BSC link) and susceptibility of the sheathing to moisture (eg, some houses use only foam - no wood sheathing).

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