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Exterior PLYWOOD; is it OK as is?

GBA Editor | Posted in Energy Efficiency and Durability on

I just read a couple of lengthy posts here, dealing w/ exterior sheathing leading to condensation problems, and I am confused. Maybe I got lost as the conversation shifted among exterior foam and several slightly tangential posts. Very possible. My concern now is this: Should I drill holes in my exterior plywood, or rely on edge gaps and plywood’s permeability to minimize or eliminate condensation? For sake of conversation, assume I will build “business as usual”: From the inside out, the parts will be 1/2″ sheet rock, a serious vapor barrier, approximately 15″ of high-density cellulose blown into a double wall system, 1/2″ CDX for shear strength, Tyvek, and then vinyl siding. I can see where that plywood could likely condense (14K heat degree days), so do I drill it full of holes (small, spaced well so as to not effect the shear strength of the plywood) to help the wall breathe? BTW, it is pretty dry up here, too. Exterior humidity is not a huge issue. Thanks. Maybe I’ve just read myself into a stupor here today…. j

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  1. GBA Editor
    Martin Holladay | | #1

    A reminder: when asking a building science question like this, it's always a good idea to mention your location. Since you have posted a previous question, I happen to know that you are building in Fairbanks, Alaska; but not everyone remembers that.

    The short answer is: no, you don't need to drill holes in the plywood. Since air barriers are good, you want to do everything you can to make the sheathing layer as airtight as possible. Many builders install wall sheathing with caulk or tape the seams.

    The small amount of seasonal condensation will be absorbed into the cellulose, which will act as a moisture buffer. In warmer weather, the wall will dry out — probably to the exterior, by diffusion through the plywood, and perhaps to the interior, if you can be convinced to omit interior polyethylene.

    In your climate, interior polyethylene can be justified. But with your proposed wall assembly, it isn't really necessary.

  2. user-723121 | | #2


    If you are interested in a serious vapor barrier, I would use Tenoarm. I have used this product in a number of new homes and would highly recommend it. Get a case of Tremco acoustical sealant, a few cans of expanding foam, some airtight electrical boxes and you are set.

  3. jklingel | | #3

    Martin: Thanks, and I will try to remember to note the location henceforth. Even with Tyvek over-lapped and goo'd, you should caulk the plywood, huh? OK. Easy to do, and once there, it is a done deal. Doug: I'll look for the Tenoarm. I've been reading about brands listed on another forum, some with an advertised perm of 0 and still a "plasticy" membrane. I used Tremco sealant ("black death") on a remodel last year, and applied it to a few scraps of 6 mil visqueen to test it. I just pulled the scraps apart, and the caulking split, but would not leg go of the visqueen; pretty sticky stuff. It is getting a bit rigid, but still flexible enough. Thanks. john

  4. user-723121 | | #4

    Conservation Technology has a sealant they recommend for use with Tenoarm, may be superior to Tremco.

  5. jklingel | | #5

    OK. Will scope it out. BTW: The gal who sold me the Tremco said "it is impossible to get it off your skin", which may be true when it dries. However, when it is wet, it comes off quite easily, just like tar, with cooking oil. I quit using gasoline on myself many years ago.

  6. Riversong | | #6

    "In your climate, interior polyethylene can be justified. But with your proposed wall assembly, it isn't really necessary."

    Isn't necessary IF interior relative humidity is well-controlled.

    Doug - there's another thing we agree on: If you're going to use a plastic vapor barrier (which I advise against in almost all situations), then Tenoarm is the way to go. But definitely use the compatible Teno Seal adhesive or their Teno Tape for seams and edges, not solvent-based acoustical sealant.

    JKlingel: If you're taping and caulking your WRB (Tyvek) as your air barrier, then you don't need to caulk or tape the sheathing - it will breathe better if the joints are open. But don't use polyurethane caulk, as it's incompatible with polyolefin housewraps.

  7. jklingel | | #7

    "But don't use polyurethane caulk, as it's incompatible with polyolefin housewraps." Excellent. I was just reading that polyurethane caulks were great for visqueen; glad to know they won't work for Tyvek. If I can get the Tenoarm up here w/out breaking the bank, I will surely get plenty of their sealing stuff, too. I think it was them that has some interesting sounding gaskets for around window sills/frame, under the Tenoarm. Perhaps they've worked out all the bugs.... Thanks again. john

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