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Community and Q&A

Vapor barrier

Jeff Caira | Posted in Energy Efficiency and Durability on

I am planning on building a house in Massachusetts. I want to minimze the thickness of the wall so as not to lose too much interior square footage, but I would like to maximize the R value of the wall (target is 40). I have been thinking about using a staggered stud wall with 2″x4″s on 2″x8″ plates. I was thinking about using 3″ of closed cell foam against a zip wall sheathing sytem with either 3 1/2″ of fiberglass batts or blown in cellulose inside. My questions are, 1) does the closed cell foam act as a vapor barrier and if so, is it enough to prevent condensation from forming in the middle of the wall in winter and, 2) does the foam also act as an air barrier and, if not, won’t an air barrier on the warm side create two vapor barriers? Thanks for any help.

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  1. Matthew Amann | | #1

    I'm not entirely sure, but it seems you may not want to sandwich the plywood between it's vapor barrier face and the vapor retarder foam... I'll be interested in what others have to say. Why not frame two 2x4 walls separated by an inch, blow in cellulose, and cut out the thermal bridging at the plates? Nonetheless, good for you building beyond the normal standard.

  2. Dick Russell | | #2

    First, your "R" math doesn't add up to R-40 with 3" of CC foam and an R-11 or 13 batt or the equivalent in cellulose. R30-33 is closer, given your 2x8 plate.

    Properly adhered, the cc foam will be both an air barrier and a vapor retarder. The water-resistive coating on the Zip sheathing, with the seams taped, and with some provision made for sealing the joints at sill and top plates, can be an air barrier, but not a vapor retarder. The specs on that product show a decent amount of permeability.

    Don't confuse air barrier and vapor barrier. An air barrier stops air leakage, while a vapor retarder or barrier retards or prevents molecular diffusion of water vapor through the material. Properly detailed, some low-perm air barrier materials can be also part of an air barrier system.

    In the March 2011 issue of Fine Homebuilding magazine, there is an article (pic on the cover, too) of "flash and batt" insulating. The article discusses thickness of foam required to prevent condensation on its surface. I think your 3" is fine in a cavity defined by a 2x8 plate. This has been discussed on this site (GBA) as well, IIRC. The FHB article also refers you to GBA.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Does the closed cell foam act as a vapor barrier?"

    A. Yes.

    Q. "If so, is it enough to prevent condensation from forming in the middle of the wall in winter?"

    A. Yes.

    Q. "Does the foam also act as an air barrier?"

    A. Yes, unless the installer is sloppy.

    Q. "Won't an air barrier on the warm side create two vapor barriers?"

    A. Many air barriers (including gypsum drywall) are not vapor barriers. Drywall is vapor-permeable.

  4. Matthew Amann | | #4

    I'm not sure if you were talking to me Dick, but I definitely understand the difference. I do question that if there is kraft paper in that coating, which is usually .7 perm if asphalt impregnated, that it is "readily" vapor permeable. The stats I saw on the web were all over the board-1, 2-3, over 8, 12-16, Really? I actually want to like the product, seems like a good quality sheathing, which these days is hard to find. Perm figures seem to come from everyone but Huber, which is concerning to me.....And like you, Dick, Huber seems to want to educate people on the differences in barriers, we just want the facts. I couldn't seem to find it on their website anywhere. Pick a number people....

  5. Jeff Caira | | #5

    Thanks for all the helpful information. There is clearly a lot of knowledge on this site.

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