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Vapor retarders

Birdie L | Posted in General Questions on

I believe I have myself very confused on vapor retarders.   I have read many articles and codes, and I am now more confused than when I started.

Climate Zone 5.

We we have Un-Vented cathedral with closed cell spray foam.

We also have 2 vented attics with blown in fiberglass, topped with cellulose for better air sealing.

All the walls have open cell spray foam.

No vapor barriers were added, such as polyethylene.  We have painted walls and ceilings, which I thought was enamel paint but just learned it is actually acrylic.

I have read closed cell spray foam is class 2 vapor retarder, but I am very unclear as to what to do, if anything, with the open cell walls and the vented fiberglass attics.

i would hate to cause problems by adding vapor barriers if not needed.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    In vented attics the vapor retardency of the ceiling doesn't really matter until your on the cold half of zone 7. In zone 5 it's not at all an issue.

    In the walls in zone 5 it ALMOST doesn't matter. If the siding is back ventilated (say vinyl siding or any rainscreened type assembly) the enhanced ability of the sheathing to dry toward the exterior combined with the capillary break of the air gap makes it not matter (in zone 5 or lower, but not zone 6). This is enshrined in code in R702.7 as the "Vented cladding over wood structural panels." exception:

    https://up.codes/viewer/wyoming/irc-2015/chapter/7/wall-covering#R702.7.1

    If the siding is tight to the sheathing it may matter, but primarily on the sunless north side, or walls that are completely shaded (say, a house located in a tall coniferous forest.)

    With the air tightness of open cell foam it's really only a vapor diffusion mechanism, which is much slower than air leaks. If you're the type to lie awake at night wondering, re-painting the walls using a "vapor barrier latex" primer base layer would bring the vapor retardency down to Class-II levels, and proven to work.

    1. Birdie L | | #2

      Thank you!

      We have Zip System sheathing, and ended up installing Everbuilt housewrap over too because we reached our max exposure days during building. Would this make any difference? Most of our cladding is vented siding, but we also have synthetic stone with the rain slicker max as the rainscreen.

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