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Community and Q&A

Dew point in walls using ZIP and ZIP-R

LawrenceMartin | Posted in Green Building Techniques on

I have a question regarding the dew point in walls using ZIP and ZIP-R sheathing in an arid Climate Zone 5 (Colorado).

In a pair of GBA and BSC articles, the minimum amount of exterior non-permeable insulation needed to prevent condensation from occurring within the fluffy part of the wall cavities is R7.5 for a 2×6 R20 wall.  Is it correct that this is only an issue if there is also an interior poly layer preventing inward drying?

I ask because if a house were built using 2×6 R20 framing with either ZIP with no insulation or ZIP-R5, the situation would be the same with the air barrier on the exterior and the dew point occurring within the cavity framing.  But, if there is no interior poly, shouldn’t this allow the assembly to dry toward the interior?
For custom homes in Climate Zone 5, we specify the Zip R12 sheathing with 2×6 R-20 fluffy insulation.  But, I occasionally work with builders who are adamant about using the code minimum, which is R20 + R5ci. And, I frequently see houses being built with the standard ZIP uninsulated sheathing. 
Thanks for any insight. 

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  1. Expert Member
    Michael Maines | | #1

    No, in fact the "ratio method" as we tend to call it here on GBA, of providing enough exterior insulation to keep the sheathing above the dewpoint temperature, is derived from IRC requirements and Building Science Corp research for walls that are safe WITHOUT poly sheeting on the interior. Poly sheeting is a class 1 vapor retarder and does not allow vapor movement in either direction. Most water vapor/condensation-related issues are due to interior moisture trying to get outside, not outdoor moisture trying to get inside.

    Drying to the interior only occurs when the vapor pressure indoors is lower than outdoors. The vapor pressure is a combination of heat and humidity. In cold climates, vapor pressure outdoors is usually lower than indoors. Basically, when you're running an air conditioner, vapor movement is likely to be toward the interior, and otherwise is is likely to be toward the exterior.

    Zip-R panels are nearly as vapor-open as Zip without insulation. The OSB itself is the least-permeable part of the assembly, at least until it becomes saturated; then the polyiso becomes the lowest-perm component. On walls without exterior insulation, the sheathing gets damp in late winter/early spring but it continues drying toward the exterior until it's dry. Having dry outdoor conditions helps speed this process.

    1. maine_tyler | | #4

      Are you suggesting the ratio rules don't apply to zip R? I've heard conflicting info on this and am unaware of any real concensus.

  2. LawrenceMartin | | #2

    Thank you, Michael. I was thinking that the OSB component when coated with ZIP's green coating and the ZIP tape was an air barrier and moisture barrier and vapor barrier. But, I guess it's not a vapor barrier until the polyiso layer is added.

    That seems very concerning that the current code requires a minimum of R5 ci in Climate Zone 5.

    1. Expert Member
      Michael Maines | | #3

      Those are common misperceptions. Zip alone is very roughly around 1 perm when dry and 10 perms when damp. The coating is around 13 perms when dry and higher perm when damp. The polyiso is around 1-2 perms. Foil-faced polyiso is a vapor barrier (<0.1 perms) but the type used on Zip panels has a vapor-open fiber facing and the foam itself allows some drying. The same is true for its close cousin, spray polyurethane foam.

      I share your concern with R5 exterior insulation; it's the same for CZ6 where most of my work is. Because the perm rating of Zip-R is similar to conventional OSB, though, it seems to work well in most cases. As a designer I am extra-cautious but the majority of homes in cold climates seem to work on with Zip-R.

      1. maxwell_mcgee | | #5

        so to summarize, what you're basically saying is that if you were to put a smart vapor permeable barrier on the on the inside (e.g., an intello+ between drywall and fluffy stuff between the studs) then a high enough Zip-R assembly (e.g., Zip R-12 in CZ 5-6) should be okay to manage the vapour condensation risk?

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