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Community and Q&A

Vapor Barrier

Randy_Williams | Posted in Building Code Questions on

The state of Minnesota makes amendments to the building code. I have a question on one of their changes. The Minnesota code reads:

A vapor retarder and air barrier shall be applied to the warm in winter side of the wall with a permeance of not greater than 1.0 in accordance with ASTM E96 procedure A and a permeance not less than 0.3 in accordance with ASTM E96 procedure B meeting the following codes…

The code goes on to detail how to attach and seal the vapor and air barrier to the framing.

What is the difference between ASTM E96 A and B?

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  1. Expert Member
    Dana Dorsett | | #1

    Method A is aka "dry cup", method B aka "wet cup" are the two prescribed methods of testing water vapor permeance. Most materials specifications listed are the dry cup test.

    If MN code is now prescribing <0.3 perms wet-cup they have basically outlawed Certainteed MemBrain smart vapor retarder- I'm not sure about Intello. That also eliminates vapor barrier latex.

  2. Randy_Williams | | #2

    Thanks Dana, I'm pretty sure the code writers in Minnesota worry more about the wall wetting potential during winter heating than summer cooling. I conduct blower door testing for several contractors, most new construction is code minimum, but they are achieving results under 2.25 ACH @ 50 pa, last house I tested was 1.7. The insulating contractors have gotten very good at air sealing with poly. My opinion is if they are going to require poly, they should also require an air gap behind the siding.

  3. GBA Editor
    Martin Holladay | | #3

    For more information on the difference between the wet cup test and the dry cup test, see "All About Vapor Diffusion."

    For more information on the wet and dry permeance of smart vapor retarders, see "Smart Vapor Retarders for Walls and Roofs."

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