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Community and Q&A

Semi-impermeable vapor retarder

Michael_Kalman | Posted in Energy Efficiency and Durability on

What is the best semi-impermeable barrier that is around 0.1-1 perms (Class II) so that I can avoid using a Class I vapor barrier. I worry that in my Zone 6 Southern Ontario climate I will have problems with mold in my walls. We don’t use air conditioning but I want to make a very clean environment as we are both mold sensitive.  I am using all borate cellulose.

Is there anything other than latex or oil based paints? And if I went the Class I route due to availability and unavailability of other products, how would I ensure that the vapor barrier can drain to the exterior and not right down to the joists?

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  1. Expert Member
    Michael Maines | | #1

    Michael, depending on your wall assembly, your best bet is likely a variable permeance membrane, sometimes called a smart vapor retarder.

    Siga Majrex is 0.23 perms when vapor is moving from interior to exterior, and 0.68 perms when vapor is moving to the interior, for improved drying ability.

    Pro Clima Intello is 0.2 perms when dry, but opens up to 6.0 perms when damp.

    There are other similar products, including Certainteed Membrain. Just make sure they actually meet your building code--I was under the impression that they might not.

    1. Michael_Kalman | | #2

      Amazing, thank you so much for letting me know about these products! When installing, does one need to provide a route for these retarders to drip out to the exterior in a tiny home?

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