PGH Book Wall Assemblies-Vapor variable membranes p.119
ar_t
| Posted in Expert Exchange Q&A on
“..these membranes are composed of materials that become less permeable as humidity increases on its interior side and more permeable as the air dries.”
”… but in general they are designed to have low permeability when the relative humidity is low and higher permeability when the humidity increases.”
This appears confusing to me. Are the authors talking about inside the building envelope or inside the wall assembly? These two sentences in the first two paragraphs are contradictory, I think, or perhaps I am not understanding the meaning in the first two paragraphs.
I love the book, by the way,
Robert
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You are right the book can be a bit confusing as it was written by a few different folks at the same time as I recall, but its great resource in general!
ProClima Intello Plus has a good breakdown on how there product operates:
Vapor Variable: Low Class II vapor retarder in winter (70% RH) with a DIN 68800-2 compliant, Hydrosafe value of 1.6 perms
Check out 475 Building Supply for more info on Intello or Siga has a similar product as well.
That is one of a few mistakes we found as well. For most variable permeance products, the higher the relative humidity, the more vapor-open the membranes become, regardless of which side of the membrane is experiencing increased humidity. Siga Majrex works a little differently, and is always more vapor-closed to moisture trying to get from indoors to outdoors, and more vapor-open for moisture trying to get from the wall cavity to the building interior. That's generally the intent of the other membranes as well, to keep most moisture out, but to allow it to dry to the interior in the event of elevated moisture within the walls or roof.