Condensation

**Grey Mouse** · 12-22-2018, 11:03

Your welcome. If you need a comparison just think of standard Viqueen polyethylene sheeting as being 6mil for thickness. Six mil is very thick for mylar. I have some long term food storage bags in 6mil mylar. No way I would wrap up in that thickness lol

**BillyBob58** · 12-22-2018, 11:36

So far, I can only find a discussion of perm rating related to building materials. I would, out of curiosity, like to see such a rating for clothing and sleep gear, just for fun. But maybe if some of my clothing is outright called "vapor barrier", that tells me the perm rating is < .1? Also- towards the opposite end: at what perm rating is clothing declared "breathable"? Not important, just curious.

https://www.energyvanguard.com/blog/...-What-the-Heck

In other words, as the thickness increases, water vapor has a harder time making it through, and the perm rating drops. That makes sense. So, materials with lower perm ratings are better at retarding the movement of water vapor. If the perm rating is low enough, we call the material a vapor retarder. If it's really low, we call it a vapor barrier.
The International Residential Code (IRC) updated its definition of vapor retarders in 2007 to include the three classes that Joe mentioned in his 2006 article.
Class I: ≤ 0.1 perm (called impermeable)
Class II: 0.1 to 1.0 perm (called semi-impermeable)
Class III: 1.0 perm to 10 perms (called semi-permeable)
The class I vapor retarders are also called vapor barriers because they pretty much stop the movement of water vapor. If the perm rating is greater than 10, it's not considered a vapor retarder. It's a permeable material.