ruminations about architecture and design

Thursday, February 13, 2014

Completely vapor open walls

Everyone still seems to be confused by  the differences between air barriers and vapor retarders. Codes, contractors, and designers have given too much focus to vapor retarders, and more specifically, vapor barriers than is warranted. As Joe and John have pointed out, if you aren't in Canada then being obsessed about vapor barriers is a waste of time. We should all be more concerned with air barriers, because if we're actually concerned about water management, then we should all be on board with the fact that pressurized air can transport much more water vapor through a the building enclosure than vapor diffusion on its own.

As a designer, I would like to have a one-stop shopping solution for the AVB and WRB. Sto Corp, among others, seems to be heading in the right direction with many of their products, but I'm a bit puzzled by the Sto Energy Guard system that is detailed above. They take responsibility for the exterior insulation, a waterproof air barrier, the substrate joint treatment, and a drain screen, but they don't take responsibility for the "Code Compliant Paper WRB. Why not? And why isn't their "waterproof" air barrier satisfying the requirements of the WRB? And why is their XPS insulation better than another product?

So, if I use a vapor impermeable, code approved WRB in this assembly in a mixed/humid climate or a cold climate I might be making a big mistake. Sto gives me a hint that I should I be using a vapor permeable "paper" WRB which implies felt paper or Tyvek, but they don't want to stick their neck out and state that explicitly.

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