Talk Perms Like a Pro: The Science of Water Vapor

Faced with managing water vapor in buildings? Who isn’t? Put down that polyethylene and learn to talk like a building scientist.

September 2, 2014

Water moves through buildings in four ways (see How Water Moves Through Buildings), and of these, vapor diffusion is the most confounding to understand and manage. Yet it’s arguably the most important, too: there are lots of ways to get a building wet, but vapor diffusion is uniquely important in allowing them to dry out.

Lesson one in the lingo of vapor is permeance vs. permeability. Both are terms for the rate of vapor transmission through a material, but permeance indicates that rate through a standard surface area at a specified pressure and material thickness (e.g., 60 mil) and is expressed in perms. This measure makes sense when judging how sheet goods respond to vapor pressure; their thickness is not only fixed but also slight.