My how far we've come. From the days of back plaster and rosin paper over wood board sheathing to fully-adhered membranes and liquid-applied air barriers, Joe Lsitiburek walks us through the steps in our collective experiences at learning to control air flow as a means of controlling energy and moisture flow.

(This paper first appeared in the ASHRAE Journal, which is also available, and perhaps an easier read, in PDF)

Throughout the postwar years practitioners were taught, incorrectly, that vapor barriers were necessary in cold climates to protect wall assemblies from moisture damage and that it was necessary to install these vapor barriers on the interior of cavity insulation. The industry saw the introduction of kraft facings and foil facings on batt insulations as a result. These vapor barriers were by their very nature discontinuous and they proved ineffective in protecting wall assemblies from vapor. Vapor is principally transported by air flow not by vapor diffusion. We needed air barriers not vapor barriers to control vapor flow. It took decades for that distinction to be appreciated.

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