Builders have worried about wintertime vapor diffusion ever since 1938, when Tyler Stewart Rogers published an influential article on condensation in the Architectural Record. Rogers’ article, “Preventing Condensation in Insulated Structures,” included this advice: “A vapor barrier undoubtedly should be employed on the warm side of any insulation as the first step in minimizing condensation.”
Rogers’ recommendation, which was eventually incorporated into most model building codes, was established dogma for over 40 years. Eventually, though, building scientists discovered that interior vapor barriers were causing more problems than they were solving.
Interior vapor barriers are rarely necessary, since wintertime vapor diffusion rarely leads to problems in walls or ceilings. A different phenomenon — summertime vapor diffusion — turns out to be a far more serious matter.
During the 1990s, summertime vapor diffusion began to wreak havoc with hundreds of North American homes. This epidemic in rotting walls was brought on by two changes in building practice: The first was the widespread adoption of air conditioning, while the second was one unleashed by Rogers himself: the use of interior polyethylene vapor barriers.
Rogers conceived of interior vapor barriers as a defense against the diffusion of water vapor from the interior of a home into cold wall cavities. Rogers failed to foresee that these vapor barriers would eventually be cooled by air conditioning — thereby turning into condensing surfaces that began dripping water into walls during the summer.
As with many scientific discoveries, it took a series of disasters to fully illuminate the phenomenon of summertime vapor diffusion.
One early victim of this type of diffusion was Cincinnati builder Zaring Homes. In the mid-1990s, Zaring Homes was a thriving mid-size builder that completed over 1,500 new homes a year. But the company’s expansion plans came to a screeching halt in 1999 when dozens of its new homes developed…