Last week’s blog answered some common questions about vapor retarders. This elicited a comment from Bill Rose, research director of the Building Research Laboratory at the University of Illinois in Champaign. “We might imagine a future in which the building code sections that address the vapor barrier would all go blank,” Rose wrote. “I bet most readers would be able to design excellent buildings that perform well and are quite durable, without using the words ‘vapor barrier’ at any point in the process.”

Rose is a renowned building scientist and an expert on the history of vapor barrier requirements in building codes. These often counterproductive requirements were introduced with little scientific evidence to support their usefulness. As Rose points out, the abandonment of all code references to vapor retarders would appear to be a low-risk proposal, since it’s very rare for any problems to be caused by the diffusion of water vapor from the interior of a home towards the exterior.

An erroneous obsession
While building codes have historically shown a curious and unjustified obsession with vapor barriers, they have (until recent years) almost entirely ignored the need for air barriers. Unfortunately, when code officials decided to require vapor barriers but ignore air barriers, they got it exactly backwards.

Things are looking up, and codes are belatedly beginning to address the need for residential air barriers. In section 402.4.2, the 2009 International Energy Conservation Code (IECC) will, for the first time, require blower-door testing of some new homes. (Builders who don’t want to arrange for blower-door testing can instead comply with the new air-sealing requirements by an alternate method that requires visual inspection of 17 areas known to contribute to air leakage.)

So we’re now in an interesting period of transition. Pressured by building scientists to eliminate nonsensical vapor retarder provisions, code officials have partially relented by expanding the climate zones where vapor retarders can be omitted and by providing more flexible ways to comply with vapor retarder requirements in cold climates. At the same time, code officials are taking the first timid steps toward logical air-sealing requirements. If this welcome trend continues, we may see increasingly stringent air tightness standards and the eventual elimination of vapor retarder requirements.

Looking back at older codes
Builders with gray hairs may remember the early 1990s, before the advent of the International Codes. Today’s International Residential Code was adapted from its precursor, a regional model code known at the CABO (Council of American Building Officials) One- and Two-Family Dwelling Code.

I recently pulled out my copy of the 1992 CABO code to refresh my memory on the book’s 18-year-old requirements. Back then, vapor retarder requirements were almost identical to those in the 2006 IRC. (As I noted in last week’s blog, the 2009 IRC has rewritten these requirements.) Like the 2006 IRC, the 1992 CABO required a vapor retarder rated at 1.0 perm or less on the warm-in-winter side of thermal insulation in walls, floors, and ceilings. (The exception for hot, humid climates was fairly limited.)

As far as I can determine, the 1992 CABO included no requirements whatsoever for air sealing — no section comparable to section N1102.4 in the 2006 IRC, which requires that “the building thermal envelope shall be durably sealed to limit infiltration.” Although the 1992 code required the use of asphalt-saturated felt as a WRB under some types of siding, including brick veneer and shingles, the code specifically noted that no asphalt felt was required under horizontal fiberboard siding or wood bevel siding (clapboard).

Interestingly, code requirements for duct tightness predate code requirements for envelope air sealing measures. The 1992 CABO not only required ducts to be designed according to Manual D, it also required that “joints of duct systems shall be substantially airtight by means of tapes, mastics, gasketing, or other means.” Unfortunately, these requirements — just like similar requirements in the current versions of the IRC — were rarely enforced.

Last week’s blog: “Vapor Retarders and Vapor Barriers.”