The transport of heat and moisture in buildings is complex. Hygrothermal transport is dynamic, multiphase, multidimensional, and interfacial in a porous media. Vapor, liquid, and ice may be present at the same time within an envelope structure. The hygrothermal performance of the envelope component is also directly dependent on the specific interior and exterior loading. These are some of the reasons why simple generalizations with regard to design elements in building envelopes do not work. One of the best examples is the selection of vapor retarders in building envelopes.

There has been considerable controversy about what needs to be done to control moisture vapor movement in the walls of US residential homes. Indeed, guidance in the selection of vapor retarders becomes even more critical for the highly insulated walls that are being proposed for the future net zero energy buildings. In 2003, the U.S. Department of Energy investigated research work to provide a series of recommendations to propose changes to the energy codes. Initial proposals were met with some resistance, primarily based on the fact that there was a perceived lack of scientific backing to the proposals. The Oak Ridge National Laboratory, working in close collaboration with Building Science Corporation, developed a plan to perform a series of hygrothermal computer simulations. These simulations validated previous understandings and provided additional insight on some of the complex interactions present in building envelopes. They also included the impact of 1% water penetration on the sheathing membrane as an additional load as well as the impact of air conditioning during the summer months. These results became the basis for the vapor retarder recommendations. This paper summarizes the scientific evidence for the recommended code changes that were submitted to the International Energy Code Council.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings X – December 2007

Units: Dual