The National Institute of Standards and Technology recently published a study that tested the permeance of various building materials. (Permeance is a measure of how quickly water diffuses through a material.) A vapor retarder is defined as a material with a permeance 1.0 or lower.

Most results confirmed common sense. For example, vinyl-covered gypsum performed better than fiberboard sheathing. However, the study contained one surprise. It showed that the permeance of the asphalt-impregnated kraft facing used as a vapor retarder on batt insulation can rise dramatically under very humid conditions.

That doesn't mean you should stop using kraft-faced batts. As the graph indicates, kraft paper's permeance doesn't increase much until the relative humidity (RH) reaches 50 percent. (Most homes have winter humidity levels of 30 to 50 percent.) In addition, the relative humidity in this study is the average of the relative humidities on both sides of the material. Thus an RH of 65 percent-which raises the kraft paper's permeance to around 2-might require an RH of 90 percent on the warm side and 40 percent on the cold side ([90 + 40]÷2 = 65). Doug Burch, a mechanical engineer at NIST who co-authored the study, points out that even with high average humidities, whether you get actual moisture in the wall will depend on the particular house. But the results suggest that in very moist areas like shower rooms, the prudent builder won't rely on kraft paper alone to keep moisture out of the walls. A better strategy is to install a continuous polyethylene vapor retarder.