ADVANCED VAPOR RETARDER

With Veridian Homes of Wisconsin, CARB evaluated the performance of CertainTeed's MemBrainTM Smart Vapor Retarder in a side-by-side comparison to a conventional polyethylene vapor retarder.

The key findings are:

• Humidity build-up consistently occurred behind both vapor retarders on summer afternoons
• The build-up resulted largely from sun driving moisture into the wall
• By allowing drying to the inside, MemBrain significantly reduced the magnitude and duration of this humidity build-up
• During the winter, both systems were equally effective at restricting room moisture from diffusing into wall cavities

In cold climates like Wisconsin, drying in wall systems usually occurs to the outside. However, if a 2x4 bottom plate is saturated due to a water leak, there is a potential for drying to both the outside and the inside. And even in Wisconsin, a wall system can only dry to the inside during humid summer afternoons.

The building science community increasingly promotes a two-pronged approach to managing moisture in exterior wall systems: minimize the wetting rate while simultaneously maximize the drying rate. The MemBrain vapor retarder allows drying to the inside in response to a build-up in wall humidity. When humidity levels are lower, such as during a Wisconsin winter, the MemBrain acts like a conventional vapor retarder and restricts water vapor from household moisture sources from entering the wall system. Monitoring results indicate the MemBrain vapor retarder significantly reduced the number of hours of high RH (over 70%) in the Veridian wall compared to the polyethylene vapor retarder that never allows for any drying to the inside.

Brainy Membrane, Home Energy, July/August 2006