The new vapor retardant is a polyamide film that can become more than 20 times more permeable at high relative humidity than during times when there is low humidity, such as in the winter.
More typical polyethylene vapor retardants allow moisture to condense on cold surfaces and accumulate in the wall cavity when the seasons change.
The concept of a “smart” vapor retardant was introduced seven years ago in Europe and has become widely accepted there, CertainTeed says.
With a high resistance to water vapor diffusion in winter, says the manufacturer, the pores of MemBrain™ react to increases in the relative humidity so that water vapor can pass through. This lowered resistance supports the drying process.