Miofol® Active is a moisture-variable membrane, which is made from non-woven polypropylene and a polyethylene-copolymer film. The membrane serves a double function: In the winter, when it is warmer inside than it is outside, vapor flows from indoors to outdoors. Relative humidity is also low in the winter, which means the polyethylene-copolymer film will be closed and serve as a moisture barrier. As a result, almost no vapor can enter the construction during the winter. In the summer, vapor primarily flows from outdoors to indoors, and relative humidity is high both indoors and outdoors, which means the polyethylene-copolymer film will be open. Because the membrane is permeable, moisture can leave the construction and flow indoors.