In an average four-person household, between 10 and 12 liters of water vapor are produced every day. Materials and building materials that are able to absorb the moisture produced by cooking, showering and sweating, for example, and then slowly release it back into the room air are known as diffusion-open materials. Because they are open to vapor diffusion, they help to regulate the indoor climate. The performance of diffusion-open materials is measured by the so-called vapor diffusion resistance coefficient µ (mü). The lower the value, the easier it is for water vapor to penetrate the material. This is an advantage because they store moisture when the room air is too humid and release it again when the air in the room is too dry.
