Water vapor resistance
According to DIN EN 343, water vapor transmission resistance R et is the “water vapor partial pressure difference between the two surfaces of a material divided by the resulting evaporation heat flow per unit area in the direction of the gradient . The evaporation heat flux can consist of diffusion as well as convection components. The water vapor transmission resistance R et , expressed in square meters times Pascal per watt , is a specific quantity for textile materials or material combinations, which determines the "latent" heat of evaporation through a given area in response to a constantly acting water vapor pressure gradient. "
In common parlance, the term “ breathability ” is also used. The lower the R et , the more breathable a material is.
In contrast to the water vapor transmission resistance , the water vapor diffusion resistance is used to evaluate the water vapor transport through materials that are (approximately) airtight and therefore do not allow a convection flow. The water vapor diffusion resistance is relevant, for example, in buildings with an airtight building envelope and with porous materials that hardly allow convection due to the material thickness.