Underlayments, also called roof foil, roof membrane or sarking membrane, are a modern product used mainly in roof construction. They form the layer between the roofing and the insulation layer of the roof. In this regard, especially 2 points are very important. Especially in the shell construction phase, these foils must keep the underlying construction dry. This means that such foils must be rainproof, and at least tested for resistance to water penetration of class W1. This class serves its purpose in normal weather conditions. In addition, since roofs are already insulated but not covered for a long period of time, a certain UV resistance of this membrane is absolutely necessary. If the roof tiles are laid later, this point is no longer so important, because the film is no longer directly exposed to UV radiation.
The second point is the permeability of water vapor, it is also called diffusion openness. The underlay is on the outside, so any water vapor must be able to escape from the inside to the outside. The so-called Sd value serves as a measure of this permeability. The lower this value, the more permeable the film is to water vapor.
Even after the shell construction phase, the roof foils are an important protection against water from the outside. Be it from rain that is pushed under the roofing with the wind, or be it when individual tiles break and there is a hole in the roofing until they are replaced. Standing water can also occur when the snow melts, if the ice and snow on the roof do not thaw evenly. Here, too, the underlayment performs its important service of keeping the roof watertight and preventing entry into the insulation layer.