Preliminary roofing underlayment films (FWK)

The main task of preliminary roofing underlayment films (FWK) is to protect the roof structure and thermal insulation from:

Condensation formed under roof coverings;

Effects of wind, such as lifted precipitation under roof coverings;

Minor leaks caused by various roof covering imperfections.

This type of roofing films is called vapor-permeable membranes because micro-funnels-shaped perforations are made in their surface to allow water vapor to pass through only in one direction.

Therefore, when laying them, it is important to pay attention to place them with the correct side up. The perforations are designed in such a way that the funnel is inverted with a wider end facing downwards (from where water vapor flows) and a narrowing towards the outer side of the film. Therefore, this type of FWK must be laid with the writing facing upwards.

IMPORTANT! All low-vapor-permeable preliminary roofing underlayment films require a space or ventilation gap separating them from the roof thermal insulation. Therefore, in sloping roofs with an attic, two ventilation gaps must function: the upper one between the roof covering and FWK, and the lower one between FWK and the roof thermal insulation.