Berechnung der Feuchte im geneigten Dach - Ansatz des 'wahren' sd-Wertes von Unterspannbahnen / Determination of Moisture Content in Pitched Roofs - Application of the 'Real' sd-value of Understating Sheets

Lately, diffusion open underslating sheets (USS) have been inserted to pitched roofs increasingly. These sheets allow moisture to escape from the construction while they remain waterproof The conventional procedures to determine the water vapour diffusion resistance of the USS result in high deviations of the test results depending on the test methods and the boundary conditions. For this reason, an ordinary, non-ventilated roof-system with insulated rafters was tested in consideration of the different s_d-values. The characteristic sd-values of underslating sheets based on experiments according to DIN 52165 were assumed to be constant and dependent on the humidity. The calculations prove that a serious danger to the roof-systems (almost by a factor of 10) is possible considering the chosen s_d- value of an USS. Thereupon recommendations for an experimental laboratory method of the 'real' s_d-value were made in order to be able to judge the roof-system in a realistic way.