Water Vapour Transfer Properties of Spunbonded Plastic Films

The water vapour permeances of two types of spunbonded plastic roofunderlay films are measured using different methods. Experiments with isother mal permeance cup procedures are discussed. Because of the high permeance of the tested films, the analysis of these measurements is not as simple as described in most standards. To obtain reliable results of the cup tests, the influence of the water vapour diffusion resistance of the air films is separately investigated. A non-isothermal per meance test was developed to measure the effect of surface condensation on the va pour permeance of the films. The paper describes the apparatus and discusses the re liability of the method. The properties measured by the non-isothermal test method are compared to those obtained with the isothermal method.