Separation of moisture flow through porous building materials into vapour and liquid components

Abstract

The prediction of the effects of moisture in buildings is now a well-established part of the building design process. A significant international effort is taking place to develop combined heat and mass transfer models which will enable the thermal and hygric performance of building envelopes to be more accurately assessed. However, the universal solution of the defining equations requires the separation of the total moisture flow into its liquid and vapour components for any specified set of humidity conditions. Until now this has not been possible and partial model solutions have required the premise that liquid water flow in porous building materials only becomes significant at humidities approaching 100%. Such an assumption is simplistic and clearly at variance with the body of experimental evidence. This paper presents an analytical and experimental procedure which allows the transport parameters defining liquid and vapour flow to be described for any given material as functions of relative humidity. Such information allows for a complete model solution without recourse to questionable assumptions regarding the initiation of capillary condensation.