Heat and mass transfer within porous building materials

The thermal and structural performance of building elements can be significantly impaired by the presence of excess moisture. At present, designers have available only simplistic steady-state techniques to predict such effects, for example that presented by Glaser in 1959. These simple models recognise moisture transport in vapour form only and do not allow information on material moisture content to be obtained directly. They are also based on the assumption that the material transport properties are independent of the prevailing environmental conditions, whereas they are in fact complex functions of parameters such as relative humidity. This research has been carried out to develop a set of model equations which account for both liquid and vapour transfer through porous structures, and which enable material moisture content profiles to be produced. The equations generated in this work are transient and enable the effects of moisture and thermal capacity to be considered. An experimental investigation has also been carried out to produce a methodology which can be used to obtain the required material properties. These equations and material properties have been combined with realistic boundary conditions to produce a finite difference model which enables simple wall structures to be analysed in terms of temperature, vapour pressure, relative humidity, moisture content and moisture flow rate. The use of this FORTRAN 77 computer code is illustrated by application to traditional and timber-framed wall constructions. The results illustrate the applicability and flexibility of such an approach and confirm the importance of its further development in the future.