Hygric properties of porous building materials (VIII): Influence of reduced air pressure

Abstract

Hygrothermal properties of porous building materials are indispensable for understanding and optimizing the hygrothermal performance of building enclosures and the built environment. In high-altitude areas, the influence of reduced air pressure on hygrothermal properties should be considered and, if needed, proper corrections should be adopted. Currently, the impact of reduced air pressure on thermal properties has been thoroughly investigated while that on hygric properties remains unclear. In this research, hygric properties of three representative porous building materials – calcium silicate, autoclaved aerated concrete and ceramic brick – are measured at three different air pressures, namely 1.00, 0.80 and 0.60 atm. The static gravimetric test, psychrometer test, cup test and capillary absorption test are performed to obtain the sorption isotherm, moisture retention curve, vapor permeability, capillary absorption coefficient and capillary moisture content. Results show that the influences of air pressure on the sorption isotherm, moisture retention curve, capillary absorption coefficient and capillary moisture content are limited, while the vapor permeability increases obviously with the decreasing air pressure. Specifically, the vapor diffusion resistance factor remains stable in the dry cup test, and this can be predicted by Schirmer's law. In the intermediate cup and wet cup tests, where liquid water contributes to the moisture transfer, the impact from air pressure becomes smaller.