The influence of surface treatment on mass transfer between air and building material

The processes of mass transfer between air and building structure and in the material influence not only the conditions within the material but also inside the connected air spaces. The material which absorbs and desorbs water vapour can be used to moderate the amplitude of indoor relative humidity and therefore to participate in the improvement of the indoor air quality and energy saving.

Many parameters influence water vapour exchange between indoor air and building material. The aim of this work is to present the change of mass transfer under different climatic and material conditions. The measurements were performed at the Technical University of Denmark (DTU), Department of Civil Engineering.

Two climatic chambers were used for the tests, the first one for dynamic and the second for steady state conditions. Two commonly used building materials exposed to the indoor environment were chosen for the experiments: gypsum board and calcium silicate. The wallpaper and paint were used as finishing materials.

Impact of the following parameters for changes of RH was studied: coating, temperature and air movement. The measurements showed that acryl paint (diffusion open) can significantly decrease mass uptake. It was shown also that higher air velocity speeds up the process of mass exchange between indoor air and materials but apparently decreases the total amount of exchanged water after a longer period.

The experiment allows not only to check the influence of surface treatment on mass transfer, but can be used also as validation for simulation programs. At the end of the article, a mass uptake calculation using the HUMIMUR model is presented.