Measured moisture buffering and latent heat capacities in CLT test houses

This research investigates the significance of the moisture buffering and latent heat capacities in exposed cross-laminated timber (CLT) walls with the respect to indoor climate and energy consumption. Hygroscopic materials have the ability to accumulate and release moisture due to change in the surrounding humidity. The moisture buffer capacity is regarded as this ability to moderate, or buffer, the indoor humidity variations. Latent heat refers to the heat of sorption due to the phase change from vapour to bound water in the material and the other way around. The indoor relative humidity (RH) is closely related to indoor comfort, more specifically to thermal and respiratory comfort and perceived air quality. Both persistently low RH (<20 %) and high RH (>75 %) can cause health threats for humans such as respiratory infections or the growth of mould. Wood is naturally hygroscopic, which enables it to act passive and efficient to stabilize the indoor humidity and thus, temperature variations. A better understanding and more deliberate implementation of these properties could potentially reduce the need for ventilation and heating without compromising the indoor comfort.  A full-scale experimental study compares the responses of two 25 m2 test houses to an applied moisture load. The test houses are identical constructions made of CLT elements, where the exposed spruce interior in module A is kept as is, while the interior in module B is covered with PE foil emulating an impermeable surface. The moisture load is applied as continuous flow of mist under several various conditions, including different magnitudes of the load and altered air change rates of the ventilation fan. The responses are measured in terms of RH, moisture content in the wood, changes in surface and air temperature and the time needed for the system to restore itself to the initial state. The software WUFI®Plus is utilized to perform a hygrothermal building performance simulation for comparison and evaluation. Both the experimental and calculated results show that exposed wood is an efficient moisture buffer capable of reducing daily fluctuations in RH. The results also show a rise in surface temperature, which is a contribution from the latent heat of sorption.