Moisture capacity of log houses can improve the indoor climate conditions

The hygroscopic capacity ot timber can signiticantly improve the indoor conditions in log houses. Relative humidity of indoor air is, along with air and surface temperatures, a key factor for thermal comfort and also for perceived indoor air quality. The ability of timber to store moisture during indoor load periods and to release it back to the indoor air during unoccupied periods makes it possible to smooth down the indoor relative humidity variations by passive, structural means. This paper presents the numerical simulations carried out to study the effect of this moisture buffering effect in log houses compared to houses without available hygroscopic material. The numerical simulations were done using a room space model that integrates the structures, indoor air, and the ventilation, heating and cooling systems. The model solves the indoor temperature and humidity values using these dynamically changing heat and moisture flows. The analysis was done for Northern climate conditions (Helsinki, Finland) and the indoor loads corresponded to a case with two persons sleeping in one room. The ventilation was set constant (air change rate 0.65 1/h) and the room occupation and load conditions were repeated every night over the one-year simulation period. The results showed that when the room had four log walls, the moisture transfer between indoor air and walls was significantly higher than the moisture transport caused by ventilation. In the case with four log walls, the yearly average relative indoor humidity during occupation was 42 % RH and the maximum 69 % RH, while in the case with non- hygroscopic structures the average indoor humidity value was 51% RH and the maximum 93 % RH. The maximum level for indoor comfort is typically 60 % RH. In a case with non- hygroscopic walls, the indoor humidity exceeded this limit value in about 20 % of the yearly occupation time, while in the log house it was about 10 % due to the moisture interaction of the walls. Also the lowest humidity levels during cold winter period were higher in the log house when compared to non-hygroscopic walls. Utilization of the moisture capacity of structures to smooth down the indoor humidity conditions offers an effective passive method to improve the indoor air conditions in an energy efficient and sustainable way. The effect can be easily applied in log houses that have high hygroscopic capacity. This paper presents the potentials and sensitivity analysis of this application.