Impact of indoor humidification on hygrothermal performance of building envelope in Northern Finland

Dry indoor air is known to increase the risk for respiratory infections, viability, transport of influenza virus and intensifies sensitivity of nasal systems. The aim of this study was to investigate the effect of humidification on the hygrothermal performance of the common timber wall structure of a day care building located in Northern Finland. Hygrothermal performance of structures under different indoor moisture loads was analysed. Numerical simulation and the Finnish mould growth model were used to evaluate the impact of humidification on moisture safety of the envelope. Largest impact of indoor air humidification is observed in the low humidity ranges on inner side of water vapour barrier. At the outer side of the water vapor barrier the indoor humidification has no effect on the hygrothermal conditions or mould growth risk. The most favourable conditions for mould growth are obtained at the corner of the building envelope, at the interface of load bearing timber and windshield where hygrothermal conditions are almost 60% of the time over 80% of relative humidity. However, the Finnish mould index does not indicate any mould growth risk. The study demonstrates that the indoor air can be humidified to RH of 35% during the cold dry period.