Indoor relative humidity: relevance for health, comfort, and choice of ventilation system

Abstract

It is generally recommended to keep the values of relative humidity (RH) indoors between 40 and 60% for comfort and health. However, the environment in our homes and offices is a lot dryer in the winter, going down to 20% or less in cold climates. We can be in such dry environments for short periods, but in the long term we might get physiological impairments such as dry or irritated eyes, nose, throat and skin, and even an increase on headaches and respiratory or skin diseases and allergies. On the other hand, too high values of relative humidity can promote growth of fungi and mites, and create moisture problems in building materials. Those problems could be solved by finding a balance between ventilation rate and indoor humidity production, in combination with moisture absorbing materials. However, these strategies work better with lower air speed and ventilation rate, which may in turn conflict with the need for fresh air to compensate for the production of CO2 and other pollutants. Typically, mechanically ventilated spaces tend to have a lower RH than those with natural ventilation, independently of the season and indoor temperature, since their main focus is providing enough fresh air to keep CO2 levels below 1000ppm. Recently, it has gained acceptance to monitor temperature, humidity and CO2 for indoor air quality and health, which has the potential to show their interactions and help find an optimal balance between them. We carried a building performance simulation (BPS)analysis of an office building with an optimized design for passive strategies and automation in cold climate. Instead of focusing on high air changes, this building uses extra high floors for stratification of temperature and pollutants, to reduce the need for ventilation in winter. Then we compared indoor RH under natural and mechanical ventilation, to reflect on the effect of the ventilation system.