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Indoor humidity: a challenge on many levels

Although the temperature often serves as a benchmark for indoor comfort, humidity plays an important role as well. It not only affects our comfort, but also our health and the condition of our buildings. That’s why it’s important to understand the challenges and effects of proper humidity management.
Indoor humidity

Indoor humidity levels

The recommended humidity levels inside a building are between 30 and 50%. This can be easily measured with a hygrometer or a thermometer that displays the relative humidity levels (i.e. the amount of water in the air relative to the temperature). Keeping the humidity within this range isn’t always that straightforward, but surely important to safeguard indoor air quality.

Low humidity

Since warm air holds more moisture than cold air, a room that isn’t heated properly can be at risk of low humidity. This can cause dry skin and more dust in the room, which in turn can aggravate allergies, asthma and other health conditions. When air is too dry, it can lead to lung and throat irritation, but also increase the risk of a viral infection.

High humidity

Although the effects of low humidity are certainly unpleasant, the risks associated with high humidity levels are more far-reaching. After all, high indoor air humidity causes an increase in the humidity of building materials, surface materials and structural materials. When these materials become moist, they are a breeding ground for mold, mildew and bacteria. Microbial growth can trigger or exacerbate allergies and cause illness.

“On top of that, mold growth and rot can also damage building structures. During the heating season, the humidity in the room air tends to transfer to the building envelope unless the vapor barrier is properly implemented. In this case, moisture condenses inside the structure and, in addition to microbial growth, causes a decrease in thermal insulation and an increase in heating energy consumption”, says Mikko Iivonen, Senior R&D Adviser at Purmo Group.

Understanding indoor humidity

To ensure a proper humidity management and avoid the risks associated with low and high humidity, it’s important to understand how moisture originates. Mikko Iivonen explains: “On the one hand, there is a small amount of moisture that enters the building from outside with ventilation air supply. The main source of moisture, however, is the residents and their housing functions. A typical family produces about 5 to 10 litres of humidity per day in the indoor air.”


The only way to reduce excessive indoor humidity is proper ventilation. In residential buildings, for example, ventilation ensures that clean outdoor air is brought into the rooms and stale, moist indoor air is removed mainly from bathrooms and kitchens. In modern window constructions with improved joint tightness or in rooms without windows, extra care should be taken to ventilate the rooms and possibly provide forced ventilation. A sufficient ventilation rate is considered to be 0,35 L/s per floor m2, which corresponds to an air change of the air volume of the dwelling once every two hours. Considering the importance of ventilation, it’s not surprising that a ventilation concept is obligatory for renovations and new buildings nowadays.

Heating systems

As much as the heating system controls the indoor temperature, indoor humidity levels also affect the heating system. Especially in wet room such as the bathroom and the kitchen, the use of steel radiators is only possible if the application possibilities and limits are observed as otherwise long-term indoor humidity (relative humidity > 60%) will trigger corrosion of the steel. In those rooms it’s better to choose heaters made of, for example, brass, copper-aluminium or stainless steel.

Moreover, it’s important to position the radiators outside the spray area, so not under a washbasin or within 1,5 metres of a shower or bathtub. If they are installed within the spray area and corrosion occurs, no warranty can be claimed. However, sometimes local conditions, such as limited space, require radiators to be installed within the spray area. In such cases special measures like galvanised surfaces, appropriate protective cladding, etc. must be taken. If you need more information on the possible measures to take, your Purmo sales representative will be glad to help you.