Published: Feb 2016 | Journal of Environmental Protection, 7, 855-873

A comparison of relative humidity between two Swedish buildings with different ventilation solutions

Alsmo T, Alsmo C


The purpose of this study was to investigate how the relative humidity differed between two buildings, one with natural ventilation and the other with mechanical ventilation.

An office building in central Sweden, mechanically ventilated, where occupants were complaining for many years about dryness and bad indoor air quality, was compared with a naturally ventilated similar building.

The climate measurements included indoor and outdoor RH and temperature in the period from October 2014 to September 2015.


The measurement results indoors show that there is generally lower relative humidity (%RH), in the mechanically ventilated building than in the building with natural ventilation (see fig. 1).

Relative humidity below 40% is an indication that health problems can affect people. The humidity drops below this level regularly from the beginning of October, in the building with the mechanically ventilated system, and is constantly below this level until the middle of June. That is, this negative situation for the indoor environment continues for seven months. There after the level is kept more frequently above 40%.

For the building with the natural ventilation system the RH level below 40% is passed in mid-November and being re-stabilized above 40% in May. That is, this situation is going on for five months.


The mechanically ventilated system creates, from a health perspective, a very low relative humidity, and this fact must be taken into consideration for the existing systems as well as the planning of new ventilation systems.

It is necessary in view of the Swedish climate conditions that henceforth, consider the relative humidity in calculations and installation of ventilation systems.

An important national (Swedish) issue to come to terms with the dry indoor air is the authority requirement for ventilation flows. This requirement is based on maintaining a carbon dioxide level that only rarely exceeds 1000 ppm, which governs the ventilation design. This situation dilutes the negative effect on the relative humidity and further dries out the indoor air. This requirement must be questioned as it has been shown that it is based on inadequately substantiated materials from the authorities and where the corresponding level in the EU is 5000 ppm.

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Doc's view...

by Walter Hugentobler

This study illustrates that ventilation alone is by no means sufficient to control humidity in a building. Both naturally ventilated and mechanically ventilated buildings suffered humidity levels below 40%RH for many months of the year.

The author cites a reason for low indoor humidity levels in Sweden is a government directive to lower indoor carbon dioxide levels and the over-ventilation that occurs as a result.

However, the real reason for low indoor humidity is the lack of pro-active humidification of the indoor environment. Without adding moisture to the indoor air with a humidifier, neither building will ever achieve a healthy humidity level of between 40-60%RH, consistently throughout the year.

Rather than loosen regulations on carbon dioxide levels to move indoor conditions closer to the ideal humidity level, but still not achieve them, legislation should be introduced to maintain humidity levels alongside controlling carbon dioxide. Both are easily achievable with the correct air conditioning strategy.

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