The amount of the relative humidity in the interior air of a building depends on the following critical variables:
- the intensity of the internal sources of moisture (e.g. flowers, cooking, drying...),
- the quantity of fresh air supplied.
The water vapour generated by the internal sources is diluted by the supply of fresh air. It doesn't matter whether outside air is brought in through cracks, windows or by a ventilation system. Nor does it matter if air is warmed up (e.g. by a heat recovery) - the amount of water contained in the air flow is unchanged by these processes.
This dilution effect is particularly strong in the winter; cold outside air contains very little water vapor (with -5°C/90% e.g. only about 3 g per cubic meter of air, see first diagram at left). If this air is brought inside, it is eventually warmed up to 20 °C and then its relative humidity amounts to only 17.6% - as long as no additional water from internal sources of is supplied to the room air. In the case of typical household moisture sources (330 g/h - varies individually) and a "standard" ventilation air volume (in the example 120 m³/h - as per DIN 1946), then the result is a relative interior air humidity of 33.5%. This level of humidity is usually comfortable if the air is fairly clean.
If during standard ventilation the air humidity appears too low to the inhabitants, there is an easy remedy: By decreasing the outside air flow rates, the relative interior air humidity rises, because the sources of water vapour are diluted less. So the occupants may just reduce the outside air quantity in the above wintertime example to on 80 m³/h - which is still permissible for sufficiently good indoor air quality - as a result, the relative humidity of the indoor air rises to approximately 41%.
Nobody should supply more fresh air than is necessary for good indoor air qualty, because it affects the comfort of the occupants by changing the humidity. Conventional ventilation planners are inclined to specify rather high ventilation amounts for dwellings. There were times when 0.5-ach (or even 0, 8-ach) was specified, precisely to keep the indoor humidity levels low during the winter in order to reduce the risk of condensation and mold. These two dangers do not exist in the...