The outdoor air in the example contains 3 g water for each m³. When the air is cold (-5°C) it is at 90% relative humidity. If that same air is warmed up to 20°C, it will be at only 17.6% relative humididty. This is because warm air can hold more water vapor (at 20 °C up to 17,3 g/m³)

The larger the volume of fresh air brought in from the outside, the lower the relative humidity in the house.

In the case of "normal ventilation" with 120 m³/h the humidity generated in the house is diluted with dry outside air resulting in a relative indoor humidity of of 33%. Which is usually acceptable. (amount of air for each person is 30 m³/h in accordance with DIN 1946, volumetric airchange of about 0.37 h^-1)

For the "increased ventilation" case with 240 m³/h the humidity generated in the house is more diluted. The relative humidity is reduced to 27%. This is felt to be too dry for comfort. An fresh air quantity of 60 m³/h for each person thus upsets the moisture balance "too much of a good thing". (volumetric air change of about 0.75 h^-1)

If one wants to keep the humidity higher in the winter time, the simplest way is to reduce the fresh air quantities, which reduces the dilution of humidity in the room air. An outside air quantity of 80 m³/h (based on to 20 m³/h for each person) is still enough for acceptable interior air quality according to experience (see measurements in different Passive Houses). With this outside air quantity, the relative humidity in the house becomes 41%. (the air change rate is about 0.25 h^-1 based on the whole house volume.


However, the indoor air quality is also sufficient. The volumetric air change rate should not drop below 0.25 h^-1. As many years of measurement of the air quality in the passive house Darmstadt Kranichstein has shown.

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 Passive House, because of the good thermal protection for all external construction components interior surfaces are so warm that even with 60% room air humidity, there is still no condensation. Therefore the fresh air quantity can be reduced somewhat in the passive house during the winter, in particular if the room air humidity is felt by the inhabitants to be too low. "reference values" for dwellings are house air changes performed by the ventilation system of between 0.3 and 0.4-ach. For Passive Houses we generally give the recommendation to specify the amount of air to the lower of the two values. Then the room air humidity remains comfortable, along with good air quality.

Remedies for indoor humidity levels:

• Reduce the amount of fresh air
• Add moisture sources in the dwellings (e.g. plants)
• Keep the interior as dust free as possible: vaccum frequently with using a vaccum with a good dust filter.

By the way: Practically dust free air even with low moisture level is not felt to be "too dry"; in the cold mountain air we feel fine. However, air in dwellings, given reasonable actions, cannot be kept dust free . Therefore there exists a lower boundary for relative humidity (with approx.. 30%). Below this, most people find the air to be too dry, so refer to points 1. and 2. as described above to solve the problem.

If maintaining comfortable levels of indoor humidity will require reducing ventilation air volumes below those necessary for good indoor air qualty, then a mechanical humidifier can be used instead, but this must be cleaned and refilled regularly.

At the Passive house conference in working group II, Ruedi Krisi presented Enthalpy recovery ventilators, which recover humidity in addion to heat. Also at the current passive house conference new results of the ventilation research are expected.

This link leads to the main site of the Introduction to Passive Houses.

(last changed: 6.05.2007 / author Dr. Wolfgang Feist; thanks to David Stecher for proof reading of the 1st edition
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