Ventilation and Humidity in Cold Air in Passive Houses

Ventilation and Humidity in Cold Air in Passive Houses

Only a small amount of moisture is contained in cold air (in the example: 3 g / m³) and this is already 90% of the moisture that air can absorb at -5°C (saturation humidity at -5°C). Warmer air can absorb significantly more water vapour, e.g. up to 17.3 g / m³ at 20°C

If cold air is brought into a room by means of ventilation and heated to 20 ° C, it contains 3 g of water per m³ with a relative humidity of only 17.6%.

The higher the quantity of external air supplied, the lower the relative air humidity in the interior space.

In normal ventilation with 120m³/h, the moisture released in the house is diluted to the point that the outside air conditions give a relative humidity of about 33% in the interior. As a rule, this is acceptable. (Air volume per person 30 m³ /h according to DIN 1946, air changes based on the volume of air about 0.37 h ¹)

With increased ventilation (240m³/h), the moisture released into the house is increasingly diluted. Now there is only a relative humidity of approximately 27%. This is generally considered to be too dry. An external air quantity of 60m³/h per person is thus too much in regard to the moisture balance. (Air changes based on the volume of air about 0.75 h ^-1)

If the air humidity is to be kept higher in the winter, the simplest way is to reduce the fresh air quantity and thus reduce the humidity of the air within the room. An external air quantity of 80m³/h corresponding to 20m³/h per person is still sufficient for acceptable indoor air hygiene (see measurements in various passive houses). With such an outside air volume, the humidity in the house is about 41%. (The air exchange is then only relative to about 0.25h^-1 of the whole house); the building should be supplied permanently with fresh air, the air quality is sufficiently less than about 0.25 h^-1. air exchange will not be achieved unless the apartment is uninhabited and the apartment is pre-washed with fresh air before the next "use" (this is an expense which we do not recommend for apartments). These recommendations result from experiences with long-term measurements of air quality in the passive house Darmstadt Kranichstein.

The relative air humidity in indoor rooms depends on two influencing variables:

• The intensity of the internal moisture sources (eg plants, cooking, drying etc)
• Of the quantity of fresh air supplied from outside.

The water vapour from the internal moisture sources is diluted by the fresh air. It does not matter whether the outside air is brought in through joints, through windows or through a ventilation system (at least as long as no humidification or dehumidification takes place). And it also does not matter whether the air is heated (eg by a heat recovery) - the mass of the water contained in the airflow does not change in all these processes.

This effect is especially strong in winter; Cold air only contains very little water vapour (at -5 ° / 90% e.g. only about 3 g per cubic metre of air). If this air is brought into the room, where it is ultimately heated to 20 °, its relative humidity is only 17.6% - as long as no further water is supplied to the room air from internal sources. In the case of household moisture sources (330g/h - individually different) and "standard" ventilation (in the example 120m³/h - here DIN 1946), a relative internal air humidity of 33.5% results under these conditions. This is generally a more comfortable value when the air is clean (little dust).

If the ambient air humidity appears to be too low for the residents, the relative humidity of the indoor air increases because the water vapour sources are less diluted. If, in the above example, the outside air volume is reduced to 80m³/h in the winter - which is still permissible in any case and still gives a sufficiently high room air quality - the air humidity rises to approx. 41%.

Conventional ventilation planners tend to give a high dimension in the air volume of residential ventilation systems; There were times when a 0.5-fold or even 0.8-fold air change was considered necessary - just to keep the room humidity low in winter; Then the risk of condensation formation is associated with the associated risk of mold fungus. However, these two dangers do not exist in the passive house because of the excellent thermal envelope, all the interior surfaces of exterior components are warm so that no condensation is formed even at 60% room humidity. Therefore, the outside air volume in the passive house can be lowered in winter, especially if the ambient air humidity is perceived as too low by the occupants. For passive houses, we generally recommend that the air volumes be oriented towards these lower values. Then the air humidity remains in the comfortable zone with good air quality.

Remedy for low ambient air humidity:

• Adjust air volumes lower
• Possibly additional sources of moisture in the living areas (eg plants)
• Keep the apartments dust-free as far as possible: clean with a vacuum cleaner with a fine dust filter.

Virtually dust-free air is not perceived as "too dry" even in very humid conditions; However, air in dwellings can not be kept dust-free at a reasonable cost. Therefore, there is actually a lower limit for the relative air humidity (at approx. 30%) below which most users feel the air is too dry. Then only the points 1 and 2 help as described above.