Recommendation for Inside Air Humidity in Winter, in Homes with Ventilation Systems, Particularly Passive Homes

 Dr. Wolfgang Feist, Passivhaus Institut - December 2000
 

The relative humidity of indoor air is particularly dependent on:

  • Quantity of humidity from indoor sources (e.g. plants, cooking, clothes dryers, etc.).
  • Quantity of fresh air supplied from outside.


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    Water vapor from indoor sources of humidity is diluted by the incoming fresh air. The greater the volume of incoming fresh air, the lower the indoor relative humidity.

    This dilution effect is particularly pronounced in winter, because cold outdoor air contains very little humidity (e.g., there are only 3 g of water per m³ air at -5°C(23°F)/90% relative humidity). When this air is brought inside and warmed to 20°C (68°F), it's relative humidity is only 17.6%, before additional moisture is added by indoor sources. With "normal" household sources (330 g/h - varies) and "normal" ventilation (e.g., 120 m³/h (4238 ft³/h), from German Industry Standard "DIN 1946") this example would result in a relative humidity of 33.5%. In general, this value will be comfortable as long as the air is reasonably clean (free of dust).

    In cases where ventilation is within standards, but occupants still perceive the air as too dry, decreasing the fresh air supply volume is an easy fix. The decrease in volume of fresh air causes the humidity to increase, because moisture from interior sources is diluted less. If the fresh air supply in the above example is decreased to 75 m³/h (2649 ft³/h), which is well within acceptable limits for acceptable air quality, the indoor humidity will increase to 44%. In the interest of maximum energy savings, the rate of ventilation with fresh air should not be higher than needed to achieve a comfortable indoor humidity.

    Conventional design tends toward higher rates of ventilation. In the past, air change rates as high as 0.5 or even 0.8 were considered necessary to keep interior humidity in winter low enough to discourage the development of condensation, which can cause mildew and damage to building components. But this risk does not exist in a passive house. Outside building elements are so well insulated that interior surfaces are too warm for condensation to occur even at 60% relative humidity; and moisture barriers and air seals prevent moist inside air from reaching building elements that could cool it to below the dew point. Therefore, fresh air volume can be lower, particularly when occupants perceive the humidity as being too low. "Appropriate" air change rates for residences are between 0.3 and 0.4. For passive houses, we generally recommend leaning toward the lower rate. This keeps the indoor air quality good, while maintaining a comfortable humidity and maximizing energy savings.

    Summarizing solutions to too low indoor air humidity:

    1. Decrease the air change rate.
    2. Consider adding sources of moisture (e.g., more plants).
    3. Keep the home as free of dust as possible: clean often with a good vacuum cleaner having a fine dust filter and make sure the ventilation air filtration (if any) is working properly.
    Incidentally, air that is practically dust-free does not feel "too dry" even if it contains very little moisture; people feel very comfortable in cold air at high elevations (where there is very little dust). Since the air in a residence cannot be kept free of dust with reasonable effort, there is a practical lower limit to relative humidity (about 30%) below which most occupants consider air too dry. When that point is reached, solutions 1. and/or 2. above should be used.