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
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
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
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.
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.
link leads to the main site of the Introduction to Passive Houses.
changed: 6.05.2007 / author Dr. Wolfgang Feist; thanks
to David Stecher for proof reading of the 1st edition
Passive House Institute;
unchanged copy is permitted, please give reference to this page)