Today an important indicator of insufficient air renewal is the occurrence of moisture
damage and mold growth on the inner surfaces of external components. There are
certainly no hygienic conditions in a habitable room if molds grow there over longer
periods on larger areas of the building structure. There can be numerous causes for
this (such as defective water, rainwater or sewage systems, rising moisture or water
damage that has not been inhibited). An important and unfortunately very common
cause today is the growth of mold on the inner surfaces of cold exterior components,
where the water activity values are too high due to excessive indoor air humidity.
Chapter 2 will deal with the ventilation requirements that arise from this question.
There is a view from various sides that ensuring the room humidity is sufficiently low is
the only remaining criterion for ventilation: This must be explicitly contradicted here.
The preliminary contributions have shown that there are further health-related indoor
air pollutants that need to be reduced through sufficient, proper home ventilation and
that secondly the elimination of odors which lead to a subjective perception of poor air
quality, are among the tasks of home ventilation. As we will see later, it cannot be
assumed that the further indoor air pollution will "automatically" be sufficiently diluted
if an adequate air change is guaranteed only for the sake of moisture removal. We
must therefore also deal with the other air pollutants as well as the smell perception
and deal with its consequences for adequate room ventilation. We will consider such
contaminations there experience has shown these are inevitably associated with living:
Formaldehyde HCHO: The emissions of HCHO from building materials have
been greatly reduced after the initial discussion. However, HCHO is also emitted
from furniture and furnishings. Reducing emissions to zero is not practical.
Usually, due to the still permissible emissions from building materials as well as
the release of HCHO from furniture and furnishings, the ventilation of the home
must be maintained on an acceptable level.
Volatile Organic Compounds (VOC): For this purpose, a target value was
formulated that should be undercut in dwellings. Today VOCs mainly come from
furnishings and household chemicals, the use of which cannot be fully controlled
by the user. Adequate basic ventilation, which must be able to dilute the usual
VOC emissions in the living space to an acceptable level (that is, if possible
below the target value), is therefore essential.
Radon (Rn): According to current knowledge, there is no threshold for the car-
cinogenic effects of radon; a linear dose-response relationship must therefore be
From the 2020 perspective we want to explicitly add a point “not allowing pollutants in the external air to enter
the occupied rooms”. In this paper, the cases for radon and fungal spores are explained. Since then, we have
learned, that in certain circumstances (eg heavy traffic outside) also particulate matter (fine dust < PM 2.5) can
be an issue. This also is well handled by a passive house ventilation system using an upfront particulate filter,
recommended is the quality ePM1(80%).
assumed. However, it is not possible to reduce the Rn activity in the room air to
"zero" because the ambient air also contains measurable radon concentrations;
non-avoidable sources in the room also increase indoor air activity. Ventilation
has the task of reducing the existing health hazard as far as reasonably prac-
ticable. We can be satisfied with the reduction when the still measurable activity
is significantly below the average of the activity in ordinary houses of the region.
Air exchange rates that lead to other disadvantages that also endanger health
must be considered "no longer reasonably achievable": this is certain to be the
case in cold periods if relative humidity is below 25%. Too low indoor air humidity
is problematic, because dry air hinders the ability of the respiratory tract in its
In addition, well-being disorders caused by unpleasant smells must be taken seriously.
Under no circumstances can we be satisfied with a room air condition that does not
have an indoor air pollution better than the guideline values, but which is felt by a large
number of residents as "bad air". This is not only a question of comfort, as is sometimes
expressed: the subjective feeling of "bad air" is rather a warning signal of human
perception. Some volatile substances, which are now classified as highly toxic, are
chronically hazardous to health, even in extremely low concentrations - even if these
are not necessarily coupled with an odor perception, this must be seen as an invitation
to take adequate precautions. Even today we cannot rule out with certainty that future
human toxicological findings for other substances will prove a chronically harmful
effect. Diluting odors to a level that the vast majority of residents consider acceptable
is therefore not just a matter of comfort, but a necessity for a preventive health policy.
Even if you don't want to follow these arguments, you have to keep in mind that
unpleasant odor nuisances always lead to a loss of quality of life and productivity, and
thus an economically quantifiable loss. The improvement in the perceived air quality is
therefore a value that justifies additional investments.
2 Moisture damage
According to [Senkpiel 1999], the germination and growth of mold is not tied to the
presence of liquid water films, but can already be used if there is sufficient capillary-
bound water; and this may already be the case if the relative humidity near the surface
is over 80%.
Because the causes of such mold growth are not immediately recognizable by the
residents, but also by many experts, appropriate precaution is particularly necessary
here: A leaked gutter or rising damp is also easy to understand by the layperson.
Remark 2020: This has been confirmed by many studies. The potential conflict of diluting indoor air pollutants
against reducing relative humidity (RH) below a no longer acceptable limit (now seen to be 30%) was
considered in the PHD theses of Gabriel Rojas [Rojas 2015]. It is noted that the occurrence of an RH below 25%
may be more prevalent in non-domestic buildings where the population density tends to be higher. In addition, it
is understood that a low RH can: 1) Supress the immune system 2) Create conditions that are favourable to
However, mold growth caused by excessive room humidity (not perceptible), for
example in a corner of the room or behind a cupboard, regularly leads to wild
speculations about the cause, which unfortunately cannot be immediately recognized
without knowledge of the underlying building physics. The Internet, magazines and
newspapers are full of such completely wrong analysis:
"The wall 'sweats' because ..."
Not correct! Here an analogy is drawn to "sweating" in a person, where the
wrong name "sweating walls" comes from. These terms should never be used by
professionals in this context, not even in popular science articles, because they
create completely wrong associations. The water that occurs (on the surface or in
the capillaries of the material) does not come from the component, but comes
from the room air. The cause is not too high temperature ('heat build-up', 'over-
insulated') or a thick coat of paint ('nylon shirt', 'plastic film') but on the contrary: a
too cold inner surface, which the too moist inner air can reach unhindered (see
"The wall is too thickly coated (... too tightly insulated, etc.), which means that the
moisture cannot be removed."
Not correct! The only correct thing is that too little moisture is actually removed
from the room in question. However, a sufficient moisture drainage is not
possible through the wall construction, even if the wall consisted exclusively of
very diffusion-permeable materials. A quantitatively sufficient moisture removal
can only be achieved by a sufficiently high air exchange (or by a technical
dehumidification). Damage to the interior wall surfaces can also be seen on
exterior walls that are completely open to diffusion - only if the surface
temperature is low enough to fall below the value for a critical humidity of 80%.
"Harmful insulation breaks the houses ..."
Not correct! On the contrary, properly installed thermal insulation can prevent
such moisture damage from the outset and even fix it permanently by retrofitting
it: because thermal insulation leads to an increase in internal surface temperatu-
res, the relative humidity on the surfaces decreases. If the insulation is good
enough and thermal bridges are avoided, conditions for mold growth do not exist
anywhere on the inner surfaces of the outer envelope, as long as the relative
humidity of the room air is not excessively high. To do this, however, the
insulation really has to be very good and the conditions for a very extensive
reduction of the thermal bridges have to be met. We will see below that this is
regularly achieved in the level of passive house insulation.
The real cause will easily be understood also by a layperson:
Indoor air also contains (gaseous) water vapor. This is evaporated when exhaling,
bathing, washing, showering, from drying towels, drying dishes, flower vases and
flower pots. Because of these indoor sources, indoor air always contains more vapor
(per unit mass of air) than outdoor air. Air cannot contain a high amount of water vapor.
The maximum amount of water that can be contained is called the saturation humidity.
The higher the temperature of the air, the more moisture can be contained, the higher
the saturation humidity. If the temperature drops and the amount of water vapor
exceeds the saturation moisture, the air "can no longer hold the water": it falls out as
liquid water: either on a cold surface (then this is called "condensation") or in the entire
air volume as a fog
These processes are also familiar to the layperson: if you take a cold bottle out of the
refrigerator, the ambient air on the surface is cooled to temperatures of 5 to 10 °C.
Even with customary room air humidity, this is usually sufficient to form condensation
on the surface of the bottle. The same applies to cold glass surfaces of a window
(especially at the edge of the window) or to the windscreen of a car, which is cooled at
night by radiation to the cold sky. Even in nature you can watch condensation on
meadows and leaves after cool nights.
The same physical process leads to the formation of condensation on cold inner
surfaces of components. The condensate provides the basis for germination and
growth for the mold spores that are present almost everywhere. According to the latest
findings, many fungi are not even dependent on visible condensation: in the capillaries,
fine cavities in the material, the so-called 'capillary condensation' sets in much earlier.
A more or less thick water film forms on the surfaces of the pores: some molds are
able to use this capillary-bound water for germination and growth. Studies in recent
years show that the risk of the occurrence of mold growth starts on the wall surface at
a relative humidity of about 80% - in addition to the moisture content, the temperature
plays a certain role [Sedlbauer 2002]. However, you can be sure that mold growth will
no longer occur on the surface at relative humidity levels below 80%.
Internal surfaces become cold when there is a high loss of heat at the point in question
or when the supply of heat from the room to the surface is impeded. In the latter case,
however, damage can only occur if the room air, despite the impediment to the supply
of heat, has access to the surface, so that the moisture in the air is still transported to
the surface. For example, this is the case:
Behind a closet standing on an outside wall. The supply of heat to the wall is
impeded here (the surface cools down), but there is still an unimpeded vapor
pressure balance between the air in the room and the air layer behind the
cabinet. The absolute moisture content of the air layer behind the cabinet is
(almost) the same as that in the room air.
Behind an interior insulation that is not contiguous and airtight: The interior
insulation reduces the heat flow to the original interior surface of the wall. As a
result, their temperature drops. However, if air can still get out of the room behind
the insulation (joints of 1 mm width are sufficient for this), condensation will
inevitably form behind the insulation. All interior insulation must therefore be
carefully sealed airtight on the room side (also at the connections to the side and
to the floor and ceiling), and this cover must not be open to diffusion, rather it
should at least have vapor-retarding properties.
examples show that under these conditions, interior damage can not only be
avoided, but can even be remedied.
Inner surfaces of external components that are internally shielded against heat from
the room can nevertheless become so cold that condensation or surface moisture >
80% occurs there. This is always the case when the heat loss to the environment is
very large: heat is then dissipated through the component so strongly to the outside,
that, because of the limitation of heat flow from the inside by the heat transfer
coefficient, the temperature cannot be raised far enough:
Obviously, this is the case for very poorly insulated components: on single-pane
glazing or on unseparated metal windows, “the water is pouring down”.
Also well known is the formation of condensation at the edge of a glazing, which
is due to the high heat loss of the thermal bridge due to the metallic spacer of the
The moisture-related damage at thermal bridges in the outer shell is already less
obvious, but easy to understand. There is an increased heat loss due to the ther-
mal bridge (e.g. on a lintel made of concrete or on an outer edge). The tempera-
tures of the inner surfaces are reduced at these points; this is shown, for example,
by a thermographic image in good agreement with the calculation. If you consider
the damage caused by moisture, it is regularly concentrated at those parts of the
surface that have the lowest temperatures due to the thermal bridge: the damage
pattern is strikingly similar to the surface temperature picture of thermography.
After the causal relationships have been correctly identified, conditions can now also
be specified under which moisture damage of the type described here can no longer
occur - of course, these measures do not help against other causes, such as a
Condition for interior surfaces without moisture damage
A sufficient condition to reliably prevent moisture damage to the inner surfaces of
external components is met if the relative humidity of the air directly on the surface is
not higher than 80% for longer periods (> 72 h). The relative humidity of the air on the
surface is in equilibrium with the so-called "water activity aw" of the surface material.
The condition can be determined by the inequality:
aw < 80% [no damage].
The condition [without damage] can be met in practice in different ways:
Remark in 2020: From later work [AKKP 32] we learned, that an alternative approach to reducing the diffusion
is the application of capillary active insulation materials. Also in these cases the internal layer has to be airtight.
1. By lowering the relative humidity in the space by, for example, reducing the source
strength of evaporation (no laundry drying in space), dehumidification, or simply the
dilution through increased air exchange with the dry external air. Due to a very high
air exchange, the room air humidity can always be reduced to almost outside air
level; then even with extreme thermal bridges there will be no condensation, since
the inner surface temperatures will normally not get smaller than the outside air
temperature (not in a heated room). By very high air exchange moisture damage
can be completely avoided. By using heat recovery, the energy losses for that could
be avoided. But even then, very high air exchange rates are undesirable because
they can lead to very dry indoor air (which is unhealthy in the other extreme) - which
in turn would be necessary to avoid damage in the case of very poorly insulated
components or strong thermal bridges.
2. By raising the temperature of the inner surface of the outer component. This can
for example by "heating" (e.g. electric surface heater or radiation heat from a
radiator) take place, whereby additional heat losses, or by an improved thermal
insulation of the component concerned - so the heat losses will be reduced,
whereby the surfaces in the sequence get increased temperatures. With this
second method, the problem can always be solved in principle: with extremely good
insulation, the inside surface temperature is practically no longer different from the
room air temperature. Then no condensation can occur because the relative humi-
dity on the surface is not higher than that in the room. There are only limits to the
practicality of this approach, since the required meter-thick insulation layers may
not appear to be practical – as well as any glazing with 5, 6 or even more panes.
In practice, a solution for hygienic conditions on the outer surfaces will require a
combination of approaches 1 and 2. In the following, this is explained by means of
practice-relevant examples: The required level of air exchange will be higher, the worse
the insulation quality of the building envelope in the endangered areas. The following
method of presentation is chosen methodically:
A practical structural condition (thermal insulation level, window quality) is
The occurring minimum surface temperatures in the endangered areas with the
critical boundary conditions (outside temperature -5 °C, inside temperature 20 °C for
Central European Climate) are determined for the respective selected condition.
From the minimum surface temperatures, the permissible relative humidity max of
the room air is determined, which definitely excludes a damaging effect (i.e. a
value less than 80% for building materials with capillaries, condensation-free for
diffusion-proof surfaces such as glass or metal).
A permissible relative humidity max can be determined for each structural condition,
below which moisture damage due to indoor air can no longer occur. The task of
domestic ventilation, of whatever type, is then to ensure relative humidity below the
value max. How high the fresh air supply must be depends on the source strength of
the moisture sources in the room. As these are subject to very high variability, a general
statement on the required fresh air requirement cannot be made. The specification of
the value max is independent of this uncertainty and is therefore preferable. However,
we can, for example, make a statement about the average fresh air requirement
required for average moisture source strengths found in living rooms.
Fig. 1 shows the situation in a "modernized" old building, as is often the case: New
windows were installed in the position of the old windows, but otherwise no insulation
added, especially not on the external wall. Under the critical boundary conditions ( e =
-5 °C; i = 20 °C) there are minimal surface temperatures in the critical areas of the
apartment down to 9 °C, even when there is no furniture in front of the outer walls.
Behind a cabinet in an outer edge, the temperatures can even be below 5 °C. If we
assume that no furniture is placed on the outer walls, the relative air humidity in the
room must be permanently less than 38% in order to reliably prevent mold damage
caused by air humidity. Such low indoor air humidity can only be achieved by
noticeably higher fresh air rates than are usually given today.
Fig. 1: Construction status "old building with new windows". Relevant areas of the inner
surfaces of external components regularly have temperatures below 9 ° C. To avoid damage,
the relative humidity of the room air must therefore be kept permanently below 38%.
In order to be able to correctly classify the expressed requirement, measured relative
room air humidity in an old building bedroom is documented in Fig. 2: As you can easily
see, the humidity levels are regularly above 60%, even reaching almost 70% at the
top. In accordance with the recommendation, cross ventilation with open windows is
carried out twice a day in the room concerned. The drops in relative humidity at these
ventilation times can be seen very well over the course of the process. It is interesting
that after the ventilation process, the air humidity quickly returns to a level that is
measurably below the initial level, but which is still far too high to meet the "without
damage" condition. The reason for the rise is the release of moisture from building and
furnishing materials in which water is bound by capillaries. If the average total air
change is sufficiently high, the moisture can also be vented from the building materials
over time - but this requires a much more frequent renewal of the air than takes place
in the example shown. We meet here for the first time the (empirically found) finding:
Cross ventilation twice a day is not enough.
Fig. 2: Measurements of the relative humidity in an old building [Munzenberg 2001]
In the following working group 24, we will systematically determine the conditions with
increasingly better thermal insulation, starting from an old building as in Fig. 1: It turns
out that with the improvement of the thermal insulation of the outer walls, the
temperatures on the inner surfaces also increase, namely also in the critical areas with
thermal bridges. Following orders of magnitude can gen
If the external wall is insulated according to current concepts (60 ... 80 mm) and if
you pay attention to properly insulated component connections, the inner surface
temperatures in the critical areas (plinth, outside corner on plinth, window reveal)
will be raised to between 12 and 13 °C. If relative air humidity levels of less than
45% can be maintained, there should be no mold problems in such areas as long
as there is no furniture on the outer walls. Temperatures can drop even further
behind furniture. Even the old building, which has been modernized according to
the usual concepts, is therefore dependent on guaranteed dehumidification
through domestic ventilation.
If you go to significantly better thermal insulation levels with U-values around
0.15 W / (m²K), as they are known from the passive house for new buildings, the
internal surface temperatures rise significantly, especially in the critical areas;
they are then in the outer wall edge, at the base and at the window connection
each above 16 °C. Even behind a cabinet in the outer edge, the surface
temperature remains above 16 °C. With such a high surface temperature, the
critical aw value is only reached above a relative room humidity of 62%. Such a
high value is also rarely achieved in practical circumstances (measurements by
Uwe Münzenberg) - interestingly enough, in a building with a passive house
insulation level, a comparatively small air exchange would also be sufficient to
rule out moisture damage to the interior surfaces. - In the passive house, the
strategy "2 times daily ventilation" regarding the dehumidification performance in
normal operation would actually suffice. Now you have to consider, however, that
in new buildings as well as in the renovation of old buildings, large amounts of
water bound in building materials are usually introduced, which further
exacerbates the situation. This means that additional ventilation for
dehumidification is also required at the beginning when a building is first
occupied. (Note that there still are other reasons to use a HRV system.)
Even if the immediate problem of moisture damage in a passive house does not exist
due to the very good thermal insulation there, there is another influence of the room
air humidity that should be taken into account: the multiplication of house dust mites is
interrupted if in a period of 2-3 Weeks of the year, indoor air humidity is permanently
below 45% [Engelhart 2000]. Since house dust allergies are becoming more
widespread and excretions of the house dust mite are the most important allergens, it
is advisable to meet the stated condition in an apartment once a year - in the winter
half-year this is easy with adequate ventilation, but with "twice daily cross ventilation"
it can usually not to be reached.
Advantages of directional flow (“piston flow”)
The task of moisture removal has a further advantage in ventilation designed according
to the criteria recommended for the passive house:
Fig. 3 shows the basic concept: supply air is introduced into the main living rooms in a
quantity dimensioned to the number of persons (rule: 30 m³ / h per person, as provided
for e.g. in DIN 1946). Regularly in these rooms there are rather low sources of moisture
(from the people themselves and possibly from indoor plants). The air exchange rates
that are set in the supply air rooms on the basis of the supply air condition are regularly
above a certain limit: in the PHPP a value of 0.3 h-1 is provided as a basic air change
that should not be undercut. This ensures regular, controlled moisture removal from
the supply air rooms in the residential building without any problems.
Fig. 3: Directed flow recommended in comfort ventilation in a passive house
Higher sources of moisture can usually be found in the wet rooms, where these
activities can take place: wash, bathe, shower, rinse and cook. In addition, there are
regularly wet surfaces (shower cubicle, bathtub) and wet towels that permanently emit
moisture into the room even outside the actual times of use. However, since the entire
air volume flows of directional passive house ventilation are discharged through the
damp rooms of the bathroom, toilet and kitchen, air exchange rates of around 2 h-1 are
available in these rooms. With such a high value - this is also shown by experience in
built houses [Betschart 2001] - the moisture load can be conveniently removed from
the damp rooms. In the article on ventilation strategies in [AkkP 23], Jürgen Schnieders
will show that the backflow of contaminants from exhaust air spaces
is practically zero if the door remains closed
reaches approximately 50% with the exhaust air room doors open .
Experience with dehumidification in inhabited passive houses
In the meantime there are more than 3,000 inhabited passive houses
total is correspondingly large and the experience is also available regarding moisture.
In fact, there has never been a problem with excessive air humidity in passive houses.
The proposed projecting ventilation rate regularly leads to a relative humidity of
significantly less than 50% in the indoor rooms, which can only be higher in the baths
for a short time only under peak loads (e.g. showers; max. 30 minutes).
Rather, there is subjective feedback that the indoor air is often perceived as “dry”.
Depending on the use, values of 30% are quite possible in the core winter in cold
climates (external air temp. 30%. In such circumstances,
humidity recovery from the exhaust air and/or an additional humidification is recommended. Ventilation units
with humidity recovery are available now with passive house certification (see also footnotes 6 and 6).
80%, by the way, at normal winter room temperatures, different air humidity is
Experience with greatly reduced air volume flows
An (involuntary) experiment has been carried out on the highly insulated low-energy
houses in Wiesbaden with regard to the possibility of reducing the supply air volume
with controlled ventilation. In the passive house settlement at this site, two different
efficiency standards were implemented by the developer:
Passive houses with very good thermal insulation of all opaque surfaces, warm
windows with triple-pane double low-e glazing and with a balanced ventilation
system with heat recovery; in these houses, the air volumes were planned and
set in accordance with DIN 1946;
Low energy houses, the exact same designs used for the opaque components,
but ordinary windows with only two-pane low-e glazing and pure exhaust fan
ventilation systems with humidity controlled outdoor air vents above the
windows. In these systems, the air volumes are set based on the automatic
humidity control built into the external air valves.
The measurement carried out on behalf of the IWU in the low-energy settlement by the
PHI is documented in a paper by Oliver Kah in [AkkP 23]. Figure 8 in that paper shows
that the air change via the exhaust air system was around 0.15 h-1.
Despite this extremely low air exchange rate, no moisture problems in these houses
have been reported. Of course, when evaluating this result, it must be taken into
account that the houses have a building envelope that is free of thermal bridges - with
the exception of the windows. The residents also have no complaints about air quality.
There is also a little bit more ventilation through opening windows than in the
neighboring passive houses.
This result supports the thesis given above, that there is a wide scope for reducing the
outdoor air rate in residential buildings as long as certain requirements (see below) are
met. However, the Passive House Institute does not recommend
such small amounts
of fresh air as measured in this project. The results show that the following aspects are
apparently crucial for a satisfactory function:
Secure exhaust air from the damp rooms, which is guaranteed by ventilation with
(low) negative pressure compared to the rest of the apartment,
Continuous operation of this exhaust air with a sufficient amount of exhaust air
for moisture removal.
Remark during revision at 2020: Because the ventilation is designed to improve indoor air quality, this was
mandatory from the very beginning; levels of VOC and Rn could only be reduced to safe values if the ventilation
rate stays above 0.3 h-1. In 2020 this proved important again during the experience with the Covid-19 pandemic.
With proper ventilation (as given in certified passive houses), the load of airborne virus can be reduced
3 Other indoor air pollution
In Chapter 2 we saw that in a passive house with a very good heat-insulating fabric
(including avoidance of thermal bridges), the controlled reduction in air humidity can
also be achieved with comparatively small amounts of fresh air. These air volumes do
not guarantee from the outset that they are also sufficient to reduce other indoor air
pollution. The state of knowledge for some important representatives is to be
summarized as examples.
The WHO precautionary value for formaldehyde (chemical formula HCHO) is 60 µg/m³.
The offgassing from building materials can be limited by careful planning; this is
particularly recommended for the materials used on the room side.
The decisive factor will then be the emission from furniture, smoking and the rest of
HCHO measurements were carried out several times in inhabited passive houses. The
following measured values resulted:
For comparison: in interiors classified as especially "low in pollutants" you can find
values between 20 and 40 µg / m³.
The effectiveness of passive house ventilation with regard to the dilution of
formaldehyde can thus be regarded as sufficient. However, the results also show that
a further reduction in the outside air volume does not seem advisable against the
background of usual background pollution with formaldehyde. With the air volumes
determined by project planning according to DIN 1946 and the additional condition
nmin > 0.3 h-1, sufficient air quality can be achieved in this aspect.
3.2 Volatile Organic Compounds (VOC)
Concerning volatile organic substances, a target value of 200 to 300 µg/m³ was
recommended for the total (TVOC). From here, emissions from building materials can
be limited through careful planning.
The main sources remain emissions from furniture, floor coverings, household
chemicals and smoking. The planner has little influence on this.
TVOC measurements are also available from inhabited passive houses:
Passive house ventilation is thus able to lower the concentrations into the range of the
target values. Due to the continuous ventilation, potentially initially higher concentra-
tions also drop faster [Munzenberg 2002].
However, the results show that there is no scope for a further reduction in the outside
air volume. A further reduction in emissions from household chemicals etc. is desirable,
but not easily enforceable across the board of all households
with project planning according to DIN 1946 (and nmin > 0.3 h-1) a reasonable result
can be achieved; smaller amounts of fresh air are not recommended.
Radon is a noble gas that occurs in the decay chains of uranium and thorium and itself
decays radioactively. It is -active with a half-life of 4 days. The decay products are
The main source of radon in buildings is regularly the supply from the ground under
the houses. However, the release from mineral building materials (natural stone,
concrete, plaster, clay, brick, etc.) can also play a role. The source is the uranium
content of the soil and rock.
The values for the indoor load fluctuate very strongly, depending on the location
(uranium in the soil), construction and tightness of the closure against the soil.
The natural radiation exposure to radon represents a significant risk. According to
[Bauph 1986], the current mean value of the radon concentration in room air is approx.
50 Bq/m³. This results in an average equivalent dose of approximately 15 mSv in the
bronchial epithelium and approximately 2 mSv in the alveolar region of the lungs.
According to [Jacobi 1984], of the approximately 50,000 lung cancer deaths per year
in the Federal Republic of Germany, a relative share of 4 - 12% is probably due to the
inhalation of radon and decay products: about 2,000 to 6,000 cases per year or 0.1 to
Remark 2020: Actually, in the European Union we have been successful in reducing sources of VOC from
furniture and other stuff used in buildings; this has reduced indoor pollution levels indoors significantly, shown
by measurements of these concentrations in the same dwellings [Feist 2019].
0.4% of all deaths. This shows that a reduction in radon concentrations must be
regarded as important for prevention.
Sealing against radon supply from the ground is particularly important (cf. [Brennan
1986]). In passive houses, this is taken into account by an airtight level in the floor slab
or opposite the basement; the effectiveness is checked here with a pressurization test
(n50-measurement). Incidentally, this is one of the reasons why the PHI does not
recommend open cellar access from ground floor apartments.
Zero exposure to radon is not possible: Therefore, a sufficient air exchange rate for
dilution with outside air containing only a little radon is important. Radon equivalents
have also been repeatedly measured in passive houses:
Comparative values for conventionally built houses in Darmstadt are 45 Bq / m³; conv.
Nuremberg 36 Bq / m³, i.e. slightly below the German average. The measured values
from the passive houses are each less than half as high, which can be seen as a good
indicator of the effectiveness of the sealing and ventilation measures taken. This
reduces the risks of radon exposure. Since there is no threshold for carcinogenicity
due to radiation exposure, every possibility of reduction should be taken. On the other
hand, the radon equivalents in outside air in Germany are between 8 and 20 Bq / m³
(up to 40 Bq / m³ in particularly polluted areas).
With the interior values measured in
passive houses, the best possible values are almost reached.
From the perspective of radon exposure is therefore not recommended to reduce the
design values of ventilation systems compared to those suggested for the passive
Hygiene measurements with regard to microbial air pollution were also carried out in
Revision 2020: The number of radon (Rn) deaths are in the same range as those from traffic incidents in
Germany 2018 and definitely higher than those from building fire (some 300 per year). Rn-prevention is
relatively cheap. In accordance with the CDC (Robert Koch Institut in Germany) and clinical experts we
therefore recommend measures to lower Rn-exposition in indoor environments.
There have been later publications which confirm the lower Rn-activities in Passive Houses: [Uhlig 2010] and
[Mc Carron 2020]
These results are not surprising when they are seen in connection with the
dehumidification ensured by passive house ventilation (see Chapter 2). Significant
microbial growth requires sufficient reduction of humidity in the material used for
construction; drying these in the occupied building would not be sufficient. A certain
background concentration of fungal spores is practically unavoidable in living rooms
(food, organic waste bins, indoor plants - especially flower potting soil). Bacteria were
only measured in the indoor air when people were also present. It is doubtful whether
a further reduction in the already very low number of background germs is desirable at
all [Schuster 2000] [Schuster 2002].
It is also important to observe the hygiene recommendations for the HVAC systems
(see Rainer Pfluger's contribution in the conference volume [AKKP 23]).
From a microbiological point of view, there is no indication that passive house
ventilation would have to be configured differently than previously recommended.
In Johannes Kasche's contribution in [AkkP 23], we saw that the subjective influence
of annoying smells can lead to considerable losses in performance and quality of life.
The sources of smells are often faeces, household chemicals, clothing, food - but also
the people themselves. The cause cannot always be determined easily. In addition,
the sources cannot generally be "switched off" and cannot be reduced at will. In the
case of smells, we have to live with a certain source strength as long as we use interiors
together with other people.
How high the subjectively perceived odor intensity is and whether it is perceived as
annoying depends on many factors. However, one thing is certain: odors can be
thinned out by sufficient outside air supply; as long as the source strengths are not
excessive, usually to the extent that an unpleasant perception no longer takes place.
Revision in 2020: This is still true at this time, even with the experience of the 2020 Covid-19-pandemic.
However, during a time period with increased viral load (if e.g. an infected person is present in the same
dwelling) a possibility for using enhanced ventilation rates is recommended (in a proper PH design this is always
3.5.1 Experience in normal ventilation operation
Experience in passive houses with regard to subjective smell perception is available
from a large number of projects: When the ventilation system is in the "normal position"
(30 m³ / (pers.h)), the air in the apartments is regularly perceived as fresh, even when
the room is re-entered
There is a further self-experiment from Darmstadt-Kranichstein: The CO2 control
operated there from 1991 to 1994 reduced the air volume flow if CO2-levels dropped
below 750 ppm to approx. 50 m³ / h (with 4 nominal persons 12.5 m³ / h (Pers.h)).
Under the conditions in this building, this means an air change based on the total house
air volume of around 0.125 h-1. The control worked exactly according to the program-
ming, as evidenced by the measurement logs of the ventilation system: after the
residents leave the house in the morning, the CO2 concentration drops exponentially
as expected. A value of 750 ppm is reached after about an hour; the control then
switches to the lower level as desired.
The subjective perception is interesting when you, as a resident, enter such a
extremely reduced ventilated apartment again in the evening: Now you have a clear
smell perception - all family members and guests agreed ("it should be ventilated
here"). The "smell mixture" is somewhere between sweaty clothes and spice rack
contents. It is precisely these odor sources that remain active regardless of the
presence of the residents and the CO2 produced. The perception of the smells is
particularly strong when entering the apartment from the outside, since the habituation
effect has not yet started.
If the ventilation system was operated in the normal position (120 m³ / h corresponds
to an air change of approx. 0.3 h-1), there was no longer disturbing smell.
Evaluation regarding the odor nuisance: The results in the normal position (project
planning according to DIN 1946) in continuous operation are assessed by the residents
as "good" to "very good". Significantly smaller amounts of air are not recommended,
however. Residents will complain, especially if they re-enter after long periods of
Remark from revision 2020: This is still an important experience; some producers praise their sophisticated
CO2-control – but this might be not advisable in dwellings due to the remaining sources of other substances
besides CO2 when occupants are not present. An option would be, to start a strong ventilation before the
occupants enter the room again; for dwellings, that is difficult (how to know the time it is needed?), but, that
strategy is used in schools and similar buildings nowadays.
From the review of the current state of research
, the following can be stated regarding
indoor air pollution:
The greatest loads are currently caused by indoor air that is too humid in winter
in connection with cold surfaces (on thermal bridges) and the resulting structural
damage by mold. A primary task of home ventilation is therefore to exclude such
But other indoor air pollution is not irrelevant. The examples of formaldehyde,
VOC and radon have shown that a sufficient minimum fresh air supply is
absolutely necessary to avoid health risks. In particular, if, as with the passive
house, the risk of condensation is low, these other air contaminants are
increasingly becoming the defining design variable for the amount of outside air.
Last but not least, the comfort in terms of air quality must also be right, ie smells
should be limited to a reasonable level.
It is therefore not permissible to rely on very low basic air exchange rates based on the
residual leaks and the random ventilation behavior of the users in dense houses
The calculation already implemented in the PHPP (based on DIN 1946) has proven
itself useful for the design case - the maximum of the required supply air or extract air
quantity or the outside air quantity is taken, which corresponds to a minimum basic air
change of 0.3 h-1.
Practical operation can usually be done with rather small amounts of air, especially if
the room air is otherwise perceived as too dry:
The practical operation is then based on the supply air criterion (30 m³ / (pers h)).
The exhaust air criterion must only be able to be met if necessary (position "strong").
This way, dry air can be avoided over long periods of time.
A control by the user is recommended, in the simplest case via a 3-step setting