This is how you ventilate properly!

Yes, but...
If you want to ensure that your home is furnished in a natural and/or environmentally friendly way and that you use cleaning products, etc., you can go with your own initiative. This means that economical ventilation, based on the perceived need for fresh air, is always sufficient to create a healthy indoor climate. However, it is also a prerequisite that the building structure is "in order" to the extent that the resulting humidity in the room does not lead to the formation of mold.

No!
If "breathing" means that air or, in particular, water vapor must be able to diffuse easily through the external wall, this question can be answered with a clear "no". When the term "breathing walls" is used, the impression is often created that a large part of the air exchange between inside and outside takes place via the walls. However, even with brickwork that is known to be highly “breathable”, water vapor diffusion only accounts for 25% of the amount of moisture removed by normal (window) ventilation.

To stay with the image: to attribute greater importance to “wall breathing” for indoor air quality than it deserves according to these percentages is as foolish as holding your mouth and nose shut and then leaving the body to breathe through your skin.

The so-called “breathing” of the wall only takes place in the first 12 cm of the inside of the wall (see next question), so external insulation (e.g. thermal skin) only influences the indoor climate insofar as it makes the walls warmer, which is perceived as comfortable.

Yes and no!
Surfaces that are in contact with the room air can absorb "excess" amounts of air pollutants. This so-called sorption capacity is best for natural fibers (carpets, curtains, upholstery), paper (wallpaper, books) and porous softwood fiberboards, good for untreated or vapor-permeable glazed wood and satisfactory for mineral lime and clay plasters.

The following have an unfavorable effect: varnish and oil paints, all plastic surfaces, surfaces made of synthetic textiles, ceramic tiles and cement plasters. This applies in particular to the buffering of peak humidity values ​​in the kitchen and bathroom and when living spaces are heavily occupied.

BUT: The absorbed vapors are released back into the room (albeit possibly with a time delay). The hope that the absorbed moisture can be "breathed away" by external walls is also deceptive (see question 2). The sorption processes discussed here only take place between the room air and the first 10-20 mm of the wall thickness, so that one can at best speak of a type of "surface respiration". Even an "organic house" can only really "breathe" by ventilating.

First: buy a hygrometer (e.g. department store, optician's shop) and check whether and when the relative humidity drops below 40%. If this is particularly the case in very cold and/or windy weather, then the best thing to do is:

Sealing window and door joints (in converted attics, significant additional leaks can occur in the wall paneling and insulation!)

In general, lowering the room temperature by 1-2°C leads to an increase in the relative humidity of up to 10 percentage points. If this is done by lowering the heating water temperature, the circulation of dust due to the buoyancy on the radiator is also reduced. This also helps to avoid mucous membrane irritation and other symptoms of "dry air syndrome".

The measures described above make humidifiers, which are questionable in terms of hygiene and whose effectiveness is controversial, superfluous.

No!
Moisture damage of this kind has two main causes: increased humidity and surface temperatures on the outside walls that are too cold. If the damp spots are of limited size (thermal bridges!), then 1.5 to 3 cm thick internal insulation is often enough to repair the damage in order to raise the surface temperature to a non-critical level.

ATTENTION: If the internal insulation in the affected area is too small, the mold growth will only be pushed to the edge of the insulation. (In any case, professional advice is recommended.)

Under no circumstances should you tackle the symptoms of mold using chemical means without eliminating the cause of the damage. This only helps for a short time and poisons the air in the room.

If such measures, or better still, more extensive measures (continuous external wall insulation) cannot be implemented (in the short term), then only a conscious and well-dosed increase in heating and active ventilation will help in the meantime, especially when outside temperatures are below +5° C.

Simply removing the joint seal leads to excessively high air exchange rates at certain times, without being able to guarantee that existing damage will dry out quickly. The better solution in such cases is often to install a simple ventilation system.

Is it better to heat more?
From an energy saving perspective, it is sensible to heat bedrooms less, i.e. to leave the heating off as a rule. Low room air temperatures also mean lower surface temperatures on the outside walls.

The widespread bad habit of opening the door to the living room in the cold season to "heat up" the bedroom is particularly unfavorable. This causes large amounts of humidity to flow in, which cannot be absorbed by the cooler bedroom air, but instead condenses on the outside walls. In this case too, thermal insulation of the cold surfaces is the best and safest solution (see question 5).

Anyone who sleeps with the window closed at night must also bear the following in mind: Two people release around 500 g of moisture per night just by breathing. Most of this water does not remain in the air, but is absorbed by absorbent surfaces (textiles, wood, wallpaper) in the room (see also question 3).

These so-called "sorption processes" have a catch, however: They are so slow that a simple morning airing is not enough to discharge the moisture buffer again.

The following rule of conduct helps: After getting up, ventilate briefly to get fresh air into the room. Then heat the room for 5-10 minutes with the window closed again (turn the radiator off again!). Only ventilate briefly and vigorously about an hour later (e.g. after breakfast). This also causes the moisture stored in the bedding and mattress to evaporate (throw back the duvet!). If the humidity is high and the weather is bad, this process must be repeated several times (with the bedroom door closed!).

This way, even if you heat sparingly, you can prevent the bedroom from getting musty.

Absolutely not!
Bathrooms are the place where the greatest moisture levels in the entire apartment occur, especially if you shower frequently. However, as bathrooms are usually well heated, the risk of condensation on cold surfaces is usually much lower than in bedrooms, for example. Rule of thumb: If you avoid constant fogging on the inside of the window pane in the bathroom, you can hardly expect the temperature to fall below the dew point permanently, even in the area of ​​thermal bridges. However, the absorption of moisture in porous surfaces (plaster, wood, towels) is much more critical (see questions 3 and 6). This is especially true if the relative humidity rises to 80% or more over a longer period of time.

There is only one thing that helps: ventilate immediately after showering or bathing so that the moisture has as little time as possible to penetrate too deeply into the materials. Because: the longer the penetration time, the longer it takes for the absorbed water to evaporate again. The time that the humidity is allowed to take effect roughly corresponds to the ventilation time required afterwards.

Continuous ventilation leads to the room cooling down unnecessarily and increases the risk of condensation.

In principle, the same rules apply to basement ventilation in winter as for living spaces: the colder it is outside, the better the dehumidification through ventilation works.

The most critical time for basement ventilation begins in late spring, when the winter cold is still in the basement walls, but the weather outside is already quite warm with a correspondingly high absolute humidity. This is when the moisture settles on the basement walls. In summer, the following applies to cellars and other cool adjoining rooms (e.g. pantries): it is best to only ventilate at night, when the outside air humidity is at its lowest.

Almost always in winter.
For example, at 20° C and 60% relative humidity, indoor air has an absolute water vapor content of 10.2 g/m³. At a temperature of less than +12° C, even "saturated" outside air (100% relative humidity) is absolutely drier than the indoor air in the example. The water content in the outside air would actually be higher if you counted the raindrops, but as long as it doesn't rain in when you ventilate, this is irrelevant for the exchange of steam.

No, if...
you also take "building psychology" problems seriously, then no energy consultant should try to talk the third of the population out of doing so who need a window that is at least slightly open for a peaceful night's sleep. The undisputed increase in heat loss caused by this constant ventilation can be kept within limits if the following is observed:

• Drafts should be avoided, i.e. the bedroom door must remain closed and ideally be fitted with a joint seal.
• During the heating period, even when there is no wind, a window must be tilted at most to keep the level of the "leading pollutant" for bad air, carbon dioxide, below the tried and tested "Pettenkofer limit".
• The colder it gets outside, the smaller the window opening needs to be to achieve the same effect (increasing thermals!).

One advantage of this well-dosed, continuous nighttime ventilation is that critical moisture accumulations in walls, etc. are avoided from the outset (see question 6).

Only if...
the fireplace has an air supply independent of the room air. This is the rule today for individual gas stoves with external wall connection and gas central heating, and an air supply shaft should definitely be planned for new tile and fireplace stoves. Coal and oil stoves, however, always get their combustion air from the room. If there is a lack of air supply, combustion is worse and large amounts of toxic carbon monoxide are produced (CO, not to be confused with the non-toxic carbon dioxide CO²). This alone is the reason for all tragic deaths known to date that have occurred after sealed windows have been installed.

But the problem of the tightness of the firebox and stove door should not be underestimated. Experience has shown that in certain weather conditions and when there is too little buoyancy in the chimney, stoves can "collapse". Therefore, if at all, stoves that are burning out should only be operated in bedrooms with the window open. In centrally heated apartments, life-threatening CO2 concentrations are impossible as a result of joint sealing.

Source: Hessian Ministry for the Environment, Energy, Agriculture and Consumer Protection