Interesting facts about mold

Molds in the apartment are not only unsightly and unhygienic, they can also cause health and structural damage. Toxic substances emitted by mold can cause allergies and serious illnesses. Mold itself can damage infested materials to the point of requiring replacement or very lengthy and costly repair or remediation. So the general rule is that mold in the home must be avoided.

Various types of mold can occur in buildings on a wide variety of materials. Whether and how harmful the different types are, can only be determined by a specialist. If mold has appeared, a specialist must be consulted to clarify further steps.



The materials of our ventilation units are selected in such a way that mold cannot develop. Regular cleaning also prevents mold from forming on the unit itself. If, despite everything, mold appears due to improper use, proceed as follows:  

If the mold has appeared inside or on ventilation devices or ventilation components and spores or the like can get into the living space from there, you should immediately turn off the ventilation device and possibly close it.

As a preventive measure, all devices and components should and must be cleaned regularly. This prevents the formation of a breeding ground for mold due to contamination. There are instructions and recommendations on how and how often devices and components should be cleaned.

Mold growth in the building and building components (walls, wooden beams, doors, window frames, etc.) or on furniture is much more complex than on contamination on ventilation equipment. For ventilation equipment and ventilation components, the key to mold prevention is regular and thorough cleaning. In and on the building, other contexts must be considered.


Die Raumluftfeuchtigkeit

Ventilation systems or ventilation itself can primarily only influence one factor of mold growth: the indoor air humidity

By removing stale indoor air to the outside through the ventilation system (or manual ventilation) and exchanging it with fresh outdoor air, the "humidity balance" in the building is also regulated.

Depending on the use and many other parameters (e.g. the number of people, the number of green plants in the house, how often and for how long people shower/bathe or dry their laundry, etc.), an individual amount of moisture is created in each apartment, which is released into the room air. Again, depending on the tightness of the building (joints, small cracks, etc.) and other factors (e.g. the position of the building in relation to wind influences, solar radiation and much more), the level of natural air exchange through the building envelope is influenced.  

This means that an individual amount of moisture is released into the room air and removed depending on the building.

The indoor air humidity (measured in relative humidity [%]) is therefore difficult to predict and must always be monitored or kept within certain limits. Room air that is too dry (e.g. due to "over-ventilation", i.e. excessive air exchange) can lead to health problems (dry mucous membranes promote infection) and discomfort or cause damage to parquet floors or wooden furniture (cracking). Excessively humid indoor air also leads to discomfort and contributes significantly to an increased risk of mold and bacteria growth.

High humidity is a factor in the "mold criterion".

This is an expression that summarizes when mould begins to grow or when there is an increased risk of mould infestation in practice. The mold criterion is considered to be fulfilled if there is a breeding ground for mold (this can be building materials, wood or dirt) and the relative humidity at this breeding ground (e.g. in a corner of the room on the wallpaper) has risen above 80%. Although initial exposure times may still apply here (e.g. the humidity on the building component must have been above 80% relative humidity for eight hours), these only play a subordinate role in practice, as this can only be ensured to a very limited extent.

To rule out an increased risk of mold growth, it is essential to ensure that the (relative) humidity on all building components and furniture in the room is kept below 80%. Although this is not 100% protection against mold, it significantly reduces the individual risk of mold growth.


How can moisture levels on the component be prevented from being too high?

The relative humidity on the building component should not be confused with the measurable relative humidity in the room. The relative humidity in the room is usually measured at positions where a good average value of the room air prevails in the occupied area (e.g. with a measuring device on a sideboard).

However, the relative humidity is largely dependent on the temperature. You can imagine the relationship in such a way that the air molecules move further apart when it is warmer and make more space for water molecules than when it is cooler and the air molecules move closer together and fewer water molecules fit between them.


Warmer air can absorb more water than colder air. And it is precisely this relationship that is taken into account with the term "relative humidity". In other words, the relative humidity changes when the temperature changes. If it gets warmer and the humidity does not change, the relative humidity drops. If it gets colder, it rises accordingly.

The relationship between how relative humidity changes, for example, is quite complex and is shown in the so-called Mollier diagram (also known as the hx diagram). For most people, the diagram is not self-explanatory and the relationship between the changes remains rather incomprehensible.

LUNOS has therefore considered how these relationships can be presented in an understandable and comprehensible way.

The result is the LUNOS-hx diagram:

Example for explanation:

On the left-hand side of the diagram, we are looking for the room temperature at which the relative humidity was measured. Example: 20°C. From there, we move along the blue line to the right until we reach the intersection with the curve of the measured relative humidity value. Example: 60% (red dot):

From there we walk vertically down one of the gray lines until we meet one of the other curves (blue dot):

From there, we move parallel to the blue lines to the left of the temperature scale. In this case, it would be around 17.5°C (green dot):

We have now determined from the diagram that a relative humidity of 60% occurs when the room temperature changes from 17.5°C to 70%. At a surface temperature of 17.5°C, the relative humidity on the building component therefore rises to 70%! And relative humidities of all temperatures can be "converted" according to exactly this scheme.

In our diagram, one curve is colored red. This line is intended to show that this is the so-called limit of 80% relative humidity. Above this value, as shown above, the risk of mold growth is significantly higher than the normal value. In our example, this would mean that from approx. 15.5°C the 80% relative humidity on the component is exceeded and we have a significantly increased risk of mold growth if there is a breeding ground on the component (e.g. dirt or the material itself can serve as a breeding ground). So we can see that even at a relative humidity of 60% (just an example, of course), mold can start to grow at temperatures of 15.5°C and above.

In a home, the surface temperature can be significantly below the measurable room temperature in room corners, window frames or at thermal bridges (wall penetrations of other components). And this is precisely where there is an increased risk of mold growth.

If we look at where mold occurs very often, these are almost always places that can be particularly cool:

  • Room corners: for physical reasons, this is always a weak point even with the best insulation
  • Window frames: very thin and therefore less well insulated compared to a wall
  • Behind furniture: the air can circulate less there and heat up the walls

The relationship is therefore physically based and only depends on the temperature of the surfaces.

The diagram can therefore be used to determine the surface temperature of building components above which there is an increased risk of mold growth. For example, a thermometer or an infrared camera could be used to determine the lowest surface temperature of a building component in the room and then use the diagram to determine how high the measurable relative room humidity should be so that this value is not exceeded.

Or, conversely, the measured value of the relative indoor humidity could be used to determine the maximum "cold" a building component should be in order to prevent an increased risk of mold growth. In practice, we can rarely change the surface temperature of building components. Although there are examples where this is possible (for example, in an apartment in which the bedroom is not heated, the absolute value of the room humidity in the entire residential unit will equalize due to the so-called partial pressure gradient and also ensure higher relative room humidity in the bedroom. If the room is not heated, many surfaces are cooler and the risk of mold growth is generally much higher in this room. Heating this room would provide a remedy), but in general only the relative indoor humidity can be significantly influenced by the user without having to make major interventions.


The release of moisture can be influenced. Specifically, how many sources of moisture are present in the house (green plants, pets, drying laundry, etc.) or how much moisture is removed in addition to the natural air exchange in the home. The former would be an intervention in the furnishings or the use of the residential unit, which can certainly be necessary sometimes if an even higher air exchange cannot be achieved. The second would be the type and duration of the users' ventilation behavior by opening windows or mechanical ventilation systems. In other words, when and how often and for how long windows are opened or what volume flows are discharged by ventilation units.

With mechanical ventilation units in particular, there are of course different concepts with different volume flows, sensors or switching options. The indoor air humidity therefore depends on the possible or set air volume flows and the combination of individual conditions (building, use). The outdoor temperature or the relative outdoor humidity does not really play a role in this consideration, at least in Germany, as the exchange of outdoor air and indoor air generally always leads to a reduction in the relative indoor humidity.


How high can the relative humidity in my home be?

Often blanket statements are made as to how high the indoor humidity in a home should be. Even measuring devices that you can buy in DIY stores often indicate a certain range of the measured value as ideal. You could therefore assume that the values in the room should be within this range and everything would then be fine. However, in many cases this is not the case!

As already mentioned, the relative humidity in the room should not be too low in order to prevent dry mucous membranes and other negative effects. Values of around 35-40% relative humidity should be observed as a minimum value.

If the individually measurable values in living areas fall below these values, it should be checked why the values are so low and counteracted if necessary (by changing the ventilation behavior, setting up room humidifiers, etc.). However, the maximum value for the relative indoor humidity can only be determined individually. General values have no meaning here, as the individual parameters in apartments and houses are too different.

As already explained, mold growth is not necessarily dependent on the indoor humidity values displayed by conventional measuring devices. It should always be determined which individual values should not be exceeded. This can be done, for example, with the method shown using an hx diagram and surface temperature measurements. However, the relative room humidity should not rise much higher than 55%. At least not for a long time. Even in modern buildings, low surface temperatures are to be expected in corners of rooms or behind furniture, which then already allow surface humidity levels close to the 80% (rel.) limit.

Short-term increases, for example after a shower or after cooking, are not a problem as long as the reaction is immediate and the moisture is removed afterwards or during this time with increased priority (increased volume flows from ventilation units or opening windows). Building defects such as rising damp in masonry or similar can further increase the risk of mold, but we have not taken these factors into account here. In the case of individual mold problems, this should be observed and discussed further by a specialist.

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