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Indoor mold


Causes, living conditions, building physics processes, preventive measures and procedures for mold growth indoors

Below you will find some information about the formation of mold indoors

Mold is often the subject of disputes in tenancy law matters.

It is important to clarify whether the building construction is responsible or whether there is incorrect user behavior.

When legally assessing the question of fault, a reliable assessment of the chain of causes is necessary!

A critical examination of the following influencing factors is necessary

  • Heating and ventilation behavior of the residents

  • Functionality of the heating system

  • Thermal protection and thermal bridges

  • Airtightness of the building envelope

  • Increased risk potential for new buildings

  • Indoor climate changes as a result of living space renovations

The mold needs the following living conditions in particular in its environment:
  • High relative humidity

    • Particularly good growth conditions are always found when condensation occurs on or in the building material/component.

    • The development of the spores takes place i. d. Usually takes place at a high relative humidity of around 80%.

  • Moisture

    • The decisive criterion for germination and growth of microorganisms is the water available.

    • The fungus can extract water vapor from both the substrate and the air.

  • Temperature

    • Fungal growth occurs primarily in a temperature range of 0 °C to 50 °C.

    • The optimal growth temperature is approx. 30 °C.

  • Sufficient nutrient content of the substrate

    • Almost all organic substances can serve as a food source for mold (e.g. wood, paper/wallpaper, paint, plastic, etc.) and promote growth.

    • Wallpaper and wallpaper paste are a favorable breeding medium for mold, as they contain cellulose and readily available organic substances. But plastic coatings of plasters and paints (emulsion paints) can also be used by mold as a food source.

  • PH value

    • The molds prefer to thrive in a slightly acidic environment with pH values between 4.5 and 6.5.

    • Some fungal species also tolerate pH values between 2 and 11.

Building physics processes that precede the formation of mold:

Due to the freely available water required for mold formation, the formation of mold can always be viewed as an indication of a defective structural or usage-related situation. Two different building physics processes often precede the formation of mold:

  • Condensation on component surfaces or in the component cross section

  • Moisture penetration of the components

Condensation on component surfaces
  • Air saturated with water vapor has a relative Humidity of 100%.

  • This saturation limit depends on the air temperature. The higher the temperature, the more water vapor the air can hold. So if moist air is heated in a closed system without water vapor flowing in, the relative humidity drops. Conversely, the relative humidity increases when the air is cooled.

  • The temperature to which a closed volume of air must fall until the relative humidity reaches 100% is defined as the "dew point".

  • Condensation occurs on room-side component surfaces if their surface temperature is below the dew point temperature of the air.

Table 1: Dew point temperature °C depending on the room air temperature and relative humidity

Table 2: Saturation concentration of water vapor in air

Example 1: Dew point

At a relative humidity of 50% and an air temperature of 20°C, the dew point is 9.3°C (see also Table 1).
Condensation forms on component surfaces that are colder than 9.3°C.

This process continues until the moisture content of the air is less than or equal to the maximum absorbable moisture content ≤ 100%.

Example 2: Water in room air

At a temperature of +20°C and a relative humidity of 50%, 17.25 x 0.5 = 8.63 g of water are contained in each m³ of room air (see also Table 2).

Example 3: rel. Humidity

As the temperature decreases, rel. Humidity (with a constant amount of water in the room air).

With the same absolute water content, the relative humidity is reduced by increasing the room air temperature and the temperature of the inner wall surfaces is increased, which reduces the risk of mold.

Example 4: Water vapor content in the air depending on temperature and relative humidity.

If the air temperature in a room is 20°C and the relative humidity is 50%, the air contains a water content of 8.63 g/m3 (see example 2). If the outside wall on the inside of the room also has a surface temperature of 20°C, the humidity here is also 50%. If the surface temperature on the inside of the wall cools down to e.g. 15°C, with the room air temperature and relative temperature remaining the same. If the air humidity is in the middle of the room, the relative humidity near the wall increases to 67% (see example 3). The absolute water content of the room air in the middle of the room is 8.63 g/m3, identical to that near the wall surface. If the relative humidity near the wall reaches 80%, the critical condition that promotes mold formation has been reached.

The wall surface temperature is then 12.6°C (point B). If the inner surface of the wall cools down further, for example to 9.3°C, the dew point is reached (point C). If the temperature on the inside of the wall falls further below, condensation appears on the wall surface (point D) because the relative humidity has reached 100%.

Moisture penetration of the components

Component moisture can occur as a result of the following causes:

  • Lack of resistance to driving rain on the outside walls, for example:

    • inadequate paint,

    • highly absorbent facing stone material,

    • highly absorbent jointing of facing stones,

    • Incorrectly sealed component connections

  • Capillary rising moisture due to missing or inadequate barrier layers

  • Moisture penetration of components in contact with the ground as a result of inadequate or inadequate building sealing

  • Residual moisture in young buildings as well as moisture penetration as a result of weather influences from the construction period

  • Roof leaks

  • Pipe breakdowns, burst pipes

When renovating areas affected by mold, the cause must always be determined first!

The 3 relevant “sections”  (mycelium, conidia carriers, spores) in the structure of a mold

1. First, a filamentous structure is formed from a mold spore that has landed on a suitable breeding ground Mycelium. This consists of microscopically small, long, thin, multi-branched fungal threads (hyphae) that spread from individual points in all directions. This mycelium is the actual mushroom.


2. Reproduction occurs via spores (conidia).

After some time, the mycelial threads produce numerous special hyphae, which Conidia carriers.


3. The Spurs or conidia sit on the tip, where they are then whirled up at the slightest breath of air. Once they are present in large quantities, they can affect the health of residents indoors.

Photos: Sanosil AG

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Preventive measures against mold infestation

A. On-site measures


  • Influence of the insulation level: The occurrence of mold on the room side of building structures depends on the surface temperature and humidity. These in turn are influenced by the heat transfer coefficient (U-value) and the heat transfer resistance as well as the hygrothermal conditions prevailing in the room. The insulation level, which is characterized using the U-value, significantly influences the surface temperature on the inner wall and thus the relative humidity there. Poor thermal insulation or a high U-value causes low surface temperatures and, with the associated increase in humidity, a high risk of mold.

  • Thermal bridges:Thermal bridges are localized weak points in the surrounding surfaces of a building through which more heat flows to the outside than in the adjacent areas, resulting in a reduction in the internal surface temperature.

    • Geometrically determined thermal bridge: The corner of the building represents a geometrically determined thermal bridge. The outer wall area (cooling) is larger than the inner wall area (warming).

    • Construction-related thermal bridge: Due to the weakening of the outer wall due to installation slots, radiator niches, window reveals, etc., more heat can flow to the outside.

    • Material-related thermal bridges: Material-related thermal bridges (e.g. concrete supports in masonry) are areas in external components that have a lower heat transferng resistance than adjacent areas (also for design reasons).

​TheConsequences of thermal bridges are (in addition to the higher energy losses) a drop in temperature, an increase in moisture on the inner surface and the risk of the temperature falling below the dew point and thus the formation of mold. 

  • Wother on-site measures

    • Compliance with the requirements of minimum thermal protection 

    • Providing protection against driving rain to protect the building's outer shell

    • Sealing the base plate using suitable barrier layers against capillary rising moisture

    • Dense roof covering and structurally correct structure of the roof structure.

    • Waterproof installations (heating and plumbing installations).

    • Reduction of residual moisture in new buildings as well as moisture penetration due to weather influences from the construction period.

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B. User-related measures


Adequate ventilation: In order to prevent the humidity from gradually rising to 100%, part of the water vapor contained in the air must be removed from the room air. I.d. This is usually done through room ventilation, i.e. i.e., part of the indoor air is replaced by outside air.
Air exchange in a building can occur in two ways:

  • Through natural ventilation by opening the doors and windows as well as through leaks in the outer shell of the building or

  • through mechanical ventilation using fans.

Ventilating the living space is the most effective means of removing moisture from the room.


Other preventive measures include:

  • Reduction of moisture production: In every apartment, humidity is constantly generated through living - through bathing, cooking, washing, drying and through houseplants. The following amounts of moisture are generated per person or process - grams of water per hour. The daily moisture production for a four-person household is around 10 - 15 liters of water per day.

  • Sufficient heating:Apartments with individual unheated rooms (usually bedrooms, hallways, etc.) are unfavorable. In these rooms there is a lower room air and therefore surface temperature, combined with high surface air humidity.

  • Furniture and curtains on/in front of external walls:Covering external walls with volume-forming furniture without floor clearance hinders the uniform temperature control of the wall surfaces by the circulating air in the room. In practice, such external wall surfaces behind the furniture have a surface temperature that in the winter months can be around 2 - 5 K below the regular room-side surface temperature of neighboring external wall surfaces. In poorly insulated old buildings, the temperature regularly falls below the dew point, resulting in a condensation failure.

Procedure for mold growth indoors

1. Determine the cause! => What is the cause of the moisture damage?

2. Remediation planning=> The renovation effort should be adapted to the extent of the damage and the type of use of the space. The following aspects, among others, play a role:

  • size of the affected area,

  • Strength of the infestation (individual spots or “thick” mold coating),

  • Depth of infestation (superficial or in deeper layers),

  • Types of mold occurring (important for the risk of allergies and infections, some types of mold produce toxic toxins),

  • Type of materials affected (on room-side materials that can be quickly removed or in masonry),

  • Type of use (storage room, living room, kindergarten, hospital).

With the help of these criteria, an overall assessment must be made with expertise. The resulting protective measures for renovation must then be formulated.


3. Construction supervision and success control

Architektonisches Gebäude

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