Causes and development of mold
Mold needs moisture to grow. If mold occurs in living spaces and spreads over a larger area, the remediation of the mold infestation can quickly lead to considerable costs. In order to be able to remove mold infestation sensibly and avoid it in the future, the causes must be clarified.
A common assumption is that living spaces start to go moldy when the residents don’t ventilate enough and don’t heat enough. Among other things, you will learn how to recognize mold through incorrect ventilation. In around 10% of cases, this is the only cause of mold in the home. In an estimated 60% of the cases, structural defects are responsible, in a good 30% the cause is a combination of structural defects and incorrect ventilation or the lack of a ventilation system. In modernized old and new buildings, there is a tendency for mold to develop more than in old buildings.
What are molds?
Molds form typical fungal threads and microscopic spores. They are part of the natural environment and can be found almost everywhere: in the soil, in the air, indoors, and in nature. Since they are involved in the decomposition of organic material, they play an important role in the cycle of nature. Mold spores can survive for months and years even under unfavorable conditions, such as dryness, but only multiply when there is sufficient moisture. This is when the typical fungal network of threads, known as mycelium, forms.
Mold is usually harmless indoors. However, if their concentration exceeds a certain level, they can cause damage to buildings and furnishings as well as health problems for the occupants. Mold not only grows on wallpaper and plaster, but can also infest masonry.
In order for mold to grow, it needs two basic conditions: nutrients and moisture. Other factors, which are less significant, include temperature and pH value. Mold does not need light or oxygen to grow.
Mold spores are undemanding and can spread even with a very low nutrient supply, which is already sufficiently available in simple house dust. Mold in the home can therefore also form on surfaces that appear unsuitable and contain no nutrients.
Accordingly, the most relevant factor in the formation of mold in living rooms or basements is moisture. It is not just the relative humidity of the room air that decides whether mold can spread, but the moisture content directly above the wall surface. How high this is depends not only on the humidity of the room air and its temperature, but also on the temperature of the wall. Most molds grow at 80 to 85% relative humidity. Under exceptional circumstances, however, mold can develop at 65% or up to almost 100% relative humidity. See also: How is humidity created?
The state-certified construction technician Mike Hahn works as a freelance expert for moisture and mold damage. According to his statement, the most common causes of mold or increased moisture in the apartment are "an unfavorable geographical location and floor plan, insufficient thermal insulation, incorrect material selection (wall structure, window), unfavorably placed radiators, construction defects, damage caused by accidents and incorrect user behavior (ventilation, heating, Furnish). Often several causes occur at the same time. "
Mike Hahn
construction engineer
Mold growth at a glance
- Molds are undemanding and occur everywhere in nature and in buildings
- Mold can develop in living spaces if there are nutrients and sufficient moisture
- Direct moisture penetration can result from leaky roofs, cellars, pipes or accidents
- Direct moisture penetration can result from leaky roofs, cellars, pipes or accidents
- Structural factors favor mold growth, especially in combination with incorrect ventilation and heating behavior or the lack of a well thought-out ventilation system
- Thermal bridges, such as on windows or balcony doors, and inadequately insulated exterior walls encourage the formation of mold on walls
How does mold develop in the home?
Damit Schimmelpilze wachsen können, brauchen sie zwei Grundbedingungen: Nährstoffe und Feuchtigkeit. Weitere, aber weniger bedeutende, Faktoren sind die Temperatur und der pH-Wert. Schimmel braucht weder Licht noch Sauerstoff, um zu wachsen.
Schimmelsporen sind anspruchslos und können sich schon mit einer sehr geringen Nährstoffzufuhr ausbreiten, die bereits durch einfachen Hausstaub ausreichend vorhanden ist. Schimmelpilz im Haus kann sich somit auch auf Oberflächen bilden, die für uns Menschen ungeeignet erscheinen und keine Nährstoffe enthalten.
Der relevanteste Faktor bei der Schimmelbildung in Wohnräumen oder Kellern ist dementsprechend die Feuchtigkeit. Dabei entscheidet nicht allein die relative Luftfeuchte der Raumluft, ob sich Schimmel ausbreiten kann, sondern der Feuchtegehalt direkt über der Wandoberfläche. Wie hoch diese ist, hängt nicht nur von der Luftfeuchtigkeit der Raumluft und deren Temperatur ab, sondern auch von der Temperatur der Wand (Wie entsteht Luftfeuchtigkeit?). Die meisten Schimmelpilze wachsen bei 80 bis 85 % relativer Luftfeuchte. Unter Ausnahmebedingungen kann sich Schimmel aber auch bereits bei 65 % oder bis annähernd 100 % relativer Luftfeuchte entwickeln.
Causes of mold in apartments and houses
In living spaces, moisture is created, which leads to the formation of mold, usually through indirect moisture penetration by condensation water that precipitates on cold surfaces. Condensation occurs because warm air can absorb more moisture than cold air and the water vapor in the air liquefies when it cools below a certain temperature. Since the temperature differences between inside and outside or between indoor air and wall are greatest in the cold season, mold can occur more frequently, especially in winter.
In addition, direct moisture penetration, such as through leaky roofs, basements or cracks in facades or leaking pipes and accidents, is often the cause of moisture in the house. Depending on where the water damage is occurring and how severe it is, it can be easily located and eliminated. If it remains undetected for a long time, this can lead to mold in the house.
Whether or not so much moisture settles through indirect moisture that mold can develop depends not only on the ventilation and heating behavior, but also on the building materials used and the number and intensity of the moisture sources in the rooms. Structural factors that contribute to an airtight design of the building envelope and promote the formation of mold are, for example, the use of building materials with low diffusibility and low water vapor absorption capacity (e.g. concrete and polystyrene), waterproof facade paints, sealed windows, synthetic wallpaper and wall paints and cold or cold paints. Thermal bridges such as on windows or balcony doors.
In order to avoid the development of mold, it is advisable to use building materials that can absorb moisture and transport it to the outside. Natural building materials such as clay and lime have excellent diffusion properties in this regard. A well thought-out ventilation concept, for example with a decentralized ventilation system in the house, can ensure the correct ventilation of the rooms, even if the structural conditions favor the formation of mold. Further tips for avoiding mold can be found in the guide to preventing and avoiding mold.
Causes of mold in specific areas of the house
Mold in the house is more likely to occur in some rooms than in others. Reasons for this are structural conditions, temperature differences in the apartment and activities with different levels of water vapor generation (formation of air humidity). The following is a list of the different rooms and how you can identify mold caused by incorrect ventilation.
Bathroom and kitchen
Especially in bathrooms and kitchens, a lot of moisture is generated by activities such as showering, washing, cooking, and washing up. In particular, if the renovation or construction of the building prevents sufficient diffusion of the resulting humidity to the outside, inadequate ventilation can quickly increase the relative humidity to such an extent that mold develops.
Drying laundry in living spaces or large aquariums also leads to increased humidity, which, as in bathrooms and kitchens, promotes mold growth.
Walls
Inadequate thermal insulation of exterior walls can result in large areas where heat can escape to the outside, causing low surface temperatures. During the cold season, this can lead to condensation on damp walls. Furniture placed directly against exterior walls further promotes poor air circulation and mold growth. This usually leads to mold on the walls. There are surfaces on which mold can develop particularly well. Woodchip wallpaper, for example, provides ideal conditions for mold to grow. Its relatively high sugar, protein, and lignin content is a veritable nutrient cocktail for mold. Gypsum-based substrates and wood paneling also provide an abundant supply of nutrients. Ensure adequate air circulation to prevent mold from growing on walls.
Bedroom
Moisture often forms in rooms that are significantly cooler than the rest of the home.
Typically, this is the bedroom, which is unheated in winter,
but utility rooms and storage rooms are also often unheated. In rooms with a temperature that is permanently or temporarily below 16 °C, the risk of condensation forming increases, and with it the risk of mold. When the doors to warmer areas of the home are opened, warmer, more humid air enters the bedroom and causes the walls to become damp as it cools.
Combined with poor ventilation and moderate to high moisture emissions from the residents’ breath and skin moisture at night or from drying laundry, unheated bedrooms are particularly susceptible to mold.
There are surfaces on which mold can develop particularly well. Woodchip wallpaper, for example, provides ideal conditions for mold to grow. Its relatively high sugar, protein, and lignin content is a veritable nutrient cocktail for mold. Gypsum-based surfaces and wood paneling also provide an abundant supply of nutrients. Ensure adequate air circulation to prevent mold from growing on walls.
Basement
Damp walls in basements are not uncommon, especially in older buildings. Basically, moisture can arise indirectly through condensation or directly through water penetrating from outside.
Condensation is caused by incorrect ventilation practices: basement walls rarely reach temperatures above 12°C. Since humidity can only be transported from warm to cold, basement rooms should only be ventilated when the temperatures outside the house are lower than in the basement rooms themselves. In our latitudes, this usually means that ventilation should take place between October and March and windows should remain closed between April and September.
If water can penetrate the basement, structural renovations are necessary. It should therefore be properly waterproofed from the outside and equipped with drainage. Waterproof coatings should never be applied indoors.
Windows
Thermal bridges are localized areas on walls, ceilings, two- and three-dimensional corners to the exterior wall, and especially windows and balcony doors, through which increased heat loss to the outside can occur. As a result, the surface temperature in winter is lower than on surrounding components. Surface moisture increases and mold can form.
ConclusionCauses and formation of mold
In many cases of mold in the living room, damp walls or corners are caused by temperature differences in the apartment. Thermal bridges, cold outside walls and unheated rooms are a problem, especially in winter, which can quickly lead to mold on walls. If there is insufficient heating and ventilation, the problem is exacerbated.