Mould infestation in buildings is not only an aesthetic problem, but also poses health risks. Mould has a strong influence on indoor air quality and can endanger or at least impair health. The severity of the toxic and allergenic effects depends on the type of mould (species) and in particular on the total quantity ingested, and mould infestation can occur after an energy-efficient renovation, which can cause unexpected trouble. This short article explains why this is possible and why thermal bridges are often the cause.

For mould spores to germinate and grow indoors, the following conditions must be met:

• Sufficient humidity
• A suitable temperature range
• Food from organic compounds (substrate)

In the vast majority of indoor spaces, the temperature is ideal for mould. A suitable substrate is also almost always available, as deposits of organic dust on the surfaces of building components are sufficient. It is therefore important to focus on humidity to prevent mould growth indoors.

The majority of moulds require a surface humidity of 70 to 98 %. The causes of such high surface moisture levels indoors are

• Excessively high indoor humidity, for example in bathrooms that are not properly ventilated
• Moisture from the building component, for example due to leaks or ingress from outside
• If the surface temperature of the interior walls is too low

If the surface temperature of the interior walls is too low, this can occur during an energy refurbishment, for example during a façade refurbishment:

External insulation is installed on the façade surfaces and the windows are replaced. This greatly reduces heat transmission through the exterior wall. However, this also leads to thermal bridges at connections that can only be insulated to a limited extent or not at all. These are often window reveals, blind boxes, balconies or external corners. Heat escapes much faster from these thermal bridges, which leads to a lower surface temperature of these components. In the worst case, the surface temperature is so low that condensation forms. This creates all the conditions for mould growth.

For this reason, it is important that thermal bridges are not neglected when renovating the energy efficiency of the building envelope. Solutions for minimising thermal bridges and preventing mould growth are often relatively simple to implement, provided they have been taken into account in the renovation planning.

Conclusion

Mould in interiors is very often caused by excessive surface moisture in building components. Thermal bridges become more relevant when refurbishing the building envelope for energy efficiency. It is therefore important to identify the weak points of the building envelope and minimise the resulting thermal bridges as early as the renovation planning stage. We will be happy to advise you on all aspects of energy-efficient refurbishment of the building envelope.