Does EPS insulation cause damp or mould in a house?

If the masonry is constantly supplied with water, covering it with a new layer will make observation more difficult. It is also worth checking when the patches appear. A link with rain, frost or the way the room is used is a clue, although in itself it is not a full diagnosis.

At the same vapour content, cooling the air at the surface raises its relative humidity. When the surface is sufficiently cold, condensation may occur. The risk of mould growth does not require visible water droplets, however. Persistently elevated humidity at the surface also matters.

External insulation usually raises the temperature of the inner surface of the masonry during the heating season. It can therefore reduce one of the risk factors. The effect depends, however, on the continuity of the layer. An uninsulated reveal, ring beam or connection to the floor slab may remain colder than the rest of the wall, even when its centre has good thermal resistance.

The term “breathing walls” mixes different phenomena. The transmission of vapour through materials is not the same as air exchange in a room. Moisture from use is removed above all by effective ventilation. A wall should not be expected to take over this function, regardless of which insulating material has been applied on the outside.

EPS resists vapour diffusion, but this does not automatically mean the wall is faulty. What matters is the arrangement of all the layers and the conditions on both sides. Nor can every building component with a more vapour-permeable material be regarded as safe. The position of the layers, the initial moisture and the possibility of drying out require separate assessment.

Energy-efficient renovation often includes sealing the windows or replacing the joinery at the same time. Unplanned air supply may then decrease. If ventilation previously relied on leaks, its inadequacy becomes apparent after the renovation. Coincidence in time does not prove that the cause is the EPS board bonded to the masonry.

Added to this is construction moisture from new plasters and screeds. The building needs conditions in which it can dry out. It is worth checking the supply air, the extract air and the flow paths between rooms. Ventilation required for the safe operation of fuel-burning appliances should not be restricted without expert advice. Such a system should be assessed by a suitably qualified specialist.

Measurements of temperature and humidity over time, an inspection of the services and a check of places exposed to rain are useful. A thermal imaging survey can indicate cooler areas, but the result depends on the measurement conditions. The camera does not automatically determine whether the cause is a lack of EPS, moisture in the masonry or a leaking connection.

In more difficult cases a hygrothermal analysis is needed. Internal insulation requires particular care, because it changes the temperature of the existing masonry and its ability to dry out. Selecting a material on the basis of lambda alone is not sufficient here. The mechanism of the problem and a realistic way of removing it must first be established.

If the cause is a leak, the repair begins with cutting off the water supply. If the problem is a cold detail, its insulation needs to be improved. Where ventilation is inadequate, proper air exchange must be ensured. Removing the film and repainting without such action may give only a short-lived cosmetic effect.

Polystyrene for correctly designed building components is offered by, among others, Termo Organika, Styropmin, Knauf Therm and Austrotherm. The choice of a particular grade is determined by the conditions of use and the documentation.

After the renovation, monitor the difficult places for the following season. Do not permanently conceal a cold corner with tightly fitted built-in furniture without assessing the conditions. Ensure that the ventilation works regularly and that leaks are dealt with promptly. Effective protection against damp results from the interaction of insulation, airtightness and the way the house is used.