FAQ

Joints can cause significant heat loss. In winter, warm, moist air escapes to the outside via areas that are not tightly sealed, i.e. joints and cracks. This leads to the air cooling down and humidity condensing, which can give rise to the formation of mould in the recesses. In order to detect heat loss, thermal imaging is recommended.

An energy retrofit of a building (insulating the façade and roof, as well as replacing the windows) seals the building more tightly and can thus improve energy performance.

Regular maintenance of a rendered façade can considerably reduce the risk of algal growth and fungal attack. Façades most at risk (north-facing and shaded façades) must be cleaned regularly by a specialist company and painted if necessary. Damaged connection and expansion joints on window sills and air outlets of ventilation systems should be renewed.

In the case of composite thermal insulation systems, the choice of material and the application of the coating can delay the formation of algae. The following materials should be preferred:

• mineral renders;
• thick renders;
• renders without the addition of colour, which require a coat of paint after application;
• smooth-textured finishing renders.

Many renders contain biocides (pesticides) that are supposed to prevent the growth of algae and fungi, but over time these biocides are washed off by rain and end up in waterways and soils. Mineral renders are an environmentally friendly alternative to renders containing biocides.

Heavy insulating materials (with high bulk density) store heat better than light insulating materials. During short periods of low temperature, the façade cools down more slowly and less condensation forms on the rendered surface, which reduces the risk of algal and fungal growth.

Heavy insulating materials are generally made of renewable raw materials: wood fibres, cellulose, hemp, etc. The Luxembourg aid scheme Klimabonus provides for a specific category for insulation materials made from renewable, environmentally friendly raw materials with a higher subsidy.

In general, ventilated curtain walls and monolithic structures without a composite thermal insulation system are less vulnerable to algal and fungal growth.

Eaves and window sills as well as the substructural areas protect façades from rain and water splashes and thus reduce the risk of algal and fungal growth. This constructive protection against moisture is part of the architectural design and must be taken into account in the design phase.

Algal or fungal growth on façades is caused by moisture from rain or condensation on the rendered surface not evaporating quickly enough. The higher the humidity of the outside air and the lower the temperature of the rendered surface, the higher the condensation level. The humidity lasts for a particularly long time on the sides less exposed to the sun (north and west) and when trees or plants cast shade on the façade.

The growth of algae and fungi is favoured by the absence of protection against rainwater, such as eaves or a window sill, or against water splashes at substructure level.

Algae and fungi can form on both insulated and non-insulated façades. As a rule, thin-layer exterior renderings on insulated façades and coarse-textured façades are more vulnerable to algal growth. On the other hand, mineral coatings covered with a suitable paint are less vulnerable.

Green pollution of rendered façades is pollution caused by algal growth or fungal attack. This pollution is an undesirable and yet natural phenomenon linked to the ageing of a façade, which occurs when certain conditions favourable to the proliferation of algae are met. In essence, it is an aesthetic problem and has no negative impact on the building itself or the health of the occupants.