FAQ

Mould growth is associated with elevated humidity levels and insufficient ventilation rather than with good insulation practice.

Indeed, the reverse is true: improved insulation quality prevents the formation of mould. This is found to be the case in many energy-efficient homes and buildings that have been retrofitted for energy efficiency. They are perfectly insulated, airtight and completely free of mould. If the external walls are not satisfactorily insulated, air moisture condenses in the cold areas, which favours the formation of mould. Effective thermal insulation of the façade tends to raise the surface temperature of the external wall and thus prevent moisture from accumulating.

It is likewise important to identify and eliminate all thermal bridges in the building (i.e. places where there are gaps in the insulation). Such bridges can be found, among other places, in recesses for radiators or on balconies. In addition, it is not advisable to place furniture in front of external walls, as this prevents the indoor air from circulating freely, meaning that the surfaces behind the furniture cool down.  
 

Within the framework of the building permit application procedure, it is not within the municipalities’ field of competence to monitor compliance with private property boundaries. The right of ownership is a fundamental right enshrined in the Constitution, therefore any encroachment of the façade insulation (considered as a fixed element of the building) on a neighbouring private property must be covered by the transfer of a real right over the plot of land in question, otherwise there is a risk of legal recourse within 30 years of the work being carried out (acquisitive prescription in real estate matters).

In the context of the building permit application procedure, it is necessary to respect the municipal planning regulations relating to setbacks from neighbouring private properties. The same provisions apply to constructions along the municipal public domain.

With regard to building permits, the municipalities systematically check that buildings are set back from the municipal public domain or plot boundaries.

As a rule, the planning regulations for setbacks are regulated by the municipalities in their General Development Plan (Plan d’Aménagement Général, PAG) or in the Special Development Plans (Plans d’Aménagement Particulier, PAP).

For municipalities whose PAG has not yet been adapted to include current legislation, the planning regulations relating to setbacks are still included in the written part of the PAG or in the building regulations. For these municipalities, the measurement of setbacks in the event of energy retrofitting is set out individually, by municipality, in those documents.

On the other hand, for those municipalities whose PAGs have been completely revised since 2011, the Grand Ducal Regulation that is the basis for the applicable definitions provides that the building setbacks are to be measured without the additional insulating layer in the case of an energy retrofit.

In both cases, the municipalities can include derogation mechanisms in their regulations in order to promote energy retrofitting, for example through a reduction in the minimum setback to be observed.

In principle, a wall does not breathe. Air renewal and moisture removal are only achieved by ventilation (via windows or a mechanical ventilation system). The effect of 'breathing' as mentioned above refers to the transfer of moisture by the diffusion of water vapour. In practice, this is minimal and does not play any role in the removal of moisture. On the other hand, it is important for interior plaster to be able to absorb moisture from the air and release it back into the interior after ventilation. The moisture regulation effect therefore only operates within the first few centimetres (on the interior side) of a wall. For example, clay plaster on internal walls provides excellent moisture regulation.

It is often façade insulation systems that are implicated and pilloried for this. It is true that on insulated walls the outer render layer has a lower temperature and slows down the drying of rainwater and condensed water. However, in order to determine the origin of the spread of micro-organisms, various factors have to be taken into consideration: weathering, orientation, proximity of trees or bushes and the lack of protection provided by protruding building elements (roof overhang, window sill, projection, eaves, etc.).

For new builds, weather protection should ideally be provided from the outset. Similarly, by opting for a thicker finishing coat, higher surface temperatures can be obtained.

On the contrary! Poor quality sealing can result in significant amounts of energy loss. However, the risk of building deterioration is an even greater factor: in winter, warm, moist air escapes to the outside through leaky areas such as joints and cracks. In this way, the air cools down and moisture condenses, which can lead to the formation of mould in the cavities. In most cases, we don't even notice it and therefore we can't react.

Airtightness is therefore a key issue. You must ensure that your house is well ventilated, either simply by opening the windows or by installing a controlled ventilation system.

The heat transmission coefficient (known as the U-value) determines how much heat passes through a building element – the lower the value, the better the insulation. Therefore, for example, a sand-lime brick wall with a thickness of 50 cm has a U-value of about 1.0 to 2.0 W/m² K. With an outer insulation thickness of 12 cm to 30 cm, the walls of well-renovated buildings today achieve U-values between 0.3 and 0.1 W/m² K.

Insulating materials are not all flammable. Building materials, and therefore also insulating materials, are divided into different fire protection categories. Moreover, a European standard regulates the classification of products and types of construction according to their fire behaviour.

This standard indicates that there are non-flammable insulating materials. Therefore, the main contractor can inquire about this beforehand, prior to deciding on the materials to be used. In principle, insulating materials made out of minerals are non-flammable. Mineral-based insulation materials are also covered by the Luxembourg Sustainability Certification for Housing (LENOZ) and can therefore be financed by the State as part of an overall project. Refits are also covered: the use of mineral-based insulation materials allows for higher financing than conventional materials.

Furthermore, there are other materials used as façade cladding elements that are flammable and these must be taken into account in this regard. This is why it is important to account for the overall make-up of the external wall in itself. Nonetheless, it may also include flame retardants (i.e. sealant, firewall, etc.) and thus provide protection against the spread of flames.