FAQ

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.

When improving building energy efficiency, insulation measures have the highest priority. No other measure can save as much energy for heating.

Several studies demonstrate that the production of insulation materials consumes much less energy than is saved later. Even with 30-centimetre thick insulation materials, such as polystyrene, the production energy required is already amortised after about three years.

If you rely on non-airtight windows to ensure good ventilation, you are sure to make a mistake and it will be an expensive error: the exchange of air takes place in a purely random way, depending on the weather conditions and air pressure. On the other hand, on the coldest days you can be sure that you will suffer from heat loss. In addition, there is a risk that moisture in the air will condense on single-glazed windows, leading to the formation of mould. Windows that are not airtight will lead to an unpleasant indoor climate and unnecessarily high heating costs.

In the case of retrofitting, new windows must be installed by a specialist who not only works with the latest materials but also ensures that existing thermal bridges are eliminated.

Mould growth is caused by poor construction standards or poor/insufficient ventilation, not by levels of insulation.

In fact, if you insulate your house properly, you will prevent mould growth. This can be seen in many passive houses and in buildings undergoing energy retrofitting. These buildings are well insulated, airtight and completely free of mould. If the external walls are not satisfactorily insulated or not insulated at all, air moisture condenses in the cold areas, thus giving rise to the development of mould. Thermal insulation of the façade tends to raise the surface temperature of the external wall and prevent moisture from accumulating.

In this context, it is important to identify and eliminate all thermal bridges in the building. These include, for example, window apertures, i.e. the wall surfaces facing into the window or door openings, against which the respective frames are placed. In addition, external walls should not be encumbered with furniture, as this prevents the indoor air from circulating, meaning that the surfaces behind any furniture cool down.

Therefore, in summary, proper insulation of the façade and adequate ventilation are key measures that prevent mould growth.

The certificate is valid for a maximum period of 10 years from its date of issue. The expiry date of the certificate is stated next to the date of issue.

In addition, the LENOZ certificate expires when the energy pass for the residential building in question expires.

In the case of a property in co-ownership, all co-owners must sign the application form for financial aid for the certificate process. The co-ownership management body can submit the application on behalf of the co-ownership.

A LENOZ certificate is issued for all buildings or parts of buildings that have a residential building energy performance certificate. The same definitions of residential and non-residential buildings apply to the LENOZ certificate and the energy passport.

A LENOZ certificate can be issued for all residential buildings with an energy passport, whether they are new or existing residential buildings.

Issuance of the certificate is associated with Class 4 “minimum sustainability” (Chapter 0.2 of the Annex to the Grand Ducal Regulation on housing sustainability certification).
Existing residential buildings can achieve this "minimum sustainability" by means of comprehensive retrofitting works.