Aerial thermography

On Wednesday 18 January 2023, a thermographic survey (aerial thermal imaging) was carried out over all the roofs in the City of Brussels. This imaging made it possible to draw up an inventory of heat loss for the roofs of the buildings overflown, with an accuracy of 40 cm.

Below you will find an interactive thermography of the roofs of the City of Brussels as seen from the sky (median resolution: one colour per roof, overall resolution: accurate to 40 cm²).

The dark blue colour indicates a 'cold' roof, which corresponds to either a very well insulated or unoccupied building. In contrast, red indicates a 'hot' roof, with significant energy loss.

This thermography is the result of an initiative by the City of Brussels, and was carried out by ACTION AIR. The results of this tool are indicative and in no way constitute an energy audit or certificate.

Presentation used at the info sessions in the quarters of the City of Brussels in March 2024 (PDF, 7.58 MB) (in French and Dutch)

This map shows the heat loss from the roofs of all buildings in the City of Brussels.

How can you find your home or building?

There are several ways:

  • Simple navigation, by moving around the map using your mouse as you would on any other online map.
  • By searching for an address, by entering your address in the search bar at the top right of the screen.

You can choose whether to display a simple map background or an aerial photograph, by ticking 'display satellite image' at the top right of the map.

The map of median values (one roof = one colour) that you have in front of you is useful for visualising at a glance the trends in terms of heat loss by building, and for the City of Brussels as a whole.

If you tick the 'global' layer, you will see a high-resolution map (40 cm) showing the distribution of different heat zones by roof.

Please note: a thermography is a snapshot of a situation at a given moment. A building whose occupants are absent (and who have turned off the heating) will appear on the map as having low heat loss (indicating a possible interpretation that the building is well insulated). Similarly, if the upper floors are unoccupied and/or unheated, the building could be incorrectly interpreted as well insulated. Conversely, a building that is well insulated but overheated could be considered poorly insulated.

Understanding the colour legend


Blue: imperceptible losses.

  • Possible explanations: The house or top floor may be unheated, extremely well insulated, or simply not occupied (temporarily or more permanently). There are two main possibilities: either unheated, or very well insulated.
  • Very low or imperceptible heat loss values are therefore sometimes more indicative of a home's heating level than of its insulation performance.

Light blue

Light blue: low heat loss.


Green: medium heat loss.


Yellow: high heat loss.


Orange: very high losses.

  • Possible explanations: the roof may be poorly insulated, with numerous thermal bridges.
  • A more precise diagnosis is required.


Red: excessive heat loss.

  • Possible explanations: excessive heat loss is often noted if there is an active flue (e.g. a burning chimney), or a ventilation system or flue that draws air to the outside. These losses are therefore very localised, and not necessarily indicative of poorer roof insulation performance. Nevertheless, it is possible to analyse the potential for reducing these losses and the effects of thermal bridges.

To give you some examples of the detailed diagnostics that can be carried out using aerial thermography, 3 case studies of different local authority buildings are presented:

An aerial thermography collects information about the radiation from a surface, so photons that escape.

Impact of materials

The denser the materials, the better their thermal inertia. In older housing (typically with small rooms and thick walls), heat exchange is less important, so the roof will appear less wasteful.

Glass, on the other hand, is a less dense material, and therefore more prone to heat loss. Materials such as wood or tar absorb a lot of energy during the day, so even a well-insulated roof will not appear blue because the materials it is made of give off energy. The presence of water, if there is infiltration in the roof or visible puddles on a flat roof for example, can have the same effect on surface radiation.

We must also take into account the phenomenon of reflectance or transmittance, which is the angle at which the sun shines on glass panes or flat surfaces, which may appear warmer because they store heat during the day. So, if a roof is made up of or surrounded by a lot of glazed elements, this can give a higher radiation index (the area appears in red) because the windows store heat well. Very smooth surfaces with high surface reflectance can have the same effect (aluminium roofs, for example).

So beware of various elements that may prevent your roof data from being interpreted as poor or good insulation: green roof, trees hanging over the roof, wooden terrace, gravel, solar panels, water, ice, roof and porch windows,...

Impact of ventilation systems

Heat loss from ventilation systems can come from chimneys, extractors, CMVs,... These appear as red dots on the roof, very distinct and identifiable by their shape, and should be compared with your knowledge of the composition of your roof.

Impact of thermal bridges

Thermal bridges are caused by changes in geometry that create places where energy is concentrated and then dissipated to the outside (for example sash window, architectural features,...).

In older buildings, these geometric structures often had a primarily aesthetic function, with little or no insulation. It is often thermal bridges that create the sensation of cold and draughts in a space.

In buildings that are considered to be thermal 'sieves', and therefore contain a lot of thermal bridges, the interpretation of the thermography will be more complicated because it is less visible at the scale of the data (40 cm).