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How much does it cost to insulate a roof?

The roof is the first place to invest in when insulating your home : a poorly insulated roof will cost you up to 30% in heat loss in a semi-detached building. 

Insulating a roof is not very expensive and the investment is quickly recouped. In addition, the Brussels Region offers insulation grants.

Cost can vary significantly

Prices vary depending on the layout of your roof and the chosen insulation materials. There are a wide number of options: minerals (rock and glass wool), synthetic materials (expanded or extruded polystyrene, polyurethane, etc.) or natural (wood wool, hemp, cellulose, cork, etc.). Thin multilayer insulators are controversial.

For general reference purposes, below is the expected price range, installation included. Please note: these do not include the additional work:

  • Insulating an inclined roof from the inside: between € 35 and 50 per m²;
  • insulating an inclined roof from the outside: € 40 to 60 per m²;
  • insulating a flat roof from the inside: € 30 to 40 per m²;
  • insulating a flat roof from the outside: € 70 to 115 per m².

Something to keep in mind
Insulating the attic floor (not habitable attic) instead of the roof is at least two times cheaper: smaller underside surface, little to no finish required, quick and simple installation.

Do not forget the additional work

Whether from the inside or outside, insulating a roof involves additional work:

  • interior finishing: plastering, installation of the panels, painting, upholstering, repairs or changes to be made to the electrical circuits;
  • outside roofing: removal and installation of tiles, replacement of battens, installation or replacement of the lining membrane.

What grants am I entitled to?

Energy grants are revised each year on January 1st. Visit the Brussels Environment website to find out the applicable grant award conditions.

Invoices dated up to 31 December may be sent in to Brussels Environment after the start of the following year (see detailed regulations for each grant).

How to calculate the profitability of these works?

A poorly insulated roof causes up to 30% heat loss in a semi-detached building. The average household in Brussels spends € 1,600 per year on energy, of which 60% for heating, i.e. € 960.
Which means that insulating your roof can reduce your gas bill from € 960 to € 672. That is a gain of at least € 288 per year.

Let us run a simulation:

  • You have a 100 m² sloping roof of a simple shape, easily accessible;
  • You are having at least 20 cm of glass or rock wool fitted;
  • The contractor bills you € 22/m² for the insulating material and € 25/m² for delivery and installation + 6% VAT);
  • You benefit from the intermediate grant because your income is average and you are not entitled to a grant bonus.

Financial outcome:


100 x 22 = € 2,200
100 x 25 = € 2,500
Total: € 4700 excluding VAT
+ 6% VAT: € 282
€ 4,982


100 x 30 = € 3,000

Actual cost

€ 1,982

Annual savings made on
heating costs

€ 288

Payback period

7 years maximum

This estimation can be greatly shortened if: 

  • your heating costs are higher than € 960/year;
  • you are on low income, in which case the grant goes up to € 40/m², or € 4,000 in our example;
  • your home is located inside the EDRLR (Enhanced Housing and Renovation Development Area) zone or in the ZRU (Urban Renewal Zone) zone, in which case you will receive an automatic bonus of 10% of the grant amount (+ € 300 of grants in our example).
  • you use natural insulating materials, in which case you get a grant bonus of € 10/m² (+ € 1,000 of grants in our example).

Is it better to hire a professional to insulate a roof?

Before embarking on this work yourself, consider the following:

  • Do you know how to properly place the insulation material?
  • Do you know what the required minimum thickness of insulation is to qualify for grants? This varies according to the performance of the chosen insulating material.
  • Do you know the difference between roof lining and vapour barrier ? Would you know how to place them correctly?
  • Do you have the time to do this work, knowing that a professional will be much faster?

If in doubt, also remember that the DIY savings are rather small.

  • you will buy the insulating material with 21% VAT (instead of 6% on the contractor's invoice)
  • you will not benefit from the same trade price from the supply dealer as a professional.
  • grants are also lower if you do the work yourself.

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