Roof insulation is the number-one energy gain area in a Belgian home. It is also the area where the choice of material has the biggest impact on the bill, comfort and grant eligibility. Here is a numbers-based comparison to choose between the 3 dominant insulators in Belgium in 2026.
Why start with the roof
In an uninsulated Belgian home, up to 30% of heat loss escapes through the roof. It is also the area offering the best cost-to-energy-savings ratio. Payback is usually reached in 6 to 9 years, against 12 to 18 years for wall insulation or window replacement.
The 3 insulators compared
| Criterion | Glass wool | PIR panel | Cellulose wadding |
|---|---|---|---|
| Conductivity (λ in W/m.K) | 0.032 to 0.040 | 0.022 to 0.024 | 0.038 to 0.040 |
| Thickness for R = 6 m².K/W | 22 to 24 cm | 13 to 14 cm | 22 to 24 cm |
| Material price (€/m² installed) | 22 to 32 € | 38 to 55 € | 28 to 40 € |
| Lifespan | 30 to 40 years | 40 to 50 years | 40 to 50 years |
| Carbon footprint | Average | High | Very good |
| Behaviour with humidity | Sensitive | Excellent | Good |
| Renolution grant Brussels 2026 | Yes (€50/m²) | Yes (€50/m²) | Yes (€50/m²) |
Glass wool: the standard
It is by far the most-used material in Belgium. Sold in rolls or semi-rigid panels, glass wool is installed between the rafters and then covered with a vapour barrier on the warm side.
When to choose it
- Pitched roof with easy access to the underside
- Tight budget (the cheapest)
- No major humidity or thermal-bridge issue
- Convertible or unused attic spaces
Limits
- Lower performance per cm than PIR (so greater thickness)
- Sensitivity to humidity: if it gets wet, it loses 50 to 70% of its performance and does not dry out properly
- Average carbon footprint (energy-intensive manufacturing)
PIR panel: maximum performance
Polyisocyanurate, a high-density rigid panel. The best lambda on the market, so the insulator that requires the least thickness to reach a given performance.
When to choose it
- Limited height under the roof (low attics, mansards)
- Flat or low-pitched roof
- Renovation where every cm counts
- Aiming for maximum performance to reach a PEB A rating
Limits
- Price: 40 to 70% more expensive than glass wool
- High carbon footprint (petrochemical material)
- Not reusable at end of life, complex recycling
- Fire reaction: class E to F, requires a fire-rated interior cladding
Cellulose wadding: the eco-friendly choice
Made from recycled newsprint. Blown loose into unused attics or injected into the cassettes of a pitched roof.
When to choose it
- Unused attics with difficult access (blowing reaches every corner)
- Strong ecological intent: negative carbon footprint (paper stores CO₂)
- Summer comfort: thermal phase shift of 8 to 12 hours, against 4 to 6 hours for glass wool
- Natural humidity regulation
Limits
- Gradual settling if poorly installed: requires a certified installer
- Needs a vapour barrier on the warm side (humidity)
- Large storage volume on site
- Sensitive to rodents if not properly protected by anti-pest grilles
The 2026 profitability calculation
On an 80 m² roof insulated to R = 6 (minimum target for grants), for a previously uninsulated home:
| Glass wool | PIR | Cellulose | |
|---|---|---|---|
| Total installed cost | €2,160 | €3,720 | €2,720 |
| Renolution grant Brussels | €4,000 | €4,000 | €4,000 |
| Net cost | €0 | €0 | €0 |
| Heating savings/year | €850 to €1,050 | €900 to €1,100 | €850 to €1,050 |
| Return on investment | Immediate | Immediate | Immediate |
Note: if you are not in the Brussels Region, Walloon or Flemish grants exist too but with different ceilings. See our article energy renovation grants 2026.
The Batizzy choice in 2026
For most Belgian homes under renovation, our panel of pros recommends:
- Unused attic with access: blown cellulose. Best price-performance-ecology ratio.
- Convertible attic with space: glass wool. Reliable standard, well-mastered installation.
- Limited height or flat roof: PIR. When every cm counts.
Avoid mixes (for example PIR + glass wool) except in very specific cases validated by a thermal engineer.
How to find a certified installer
To benefit from Renolution grants in the Brussels Region, the installer must be RESCERT-certified. In Wallonia, you must go through a BCE-registered contractor. In Flanders, the "EPB-premie" requires an EPB installation certificate.
On Batizzy, you filter pros by certification, area and customer reviews. Find an insulation installer near you.
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