Energy & insulation
Roof insulation: glass wool, PIR or cellulose?
Glass wool, PIR or cellulose: thickness for R = 6, installed price per m², moisture behaviour, summer comfort, and which insulation to choose for your roof.
By the Batizzy teamPublished Updated 5 min read
Nobody picks roof insulation from a catalogue. It all depends on the space under the roof, access to the loft and what you expect from the house in the middle of July. In Belgium, three materials come up in most quotes: glass wool, PIR boards and cellulose. They don't cost the same, don't need the same thickness and don't react to moisture in the same way. Here is what you need to decide.
Why start with the roof
In an uninsulated house, the roof is among the biggest sources of heat loss: warm air rises, and a loft without insulation lets it escape. It is also a fairly simple job when the loft is accessible, which often makes it the first step of an energy renovation.
The three materials compared
| Criterion | Glass wool | PIR board | Cellulose |
|---|---|---|---|
| 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 | 19 to 24 cm | 13 to 14 cm | 23 to 24 cm |
| Installed price (per m²) | €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 | good |
| Behaviour with moisture | sensitive | very good | good |
The thickness follows directly from the conductivity: to reach R = 6, multiply 6 by the material's λ. The lower the λ, the thinner the layer can be.
Glass wool, the standard
It is the most common insulation. Sold in rolls or semi-rigid slabs, it goes between the rafters and is then covered with a vapour barrier on the warm side.
It suits a pitched roof with an accessible underlay, a habitable or unused loft with no damp problem, and tight budgets: it is the cheapest of the three.
Its limits: you need a good thickness, it copes badly with getting wet (damp wool loses much of its effectiveness), and making it takes a lot of energy.
PIR boards, performance per centimetre
Polyisocyanurate is a rigid, high-density board. Its λ is the lowest of the three, which makes it the material that needs the least thickness for a given performance.
You choose it when every centimetre counts: low lofts, attic rooms, a flat or shallow-pitched roof, a renovation where you don't want to lose headroom.
Its limits: it costs noticeably more per installed m² (see the table), it comes from petrochemicals, is hard to reuse at end of life, and its fire behaviour depends on the board and the inner lining. Have the fire classification stated in the quote.
Cellulose, the ecological choice
Made from recycled newspaper, cellulose is blown loose into unused lofts or packed into the cavities of a pitched roof.
It has two real strengths: a favourable carbon footprint, and good summer comfort thanks to its density, which slows heat on hot days. Blown in, it also reaches awkward corners.
Its limits: badly installed, it settles over time, hence the need for an experienced installer. It needs a vapour barrier on the warm side, storage space on site and protection against rodents.
What 80 m² of roof costs
Illustrative example for 80 m² of roof insulated to R = 6, at the middle of each price bracket:
| Glass wool | PIR | Cellulose | |
|---|---|---|---|
| Installed price per m² | €27 | €46.50 | €34 |
| Total for 80 m² | €2,160 | €3,720 | €2,720 |
For a private home more than 10 years old, VAT is 6% if the contractor supplies and installs the insulation. What you save on heating depends too much on the house to put a figure on here. In Wallonia, the PEB certificate now shows how the label could change after the recommended work.
Aid in 2026
The per-job grants that used to fund roof insulation are gone in Wallonia and on hold in Brussels:
- In Brussels, no Renolution grant can be claimed for work invoiced in 2025 or 2026. The Ecoreno loan remains available under conditions. See our update on Brussels support.
- In Wallonia, the Prime Habitation ended on 30 September 2026. Rénopack finances insulation within a whole project that gains a PEB label, and adds 5% to the share you never repay when all envelope work uses eco-materials or bio-based materials. If you are aiming for that top-up, check which insulation products qualify before you choose. It is all in our article on the end of the Prime Habitation.
- In Flanders, Mijn VerbouwPremie supports roof insulation, with technical requirements and amounts that depend on your profile.
Our advice, roof by roof
For most houses being renovated:
- Accessible unused loft: blown cellulose. The best balance of price, performance and summer comfort.
- Habitable loft with space: glass wool. Reliable, known to every installer, and the cheapest.
- Limited height or flat roof: PIR, when every centimetre counts.
Avoid mixing materials (PIR and glass wool, for instance) without a thermal engineer's advice.
What to demand in the quote
Ask for the target R-value, the thickness and the exact product reference, in black and white. Those are the details an auditor, a lender or Mijn VerbouwPremie will ask you for, and they let you compare quotes at equal performance. For blown cellulose, ask how the installer prevents settling.
On Batizzy, post your insulation project and compare quotes from contractors in your area side by side: find an insulation contractor. For the budget, also read our guide to insulation prices and the one on 6% VAT.