Wallonia is the only region explicitly rewarding bio-based insulation: the grant's base amount rises from €20 to €26/m² for roofs, from €8.80 to €12/m² for walls and from €6 to €8/m² for floors. That is an uplift of roughly 30 to 35%, then multiplied by the income coefficient. The trade-off is a composition requirement: at least 70% bio-based content for boards, flexible insulation and loose fill.
What bio-based insulation is
Bio-based insulation is made from natural fibres of plant or animal origin, as opposed to mineral insulation (glass wool, rock wool) and synthetics (polystyrene, polyurethane). The most common on the Belgian market:
- Cellulose wadding — recycled paper, blown or injected. Excellent performance-to-price ratio, widely used in lofts.
- Wood fibre — in rigid or semi-rigid boards. The best thermal inertia of the group, valuable against summer overheating.
- Hemp and flax — in rolls or boards. Good moisture regulation.
- Cork — rot-proof, suited to moisture-sensitive locations.
Their interest is not limited to the carbon balance. At equal thermal performance they generally offer better decrement delay — the time heat takes to cross the element. In practice, under a well-exposed roof, bio-based insulation keeps the loft noticeably more comfortable in summer than mineral wool of the same thickness.
The Walloon uplift, in figures
The mechanism is simple: it is the base amount of the Prime Habitation that is raised, before the income coefficient is applied.
| Item | Conventional insulation | Bio-based insulation | Difference |
|---|---|---|---|
| Roof | €20/m² | €26/m² | +30% |
| Walls | €8.80/m² | €12/m² | +36% |
| Floor | €6/m² | €8/m² | +33% |
These amounts are then multiplied by the coefficient of your income category (× 6 in R1, × 4 in R2, × 3 in R3, × 2 in R4), within the limit of 70% of the invoice including VAT in R1-R2 and 50% in R3-R4. In practice it is often that cap which determines what is actually paid.
⚠️ The composition condition. To be recognised as bio-based, the material must contain at least 70% bio-based content for boards, flexible insulation and loose fill, or at least 25% for load-bearing elements, under standard NBN EN 16785-2:2018. This characteristic must be documented by the product datasheet: ask your contractor for it before ordering and attach it to the file.
The real calculation: does the uplift cover the extra cost?
That is the real question, and the answer depends on the item. Bio-based insulation is generally more expensive to buy than mineral wool — the gap varies widely by material and format.
The grant uplift is proportional to surface area. It therefore compensates all the better as the treated area is large and your income coefficient high. In R1 (× 6), the difference between €20 and €26/m² on a roof represents €36/m² of additional grant — an amount that comfortably covers the material premium in most cases. In R4 (× 2) the gap falls to €12/m², and the trade-off becomes tighter.
Two non-financial advantages also weigh in the decision: summer comfort, genuinely superior, and indoor air quality, since bio-based materials regulate moisture better.
Flanders and Brussels
Neither Flanders nor Brussels provides a specific uplift for bio-based insulation. In Flanders, Mijn VerbouwPremie works on the Rd value achieved and a percentage of the invoice, regardless of the material used — and since 1 March 2026 the insulation grant is only available to income categories 3 and 4. In Brussels, Renolution grants have been suspended since 1 January 2025.
Choosing bio-based insulation there therefore remains a purely technical and environmental decision, with no regional financial incentive.