Replacing windows is the item where people are most easily caught out: the grant does not depend on the price paid but on a technical coefficient that must appear on the quote. In Wallonia you need Uw ≤ 1.50 W/m²K for the complete window and Ug ≤ 1.10 W/m²K for glazing alone; the grant starts at €26/m² of opening, multiplied by the income coefficient. In Flanders the glazing grant is €64/m² capped at €1,280, but restricted to categories 3 and 4 since March 2026.
Uw, Ug, Uf: the vocabulary that decides the grant
Three coefficients circulate on quotes, and confusing them is expensive. They all express thermal transmittance in W/m²K — the lower the figure, the better the window insulates.
- Ug (glass): the glazing alone. This is the value manufacturers push, because it is the most flattering.
- Uf (frame): the frame and sash alone.
- Uw (window): the complete window, glazing and frame assembled. It is the only value that really counts, and the one administrations require.
⚠️ The classic trap. A quote advertising "Ug 1.0 glazing" without mentioning the Uw proves nothing: excellent glazing fitted in a mediocre frame can produce a Uw above the threshold and get the grant refused. Insist on the Uw value in writing on the quote, before signing — not on the invoice, where it will be too late.
Wallonia: €26/m² of opening, two thresholds to meet
Walloon technical requirements are twofold: Uw ≤ 1.50 W/m²K for a complete window, and Ug ≤ 1.10 W/m²K when replacing glazing only. The grant starts from a base amount of €26/m² of opening, multiplied by the coefficient of your income category: × 6 in R1, × 4 in R2, × 3 in R3 and × 2 in R4, giving roughly €52 to €156/m² of opening.
As with other items, the grant cannot exceed 70% of the invoice including VAT in R1 and R2, nor 50% in R3 and R4. And crucially: since 14 February 2025 the home energy audit is mandatory before the works — windows do not benefit from the exemption granted to roofs.
The scheme closes on 30 September 2026, final invoice included. Given window manufacturing lead times, often several weeks, that is a concrete constraint to factor in now.
Flanders and Brussels
In Flanders, the grant for high-performance glazing is €64/m², capped at €1,280, with no limit on the number of square metres within the invoice cap. Since 1 March 2026 it is reserved for income categories 3 and 4: categories 1 and 2 no longer qualify.
In Brussels, Renolution grants have been suspended since 1 January 2025. What remains: 6% VAT for a home older than ten years, municipal grants and the ECORENO credit.
Should you really start with the windows?
It is the most visible and most heavily sold measure — rarely the most profitable. Per euro invested, the energy gain from windows is markedly lower than from roof insulation, and often lower than from wall insulation.
Three cases where replacement is nevertheless justified:
- Single glazing — there the gain is massive and immediate, on comfort as much as on the bill.
- Windows at end of life — rotten timber, dead seals, broken hardware: replacement is needed anyway, so capture the grant.
- Double glazing from the 1980s-1990s — performance is far below today's standards.
Moving from recent, high-performance double glazing to triple glazing, by contrast, rarely pays for itself on the energy bill alone. Triple glazing is mainly justified in a house that is otherwise very well insulated, or for acoustic comfort.
New windows make a house far more airtight. Without suitable ventilation, moisture no longer escapes: condensation, mould, stale air. If you replace all your windows, the ventilation question is not optional — it is part of the job.