Walls are the second largest source of heat loss after the roof, but the job is considerably heavier. Three techniques exist — external, internal and cavity fill — and the choice is not a matter of taste: it depends on whether you have a cavity wall, on planning constraints, and on how much living space you are willing to lose. In Wallonia the grant starts at €8.80/m² (€12/m² with bio-based insulation). In Flanders it has only been available to categories 3 and 4 since 1 March 2026.
The three techniques, and which to choose
External — the best, the most expensive
The insulation goes on the façade and is then covered with render or cladding. Technically it is the superior solution: it eliminates thermal bridges, costs no living space, protects the wall from thermal shock and lets you stay at home during the works.
Its limits are administrative and aesthetic. It changes the appearance of the façade, which almost always requires a planning permit and runs into municipal regulations, listed or exposed-brick façades, and building-line constraints on terraced houses. It is also the most expensive.
Internal — the fallback
The insulation goes on the inside, behind a stud wall. Cheaper, permit-free in most cases, doable room by room — but with three real drawbacks: loss of living space (5 to 12 cm per wall), thermal bridges remain at floors and partition walls, and a condensation risk inside the wall if the vapour barrier is poorly fitted.
That last point is not theoretical: badly executed internal insulation can damage the wall. It is the technique where installation quality matters most.
Cavity fill — the best cost-to-gain ratio, where possible
If your house has a cavity wall (two leaves separated by a gap, common from the 1930s-1950s onward), insulation is injected into the cavity from outside through small drill holes. No permit, no loss of space, a single day's work for an average house, and a cost far below the other two techniques.
Two conditions: the cavity must be wide and clean enough, and the outer leaf in good condition. A prior endoscopic inspection is essential — filling a damp or obstructed cavity creates lasting problems.
Wallonia: €8.80/m² base, audit mandatory
The wall insulation grant starts from a base amount of €8.80/m², raised to €12/m² if you use bio-based insulation. That amount is multiplied by the coefficient of your income category:
| Category | Reference income | Coefficient | Invoice cap |
|---|---|---|---|
| R1 | ≤ €28,900 | × 6 | 70% incl. VAT |
| R2 | €28,900 – €41,100 | × 4 | 70% incl. VAT |
| R3 | €41,100 – €54,300 | × 3 | 50% incl. VAT |
| R4 | €54,300 – €122,800 | × 2 | 50% incl. VAT |
Unlike the roof, wall insulation is not exempt from the home energy audit. It must be carried out by an accredited auditor and the report registered before the invoice for the works, failing which the file is rejected.
⚠️ Walloon deadline of 30 September 2026. The current Primes Habitation scheme runs from 14 February 2025 to 30 September 2026. Every application — audit as well as works, final invoice included — must be submitted before that date. A reform follows, whose terms are not settled.
Flanders and Brussels
In Flanders, the tightening of 1 March 2026 removed the insulation grant for income categories 1 and 2. It remains available to categories 3 and 4: category 4, the lowest incomes, receives 50% of the invoice within a €6,000 works limit, i.e. a maximum grant of €3,000; category 3 receives 35% within a €4,200 limit, i.e. €1,470.
In Brussels, Renolution grants have been suspended since 1 January 2025 and no replacement has been decided. What remains: 6% VAT for a home older than ten years, municipal grants and the ECORENO credit.
Verdict
Cavity wall? Have the cavity inspected first. If it is usable, this is by far the best euro spent in the whole energy renovation — and the shortest job.
Solid wall and unconstrained façade? External insulation is the right technical answer. Plan for the planning permit, which noticeably lengthens the timeline — worth factoring in if you are targeting the Walloon September 2026 deadline.
Constrained façade? Internal insulation remains valid, provided the vapour barrier is fitted by a professional who knows what they are doing. It is the technique least tolerant of approximation.