Interior vs exterior wall insulation: cost per m² in Belgium (2026)
Short answer: According to isolatie-info.be, cavity wall insulation costs about €15–35 per m², interior insulation €40–95 per m² and exterior insulation with a new facade finish €120–300 per m² (all incl. VAT and installation). Bobex quotes €100–200 per m² outside (incl. VAT and finish). Exterior costs more but limits thermal bridges; cavity fill is the lowest-cost route when the cavity is suitable.
Prices in this article are indicative Belgian market ranges, not Alpha Services Benelux's price list. Your fixed price is in the quote.
What do interior and exterior wall insulation cost per m²?
Cavity fill is the lowest-cost method, interior insulation sits in the middle and exterior insulation costs most because you also pay for a new facade finish. These are Belgian market ranges, not a price list from a specific contractor.
| Method | Price per m² |
|---|---|
| Cavity wall insulation (injection) | €15–35 |
| Interior insulation (no finish) | €40–95 |
| Exterior insulation with new facade finish | €120–300 |
Mind the VAT basis: Bobex quotes €100–200 per m² incl. VAT for exterior and €30–100 for interior (VAT basis not stated); all-isol.be gives €80–250 outside and €55–80 for an inner lining, but excl. VAT. Always compare incl. and excl. VAT separately.
What drives the price
With exterior insulation the facade finish drives the price, far more than the insulation board itself.
| Finish | Price per m² |
|---|---|
| Render (crepi) | €120–155 |
| Timber cladding | €145–240 |
| Brick slips | €240–300 |
| Facing brick | from €300 |
Bobex is lower for render (€100–130) and quotes €200–250 for brick slips. Such gaps are normal.
- Scaffolding and access: tall facades or tight access add cost.
- Window sills and roof edges: often need adapting with exterior insulation.
- Substrate: damaged pointing or damp should be dealt with first.
- Cavity: rubble or old insulation may need clearing before injection.
Pros, cons and thermal bridges
Exterior insulation wraps the wall and limits thermal bridges best; interior insulation is cheaper but takes floor space and needs careful junction details.
- Exterior, pros: continuous layer, no loss of living space. The Walloon energy portal advises insulating as far outward as possible.
- Exterior, cons: highest price, new finish, a permit may be needed, not always possible on a protected or heritage facade.
- Interior, pros: lower price, room by room, facade unchanged.
- Interior, cons: all-isol.be says 5–10 cm lost per wall and a higher thermal-bridge risk if poorly executed. The Walloon portal advises against it with a vapour-resistant facing, a cracked facade, driving rain, or when junctions (wall-floor, wall-roof) cannot be continuous.
Have a professional assess the facade before you choose, especially with any damp.
How thick should the insulation be?
In Wallonia, for works that need a permit, the energy regulation sets a maximum U-value of 0.24 W/m²K for external walls, which means roughly 14 cm of mineral wool or 10 cm of PUR.
These figures come from the official portal energie.wallonie.be: 14 cm of mineral wool (lambda 0.045) or 10 cm of polyurethane (lambda 0.023). Bobex also uses 0.24 W/m²K as a reference value. Real thickness depends on the material and the existing wall; for Flanders and Brussels, check the requirements with your municipality or region. An uninsulated external wall can account for up to 25% of a home's energy loss.
Permits and facade finish by region
In Flanders, exterior insulation has been permit-exempt under conditions since 1 March 2026; Brussels and Wallonia follow other rules, and the municipality can also matter.
| Region | Visible facade | Thickness limit |
|---|---|---|
| Flanders | permit-exempt as long as the building line (rooilijn) is not crossed (a front facade standing on the building line is therefore usually not covered); exceptions e.g. for heritage, plus allotment and permit conditions | 26 cm, usual finish included |
| Brussels | permit required for a facade visible from the public road | none stated |
| Wallonia | no permit if the look stays the same | 30 cm |
In Brussels and Wallonia, a new visible finish (different cladding, brick slips) often needs a permit according to Bobex. For Flanders, the conditions of the exemption decree decide, for example for heritage buildings; always check with your municipality. More on our work: renovation.
When is cavity wall insulation the lowest-cost option?
If the house has an empty, dry and suitable cavity, injection at €15–35 per m² (isolatie-info.be) is by far the lowest-cost option.
nu-isoleren.be quotes €21–25 per m² excl. VAT (about €22–26.50 with 6% VAT) and savings of up to roughly 25% on heating. Since 1 March 2026 only income categories 3 and 4 can still get the Flemish Mijn VerbouwPremie for wall insulation; the cavity must then be at least 50 mm wide and filled to STS 71-1 (isolatie-info.be). Check vlaanderen.be/mijn-verbouwpremie. Checks beforehand: is the cavity dry, free of rubble or old insulation, and is the pointing in good order? A solid wall without cavity (often in older homes) rules this out, leaving inside or outside; let the contractor assess the wall build-up.
Comparing quotes, VAT and subsidies
Compare quotes on the same basis: area in m², insulation material and thickness, finish, scaffolding, VAT rate and what repairs are included.
- Ask for a price per m² and a total, stating incl. or excl. VAT.
- Window sills, roof edges, downpipes: included or not?
- Thickness and lambda value of the insulation should be on the quote.
- Finish: render, brick slips or cladding differ widely in price.
The 6% VAT rate applies to dwellings at least 10 years old, for private use, where a registered contractor invoices the end consumer; materials you buy yourself stay at 21%. On subsidies, amounts change often: check the official portals (vlaanderen.be, renolution.brussels and Homegrade, energie.wallonie.be) before you start. For a fixed written quote after a site visit, see our renovation page.
Fixed quote after a site visit
Alpha Services Benelux does the work with its own team, active since 2014. VAT BE 0541.320.178.
Request a quoteFrequently asked questions
Is interior or exterior wall insulation better?
Exterior is better: continuous layer, fewer thermal bridges, no lost floor space. It costs €120–300 per m² versus €40–95 for interior (isolatie-info.be). Interior is the fallback when exterior is not possible.
How much does wall insulation cost per m² in Belgium?
Indicative prices incl. VAT: cavity €15–35, interior €40–95, exterior with finish €120–300 per m² (isolatie-info.be). Bobex quotes €100–200 for exterior. Finish, scaffolding and substrate drive the price.
Do I need a permit for exterior insulation?
In Flanders, exterior insulation up to 26 cm (usual finish included) is exempt as long as the building line (rooilijn) is not crossed (omgeving.vlaanderen.be). In Brussels and Wallonia the rules differ; check with your municipality.
How thick should facade insulation be?
For U ≤ 0.24 W/m²K the Walloon energy portal gives about 14 cm of mineral wool or 10 cm of PUR. Exact thickness depends on the material and the existing wall; a contractor calculates it for your house.
Is cavity wall insulation really the lowest-cost option?
Yes, if a professional finds the cavity suitable (empty and dry): €15–35 per m² incl. VAT (isolatie-info.be), with up to about 25% heating savings (nu-isoleren.be). Without a suitable cavity it is not an option.
Sources
- Gevelisolatie prijs: overzicht per isolatiemethode — isolatie-info.be
- Vrijstellingenbesluit (BVR, februari 2026) — Departement Omgeving, Vlaanderen (omgeving.vlaanderen.be)
- Kansen voor energie en klimaat in het nieuwe vrijstellingenbesluit — VVSG (Vereniging van Vlaamse Steden en Gemeenten)
- Isolation des façades extérieures en Belgique : prix au m² et primes en 2026 — Bobex
- Les normes en vigueur pour l'isolation des murs en Belgique [2026] — Bobex (updated 12/08/2025)
- Les murs - L'Énergie en Wallonie — Service public de Wallonie (energie.wallonie.be)
- Spouwmuurisolatie prijs 2026: kosten per m² & besparing — nu-isoleren.be (updated 11 Oct 2026)
- Gevelisolatie: buitengevel, voorzetwand en prijs 2026 — all-isol.be
- 6% btw op renovatie in België: voorwaarden en recente wijzigingen — renocore.be (secondary; the official finance.belgium.be page was unreachable and should be re-checked)
- Mijn VerbouwPremie (in English) — Vlaanderen.be (portal pointer only, no figures taken)