Wall insulation: cavity, external or internal?

Three routes for external walls

The three routes are cavity filling, external insulation and internal insulation. A suitable cavity and the finishes that must remain help determine the choice.

Identify the wall before choosing insulation

A solid wall has no cavity to fill. Building age offers clues but does not prove the construction. Extensions and original rooms may have different wall types. Ask for the proposed method to be identified for each elevation rather than assuming one solution suits the whole home.

Can different walls use different insulation methods?

Yes. A protected street facade may need an internal system, while an accessible rear wall can sometimes be insulated externally. A suitable cavity in an extension may allow a third approach. Each wall needs its own assessment, and the systems must connect at corners, floors and roof edges.

Choosing different methods does not remove the need for one coherent thermal and air-control boundary. The internal wall guide covers room-space and moisture implications; the external wall guide covers facade finishes and altered junctions.

Damp and thermal bridges need attention

Damp patches, damaged pointing and leaking rainwater pipes need investigation. Insulation does not repair these defects. Record where and when damp appears and mention earlier repairs.

Junctions at floors, windows and the roof can interrupt the insulation. These thermal bridges need details, especially when you renovate in stages. Coordinate new windows and insulation before fixing sill sizes or finishing the facade.

Specify a system, not just a material

EPS, mineral wool, PIR and bio-based materials have different applications. Fixings, vapour control and finishes form part of the system. Ask for the complete construction and thermal resistance. If air leakage is reduced or windows are replaced, review how the rooms will be ventilated.

Costs and price factors

Indicative installed costs are €15 to €40/m² for cavity filling, €45 to €65/m² for an insulated internal lining and €100 to €140/m² for external insulation with render, all excluding VAT. These are different wall systems. Repairs, altered services and complex junctions may add to the budget.

Stroomversneller is a quote request platform for insulation companies. Describe your home and intended work; the company assesses what is suitable and prepares its proposal.

Grants: check the region and timing

Belgian support depends on the property’s region, applicant, work and application date. UK grant schemes do not apply automatically. Check the procedure before committing to the work, including any required prior audit. Read our dated overview for Flanders, Wallonia and Brussels. A quote request is not a grant application or confirmation of eligibility.

Frequently asked questions

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