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Install internal wall insulation.
For damp solid walls — IWI with vapour control, breathable systems, or insulated plasterboard.
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Damp trapped behind insulation
Vapour from inside the home condenses on cold brick behind insulation, soaking the wall. Cause of many IWI failures.
Cold bridges
Where insulation stops at floor, ceiling, window — heat escapes and condensation forms. Continuous insulation essential.
Cement render externally + IWI internally
Wall can't breathe outward (cement) or inward (insulation). Solid-wall buildings need breathable solution.
Lost floor area
IWI eats 80–150mm of room. Worth calculating before committing.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
PIR insulation board per m²Celotex, Kingspan, EcoTherm
£20–£40/m²
Insulated plasterboard per m²Knauf XPS, British Gypsum
£25–£50/m²
Wood fibre breathable per m²Steico, Pavadry, Diffutherm
£40–£90/m²
Vapour control layer per m²Plastic VCL or breathable
£5–£15/m²
Install labour per m²Plus material
£20–£50/m²
Single wall (20 m²) PIR systemIncluding labour
£700–£1.5k
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Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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Internal wall insulation (IWI) cuts heat loss through solid walls — common in pre-1919 UK homes. But poorly specified IWI can trap damp behind it. Expect to pay £40–£120 per m² in the UK for solid wall internal insulation.
What Does the Work Involve?
Existing wall stripped to brick or original plaster. Vapour control layer applied (or breathable system used). Insulation layer: PIR boards (Celotex, Kingspan, 50–100mm thick), insulated plasterboard (Knauf XPS), or breathable wood fibre (Steico) for heritage. Plasterboard or skim finish. Critical: cold bridging at floor, ceiling, window reveals must be designed out.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| PIR insulation board per m² | 20 | 40 | Celotex, Kingspan, EcoTherm |
| Insulated plasterboard per m² | 25 | 50 | Knauf XPS, British Gypsum |
| Wood fibre breathable per m² | 40 | 90 | Steico, Pavadry, Diffutherm |
| Vapour control layer per m² | 5 | 15 | Plastic VCL or breathable |
| Install labour per m² | 20 | 50 | Plus material |
| Single wall (20 m²) PIR system | 700 | 1,500 | Including labour |
| Single wall heritage wood fibre | 1,500 | 3,500 | Premium specification |
| Whole house IWI (Victorian terrace) | 8,000 | 20,000 | All external walls |
How Long Does It Take?
Single wall (20 m²): 3–5 days including plastering and decoration. Whole house: 4–8 weeks.
DIY or Professional?
Professional. Vapour barrier detail, cold-bridge management, and breathability all need experienced contractor. Cheap install causes damp problems.
Choosing the Right Tradesperson
PCA insulation specialist or qualified plasterer/builder. SPAB-accredited for heritage homes. Confirm breathable strategy if solid-wall pre-1919. Get warranty in writing.
UK Regulations
Building Regs Part L thermal compliance for retrofitted walls. Listed buildings need consent. Ofgem ECO grants may cover for qualifying households.
Common Problems
- Damp trapped behind insulation: Vapour from inside the home condenses on cold brick behind insulation, soaking the wall. Cause of many IWI failures.
- Cold bridges: Where insulation stops at floor, ceiling, window — heat escapes and condensation forms. Continuous insulation essential.
- Cement render externally + IWI internally: Wall can't breathe outward (cement) or inward (insulation). Solid-wall buildings need breathable solution.
- Lost floor area: IWI eats 80–150mm of room. Worth calculating before committing.
Frequently Asked Questions
Will internal wall insulation cause damp?
Poorly specified, yes. Properly specified with appropriate vapour barriers or breathable systems, no. Solid-wall homes especially need breathable IWI.
PIR or wood fibre insulation?
PIR (Celotex, Kingspan): higher R-value per mm, cheaper, requires vapour barrier. Wood fibre (Steico): breathable, suits solid walls, more expensive. Modern UK heritage practice uses wood fibre.
How much will IWI save on heating?
25–40% reduction in heat loss through external walls. Combined with other upgrades (loft, windows): can halve heating bills.
External vs internal wall insulation?
External (EWI): more expensive (£80–£150/m²), no loss of floor area, doesn't change interior. Internal (IWI): cheaper, easier per-room phased upgrade. EWI is the cleaner long-term solution where possible.
Will IWI affect my property value?
Quality work increases value through energy efficiency. Poor work that causes damp decreases value significantly.
Can I get grants for IWI?
ECO4 and GBIS for qualifying households (means-tested or low EPC rating). Some local authorities have their own grants. Check via energy supplier.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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