Internal vs external solid wall insulation costs roughly £5,000 to £13,000 more for external, but it saves more energy.
If your home has solid walls, you have two main options for insulation: internal or external. The choice often comes down to cost versus disruption and energy savings.
External solid wall insulation typically costs £8,000–£22,000 for a semi-detached house, while internal solid wall insulation costs £4,000–£10,000 (Energy Saving Trust, 2026). External insulation can cut heat loss by 40–50%, compared to 35–45% for internal insulation (DESNZ, 2026).
The higher price of external insulation covers scaffolding, specialist render, and full wall coverage. Internal insulation is cheaper because it avoids scaffolding and uses simpler materials, but it reduces floor space by 50–100mm per wall. The energy savings gap is narrow, so your decision may depend more on budget and disruption.
How solid wall insulation works and the two methods differ
Solid walls have no cavity, so they lose heat much faster than cavity walls. A typical solid wall has a U-value of 2.1 W/m²K, while insulated solid walls achieve 0.30–0.35 W/m²K (Building Regulations Approved Document L1A, 2026 edition).
External insulation involves fixing rigid insulation boards (typically mineral wool or PIR) to the outside wall, then rendering or cladding over them. Internal insulation uses insulation boards (PIR, phenolic, or wood fibre) fixed to the inside wall, then plastered or drylined.
The key difference is that external insulation wraps the whole wall, eliminating thermal bridges where heat escapes at floor and ceiling junctions. Internal insulation leaves cold spots at these junctions, which can reduce overall performance.
Quick numbers, cost, energy saving, and U-value comparison table
| Measure | Internal insulation | External insulation | Source |
|---|---|---|---|
| Typical cost (semi-detached house) | £4,000–£10,000 | £8,000–£22,000 | Energy Saving Trust cost table, 2026 |
| Floor space lost | 50–100mm per wall | None | Building Research Establishment (BRE) guidance |
| Achievable U-value | 0.30–0.35 W/m²K | 0.30–0.35 W/m²K | Building Regulations Approved Document L1A |
| Typical energy saving | 35–45% | 40–50% | DESNZ Solid Wall Insulation Impact Assessment |
The direct answer to “what is solid wall insulation”, a layer of insulation fixed to a wall without a cavity
Solid wall insulation is a continuous layer of insulating material applied to either the inside or outside of a wall that has no cavity. Its purpose is to reduce heat loss through the wall, lowering energy bills and carbon emissions.
The insulation layer is typically 50–100mm thick, achieving a U-value of 0.30 W/m²K (Building Regulations Approved Document L1A). It is the primary method for insulating pre-1920s homes, which make up roughly 30% of UK housing stock (English Housing Survey, 2025–26).
You can also read more about comparing insulation types for older homes.
Eligibility, certification, and how to verify an installer
Installers must be MCS-certified for any work that involves renewable energy systems such as heat pumps, but not for insulation alone. However, the PAS 2030:2023 standard applies to solid wall insulation under the Energy Company Obligation (ECO4) scheme (GOV.UK, 2026).
For external insulation, the installer should hold a BBA-certified system (British Board of Agrément) for the specific insulation product used. For internal insulation, check the installer is TrustMark-registered and has a CSCS card for insulation work.
Always request a full written quote with U-value calculations and a 10-year insurance-backed guarantee for the insulation system. For ECO4-funded installations, the installer must be PAS 2030:2023 certified and the property must have an EPC rating of E, F, or G (Ofgem, 2026).
How internal insulation disrupts your home and what to plan for
Internal insulation requires moving radiators, sockets, and light switches away from the wall. This can cost £500–£1,500 extra per room (Energy Saving Trust cost table, 2026).
It reduces floor space by 50–100mm per wall, which can make small rooms feel noticeably smaller. The work is messy, takes 2–5 days per room, and the room cannot be used during installation.
Condensation risk increases if a vapour barrier is not correctly installed. This is a common issue that leads to mould (BRE, 2022). You may also want to read how to prevent condensation after internal insulation.
How external insulation changes your home’s appearance and planning rules
External insulation adds 100–150mm to the wall thickness, changing the house’s external look. The render or cladding colour and texture must match nearby properties.
Planning permission may be required if the house is in a conservation area, is a listed building, or if the insulation projects beyond the building line (GOV.UK, 2026). It is usually exempt from planning permission for most detached and semi-detached houses, but building regulations approval is mandatory for the structural fixings and fire safety.
The work takes 2–4 weeks for a semi-detached house, and scaffolding is needed for the entire duration.
