Solid Wall Insulation for London Homes: Internal vs External, Costs, Planning, and Energy Savings

Key Takeaways

• ✓Internal wall insulation (IWI) is applied to the inside face of the external wall. The insulation is fixed directly to the existing brick wall (using adhesive and/or mechanical fixings) and then finished with a vapour control layer and plasterboard, or with a specialist insulated plasterboard (a composite board with insulation bonded to the back of the plasterboard). IWI reduces the internal floor area of each room treated (typically 75–125mm per wall, depending on the insulation thickness and system used — for a standard 3.5m x 4.5m Victorian bedroom, a 75mm IWI installation on the external wall reduces the room floor area by approximately 0.34m²). IWI requires all skirtings, architraves, window reveals, and electrical sockets on the insulated wall to be relocated or extended. IWI does not affect the external appearance of the building (no change to the external face, no planning implications in most cases), making it the default choice for conservation areas or where the external elevations cannot be altered. The most important technical challenge with IWI is the vapour control layer — the insulation must be designed with a correctly positioned vapour control layer to prevent interstitial condensation (condensation forming within the wall build-up where warm moist air from the room interior meets the cold outer brick). A poorly detailed or installed IWI system without an adequate vapour control layer can cause interstitial condensation and associated mould growth within the wall structure
• ✓External wall insulation (EWI) is applied to the outside face of the building. The insulation (typically expanded polystyrene (EPS), mineral wool, or phenolic board) is mechanically fixed and adhesive-bonded to the outside of the existing brick wall, and then finished with a proprietary render system (a basecoat with reinforcing mesh, and a final decorative render or silicone render topcoat). EWI eliminates the cold bridge at floor joist/ceiling joist level (where the timber floor structure intersects with the external wall in a Victorian building, creating a direct thermal path through the wall) — a significant advantage over IWI, which cannot easily address these thermal bridges within the floor zone. EWI does not reduce internal room sizes. EWI changes the external appearance of the building (the building becomes slightly wider and the wall surface texture changes from bare brick or painted brick to a rendered finish) — which is why EWI almost always requires planning permission in a conservation area (and often requires planning permission outside conservation areas where the change of external appearance affects the character of the street). For most London Victorian terrace front elevations in conservation areas, EWI on the front elevation is unlikely to receive planning permission; EWI on the rear elevation (where the change is not visible from the public street) is more likely to be acceptable. A combination approach (EWI on the rear and side return elevation where planning allows, IWI on the front elevation) is common in conservation area Victorian terraces
• ✓ECO4 and the Great British Insulation Scheme (GBIS) are UK Government-backed funding programmes that can contribute to or fully fund solid wall insulation for eligible households. ECO4 (Energy Company Obligation scheme 4): energy suppliers are obliged under ECO4 to fund energy efficiency improvements for low-income or fuel-poor households. Owner-occupiers and private renters in receipt of certain means-tested benefits, or with an EPC rating of D, E, F, or G, may be eligible for ECO4-funded solid wall insulation (the work is procured by the energy company's installer network, at no cost to the eligible household). To check eligibility and apply, contact the main energy suppliers (British Gas, E.ON, EDF, Octopus, etc.) or use the ECO4 eligibility checker at the Simple Energy Advice website. Great British Insulation Scheme (GBIS): a separate scheme targeted at households in the middle of the income distribution (not just the lowest-income households). GBIS provides partial or full funding for solid wall insulation for eligible households, with funding delivered through local authorities. Both schemes have time-limited funding allocations — check the current status of both schemes at the time of planning your insulation project, as eligibility criteria and funding availability change regularly. Where ECO4 or GBIS funding is not available, some local authorities operate their own area-based insulation schemes or provide loans for energy efficiency improvement — contact your London borough's energy efficiency or housing team for current local schemes
• ✓The thermal performance improvement from solid wall insulation depends on the insulation thickness and the U-value achieved. Indicative U-values achievable for different insulation solutions (approximate values for a 225mm solid brick wall): uninsulated 225mm solid brick wall: approximately 2.1 W/m²K; IWI with 50mm phenolic board (total wall build-up approximately 65mm): approximately 0.45 W/m²K; IWI with 100mm phenolic board (total wall build-up approximately 115mm): approximately 0.25 W/m²K; IWI with 70mm mineral wool or EPS plus plasterboard (total wall build-up approximately 100mm): approximately 0.35–0.40 W/m²K; EWI with 100mm EPS plus render (total wall thickness increase approximately 150mm): approximately 0.30 W/m²K; EWI with 60mm phenolic board plus render (total wall thickness increase approximately 100mm): approximately 0.25 W/m²K. The current Building Regulations Part L target U-value for external walls in new buildings is 0.18 W/m²K — significant insulation thicknesses are required to approach this standard in a solid brick retrofit. For most existing London Victorian terraces, a practical target of 0.25–0.45 W/m²K is achievable with solid wall insulation without unacceptably reducing room sizes or requiring very thick EWI systems. The EPC improvement from solid wall insulation on a Victorian terrace is typically 10–20 EPC points (for example, from EPC E (48) to EPC D (65) or C (75)), depending on the wall area treated and the baseline insulation level
• ✓Typical costs for solid wall insulation in London (2025–2026): internal wall insulation (IWI) for a mid-terrace Victorian house (treating rear elevation and side wall where present, excluding front elevation): £8,000–£18,000 supply and install, including plasterboarding and skimming, relocation of sockets and switches, and making good at reveals and skirtings. External wall insulation (EWI) for a mid-terrace Victorian house (treating rear elevation only, given conservation area front elevation restrictions): £6,000–£14,000 supply and install, including insulation boards, basecoat, render, and making good of window and door reveals. EWI on the full external surface of a detached or end-of-terrace Victorian house (all four elevations, no conservation area restriction): £15,000–£30,000. IWI for the full house (all external-facing rooms): £14,000–£28,000. These costs do not include ECO4 or GBIS grant funding where applicable (which can reduce or eliminate the homeowner's cost). The cost-benefit analysis of solid wall insulation (the payback period from energy savings) is approximately 15–30 years at current energy prices for a self-funded installation — but improves significantly where ECO4 funding is available (payback effectively immediate), where the installation is combined with other works (extension or refurbishment where the walls are already being opened up), or where the EPC improvement enables the property to qualify for a green mortgage product (which may offer a lower interest rate)