Why solid wall insulation matters in London
Most pre-1930 London housing has solid 9-inch (225mm) brick walls — U-value approximately 2.0 W/m²K. Modern Part L target for refurbished walls is 0.30 W/m²K — a 6.5× improvement. Heat loss through uninsulated solid walls accounts for 30–40% of total heat loss in a typical Victorian terrace. Insulating saves 20–30% on heating bills, raises EPC rating significantly (often 1–2 bands), and pairs essentially with any heat pump install.
Internal wall insulation (IWI)
Build-up: existing brick wall → vapour control layer → PIR or PHENOLIC insulation board (typical 60–90mm gives 0.30 W/m²K) → plasterboard → skim. Total wall thickness gain: 80–110mm room-side, so room dimensions shrink. Cost £80–£150 per m² of wall face. Detailing critical to avoid interstitial condensation — vapour barrier on warm side, careful detail at floor/ceiling/window reveals. Cold-bridging at junctions causes mould risk. Best done as part of full room renovation when finishes are off.
External wall insulation (EWI)
Build-up: existing brick wall → mineral wool or EPS insulation (90–150mm) → render system (silicone, acrylic or mineral) or brick-slip facing. Thermal performance better than IWI (no interruption at floors/ceilings), no room size loss, no internal disruption. But: changes external appearance (almost always refused planning in conservation areas), requires render maintenance every 10–15 years, and adds 100–180mm projection requiring eaves/sill/window detailing rework. Cost £140–£220 per m².
