The UK's housing stock is among the least energy efficient in Europe. Around 29% of homes have solid walls and no cavity to insulate; many have inadequate loft insulation; most ground floors lose significant heat through the slab or joists. Retrofitting insulation to an existing home is the highest-return energy efficiency investment available — but the priority order matters. Insulating the ceiling before the walls before the floor is the standard retrofit sequence, but it depends on your specific house type and current state.
Priority Order
1. Loft insulation — the highest return retrofit measure for most UK homes. A semi-detached house with no loft insulation loses around 25% of its heat through the ceiling. Adding 270mm of mineral wool (the recommended depth) costs £300–£600 as a DIY project, or £500–£1,000 professionally installed. Payback period: 2–4 years. If you have any loft insulation but less than 270mm, topping it up is almost always worthwhile.
2. Cavity wall insulation — for post-1920s homes with a cavity. Blown mineral wool or bead insulation fills the cavity from the outside through drilled holes. Cost: £400–£1,000 for an average semi. Grants are often available — check the ECO4 scheme and Great British Insulation Scheme for eligibility. Warning: cavity wall insulation in exposed, rain-driven locations or where the wall already has damp issues can make problems worse. Get a survey first.
3. Solid wall insulation — for pre-1920s solid wall homes. External insulation (renders or cladding over an insulation board) is more effective but changes the appearance of the building. Internal insulation (dry lining with insulated plasterboard) reduces floor area slightly and involves boxing out window and door reveals. Cost: £8,000–£25,000 for an average semi, depending on method. Grants available under ECO4 and Social Housing Decarbonisation Fund.
4. Floor insulation — for suspended timber floors (wooden boards over a void). Rigid insulation boards between joists, spray foam, or blown mineral wool from below. For solid concrete floors, rigid insulation over the slab with a new screed on top. Cost: £1,500–£5,000+ depending on access and method.
Grants in 2026
The ECO4 scheme (Energy Company Obligation) provides fully or partly funded insulation for eligible households (low income, benefits, EPC D–G). The Great British Insulation Scheme (GBIS) offers subsidised insulation for homes with EPC D–G regardless of income. Local authority Flex funding can provide insulation to households in fuel poverty. Check the Ofgem website for current eligibility criteria.