Where to start
Heat leaves a house through every surface, and the least costly surface to treat is almost always the roof. The usual order is loft, walls, floor and draughts, with windows last, because glazing is the most costly improvement for the heat it saves.
The loft
The recommended depth of mineral wool insulation in a loft is 270 millimetres, the figure used by the Energy Saving Trust. Many lofts insulated decades ago have 100 millimetres or less. The first layer is laid between the joists and the second across them, at right angles, to cover the timber.
- Insulating the loft floor makes the loft itself colder. Water tanks and pipes up there are insulated too, or they can freeze.
- The gap at the eaves is left clear, so that air can move through the roof space and carry moisture away.
- The loft hatch is insulated and draught-proofed.
- Insulation squashed under boards loses much of its effect. Raised supports allow boarding above the full depth.
- A damp loft is cured before it is insulated.
Walls: first find out which kind
What can be done to a wall depends on how it was built.
- Cavity walls are two skins of brick or block with a gap between. They became standard between the wars. In a cavity wall every brick shows its long side, and the wall is more than about 260 millimetres thick at a door or window.
- Solid walls have no gap. The brickwork shows a pattern of long sides and short ends, the ends being bricks laid across the wall to tie it together. Stone walls are solid, and thicker.
Cavity wall insulation is blown in through small holes drilled in the outer skin, by a specialist installer, in a few hours. It is not suitable for every wall: where the outer skin is in poor repair, or faces severe driving rain, filling the cavity can carry damp across to the inside. A survey comes first, and the work should carry an independent guarantee.
Solid wall insulation is applied to one face of the wall. Fixed outside, it is covered with render or cladding, which changes the look of the house and may need planning permission. Fixed inside, it takes a few centimetres off each room and means refitting skirting boards, sockets and radiators. Either is a large job. Old solid walls depend on moisture being able to evaporate, and need materials chosen with that in mind, as explained under painting the outside of a house.
Floors
Suspended timber ground floors, found in most older houses, can be insulated between the joists from below where there is a cellar or crawl space, or by lifting the boards. The airbricks in the outside wall are never blocked: they keep the floor timbers dry. Solid concrete floors are insulated when they are replaced.
Draughts and ventilation
Sealing gaps round windows, doors, letterboxes, loft hatches and unused chimneys is quick and simple. It has a limit. A house needs ventilation to carry away moisture, and a room with an open fire or an open-flued appliance needs air for safety. Trickle vents, extractor fans and airbricks are left working. A house made airtight without ventilation gets condensation and mould, described under damp and mould.
Spray foam
Foam sprayed onto the underside of a roof is sold as insulation. It can hide the roof timbers from inspection and trap moisture against them, and some mortgage lenders and equity release providers decline homes that have it. It is worth independent advice before, not after.
Help with the cost
Schemes change often, and the position in England in late 2026 is this:
- The Great British Insulation Scheme closed on 31 March 2026.
- ECO4, the obligation on energy suppliers to fund insulation and heating for low-income households, was extended to 31 December 2026, and the government's Warm Homes Plan, published on 21 January 2026, announced no successor to it.
- Warm Homes: Local Grant is run through local councils for low-income households in privately owned homes with an energy performance rating of D to G.
- Social landlords are funded separately to improve their own homes.
Scotland, Wales and Northern Ireland run their own schemes. The official starting point for England and Wales is the government's energy efficiency service.
Nobody offering free insulation at the door or by telephone is acting for the government. Work under the official schemes is carried out by installers registered with TrustMark, the government-endorsed quality scheme.
Rented homes
A privately rented home in England and Wales must normally have an energy performance rating of at least E before it can be let.