Insulating the floor above a cellar or crawl space

A suspended timber floor rests on joists above a void, with airbricks in the outside walls ventilating the space. You can often tell by the hollow sound, gaps between boards and airbricks just above ground level. A solid concrete floor has none of these and is insulated in a completely different way, usually only during a major renovation because it means raising the floor level.

Some houses have a proper cellar below part of the ground floor and a shallow void under the rest. Each part may need a different approach, and the survey should say which rooms are over which.

From a cellar or a void deep enough to crawl into, insulation can be fixed between the joists from below without disturbing the rooms above. This is the least disruptive option and the one this service is named after.

Where there is no access below, the floorboards are lifted room by room, insulation is supported between the joists and the boards relaid. It means emptying rooms and often replacing some boards, so it is often combined with a new floor finish.

In some voids, installers use systems that spray or fix insulation to the underside of the boards robotically through a small opening. Ask how the result is checked and what guarantee comes with it, because the installer cannot see every corner directly.

Airbricks keep the joists dry. Insulating the floor makes the void colder, so good cross-ventilation matters more afterwards, not less. Blocked or buried airbricks, raised ground or patios built up against the wall are common causes of rot in timber floors, and the installer should check them before insulating.

Pipes in the void are now on the cold side of the insulation. They need to be lagged, and any leaks fixed first. Electrical cables should not be buried in insulation where they could overheat; an electrician may need to look at old wiring before the work starts.

• access route to the void and how deep it is
• number and condition of airbricks, and whether any need clearing or adding
• moisture readings of joists and any signs of rot or woodworm
• the insulation material and how it is held in place
• how draughts at the skirting and around pipes will be sealed
• what happens to floor finishes and furniture if boards are lifted

If the cellar stays unheated, insulating its ceiling keeps the rooms above warmer and makes the cellar colder. Check what the cellar is used for: a boiler, a meter or a washing machine may need the extra warmth, and damp cellars can become damper when they get colder.

If you plan to convert the cellar into living space later, insulating its ceiling now is the wrong order. The walls and floor would then need insulating and waterproofing instead, which is a basement conversion rather than floor insulation.

Underfloor insulation is one of the measures named in England's Warm Homes: Local Grant and in the Home Energy Scotland grant, which lists up to £1,500 for loft, cavity and underfloor insulation. Low-income households may also qualify through ECO4 before it ends. The installation of insulation in homes is currently zero-rated for VAT.

Source: gov.uk

Say which rooms are affected, whether there is a cellar or a hatch to the void, and when the house was built. The request goes, without your name, to insulation installers in your area; one can buy it. We tell you by email who that is before the installer sees your contact details. It is free for you, and you can turn down any quote.