Roof insulation: at the ceiling or along the slope

Insulating the loft floor, between and over the ceiling joists, is the classic approach. The loft above stays cold and ventilated, the rooms below stay warm, and the work does not touch the roof covering. It suits lofts used only for occasional storage. Boarding over the insulation for storage should be done on raised supports so the insulation is not squashed.

Insulating along the roof slope, between and under or over the rafters, brings the loft inside the warm part of the house. That is the route for rooms in the roof, lofts that hold a boiler, water tanks or ductwork, and any future conversion. It can be done from inside the loft, or from above during a re-roof, where rigid insulation over the rafters avoids cold bridging through the timbers.

Warm indoor air carries moisture. When it meets a cold surface in the roof it condenses, and over years that can rot battens and rafters. A cold loft relies on ventilation at the eaves and sometimes at the ridge, which is why insulation must not be pushed into the eaves so that it blocks the airflow. A warm roof relies on a vapour control layer on the warm side and a breathable membrane on the cold side, installed with care around every pipe and cable.

Downlights, loft hatches and pipes through the ceiling are the usual leaks of warm, moist air. A careful installer seals them or fits covers designed for the purpose. Older houses with no roofing felt at all behave differently again, and need an approach that lets the roof keep drying out.

Spray foam has been sold widely as a quick fix for roofs. Research commissioned by the government found low risk for open-cell foam applied in line with the relevant British Standard, but higher moisture and timber decay risk in several combinations, particularly in colder climates, where high-resistance underlays are used without a vapour control layer, and above all where foam is sprayed directly onto the tiles or slates. Separate research looked at how spray foam affects a roof's fire performance.

Some lenders and surveyors also raise spray foam during a sale because the timbers behind it cannot be inspected. If a salesperson calls unannounced offering spray foam, ask for a survey report, the product certificate and the ventilation plan in writing, and take your time.

Adding insulation to a loft on its own does not need Building Regulations approval, and GOV.UK lists insulation in roof spaces among exempt projects. Once insulation is part of renovating the roof, for example when more than a quarter of the roof is being renewed according to the Planning Portal guidance, it has to meet the standard in the Approved Documents, or the best that can be achieved within a reasonable payback where the full standard is not feasible.

Planning permission is not normally needed where the outside of the house does not change. Insulating over the rafters raises the roof line slightly, which can matter in a conservation area or on a listed building.

The installation of insulation in homes is currently zero-rated for VAT, and low-income households may qualify for help through the schemes listed on this page. Check eligibility with the scheme or your council before you sign.

• where the insulation goes: joists, between rafters, over or under rafters
• material, thickness and the resulting insulation standard
• how the eaves and ridge ventilation is kept or provided
• vapour control layer and membrane, and how penetrations are sealed
• loft hatch insulation and draught-proofing, and treatment of downlights
• what happens to stored items, old insulation and any water tanks and pipes

The request reaches roofing and insulation businesses without your name, number or email. One business, and only one, can buy it; the request then disappears from the list. Before it sees your details, we email you who it is, including its address and phone number. There is nothing to pay on your side.

It can happen that nobody buys the request within 21 days, in which case it is withdrawn, and you never have to accept a quote. Offrano checks that buyers are registered businesses but does not check their workmanship, so if a grant is involved, confirm that the installer meets that scheme's requirements.