Roof insulation at the rafters, in rooms in the roof and on flat roofs

If the loft is an unheated storage space, insulating the loft floor between and over the ceiling joists is almost always simpler and cheaper; see the loft insulation page. Insulating at rafter level makes sense when the roof space is heated or will be, or when there are rooms with sloping ceilings, dormer cheeks and short knee walls that need treating as a whole.

A room in the roof is several jobs in one: sloping ceilings, the flat ceiling above the collar, the walls of dormers, the knee walls and the floor of the triangular spaces behind them. Gaps at the junctions are where heat escapes and where cold spots and mould appear, so a good quote describes each surface.

Warm, moist air from the house must not condense on cold roof timbers. With traditional underlays, insulation between the rafters needs a ventilated gap above it, and Planning Portal notes that ridge vent tiles may then be needed for through ventilation. Modern breathable membranes change the detailing, but the installer must know which underlay the roof actually has before choosing a method.

A vapour control layer on the warm side, sealed at joints and around services, reduces the moisture reaching the insulation. Rooms in the roof in older houses often have lath and plaster that is left in place, which needs careful thought about where moisture will go.

Be cautious about spray foam applied to the underside of the roof. It can hide the state of the timbers and trap moisture against them, and some lenders and surveyors are wary of it. Home Energy Scotland does not fund spray foam insulation.

Flat roofs can be insulated above the deck as a warm roof, with insulation boards under a new waterproof covering, or below it as a cold roof with a ventilated void. Warm roofs are often chosen for upgrades, but they raise the roof level, which affects upstands, door thresholds and parapets. It is usually done when the covering is due for replacement anyway.

Renovating a quarter or more of a roof, for example stripping and re-covering it, normally brings in the Building Regulations requirement to upgrade the insulation. In England, Approved Document L sets an improved U-value of 0.16 W/m²K for roofs when an existing element is upgraded. Where that would limit headroom, a lesser standard can be accepted, but the insulation plus any air gap should at least fill the depth of the rafters, using the insulant that gives the lowest practicable U-value.

A loft conversion is a separate project with its own Building Regulations approval covering structure, fire safety, stairs and insulation. Planning permission is not normally needed just to insulate a roof, but anything that changes the outside, such as raising a flat roof, may need it in a conservation area or on a listed building.

• which surfaces are insulated: rafters, collar, dormer cheeks, knee walls, floors
• insulation type and thickness, and the target U-value
• how ventilation is kept or whether the underlay is breathable
• vapour control layer and how junctions are sealed
• making good: plasterboard, skim, decoration
• Building Regulations route if a quarter or more of the roof is being renewed

Tell us how the roof space is used, the roof type and roughly when the house was built. Insulation installers in your area see the request with your personal details removed; exactly one can buy it. It pays Offrano, and you pay nothing.

Before your details are shared, we email you that installer's name, address and phone number. The request may not be taken, and if no one buys it within 21 days it is withdrawn. Offrano does not inspect insulation work, so ask the installer to explain its ventilation and moisture strategy in writing.