Loft insulation glossary

Airtightness: How well a building stops air leaking through gaps. Sealing gaps between the house and the loft cuts both heat loss and loft condensation.
Approved Document L: The government guidance on how to meet the energy efficiency part of the building regulations in England. Wales has its own version.
Batts: Pre-cut slabs of insulation, as opposed to rolls. Common with wood fibre and some mineral wool products.
Bitumen felt: The black sheet under the tiles of many older roofs. It does not let vapour through, so lofts under it rely on ventilation.
Breathable membrane: A roof underlay that lets water vapour pass but keeps liquid water out. Fitted to most roofs since the 2000s.
Building notice: A way of telling building control about work before it starts, without submitting full plans. Used for smaller jobs in England and Wales.
Building warrant: The approval needed before many types of building work in Scotland.
Cold bridge: A part of the structure that conducts heat more easily than the insulation around it, such as a joist left uncovered.
Cold loft: A loft insulated at floor level, with the roof space above left cold and ventilated.
Compression: Squashing insulation, for example under boards. It drives out the trapped air and reduces performance.
Condensation: Water that forms when warm, moist air meets a colder surface, such as the underside of the roof in winter.
Cross layer: The second layer of loft insulation, laid across the joists at right angles to cover the timber.
Crawl board: A board laid across several joists so installers can kneel without stepping on the ceiling.
Cut roof: A traditional roof built on site from rafters, purlins and ceiling joists, common before the 1960s.
Dormer: A window that projects from a sloping roof, with its own small roof and walls (cheeks).
Downlight cover: A fire-rated hood that fits over a recessed light so insulation can be laid over it safely.
Eaves: The lower edge of the roof where it meets the walls. Most cold lofts are ventilated here.
Eaves tray: A rigid tray or baffle that holds insulation back from the eaves so air can still flow.
ECO4: The Energy Company Obligation, a scheme run through energy suppliers in Great Britain. It runs until 31 December 2026.
Energy-saving materials: Items such as insulation that HMRC allows to be installed in homes at a zero or reduced rate of VAT.
EPC: Energy Performance Certificate. Rates a home from A to G and lists recommended improvements.
Gable wall: The triangular end wall of a pitched roof.
Joist: A horizontal timber that carries the ceiling and the loft floor.
Knee wall: A short wall in a room-in-roof, between the floor and the sloping ceiling. Also called a dwarf wall.
Lagging: Insulation wrapped around pipes and tanks to stop heat loss and freezing.
Lath and plaster: An older ceiling made of thin timber strips covered in lime plaster. It can be fragile.
Loft hatch: The opening and door into the loft. An uninsulated hatch is a common source of heat loss.
Loft legs: Plastic or timber supports that raise loft boards above the insulation.
Loose fill: Insulation supplied as loose fibres or granules and poured or blown into place.
Mineral wool: Insulation made from spun glass or rock fibres. The most common loft insulation.
NISEP: Northern Ireland Sustainable Energy Programme, which funds energy efficiency measures.
Party wall: A wall shared with a neighbouring house, often running up into the loft.
PIR board: Polyisocyanurate, a rigid foam insulation board, usually foil-faced. Used between rafters.
Purlin: A large horizontal timber that supports rafters part-way up the roof.
Rafter: A sloping timber that runs from the eaves to the ridge and carries the roof covering.
Room-in-roof: A living space built into the roof, with sloping ceilings and often dormers.
Sarking: Boards or a membrane laid over the rafters under the roof covering, common in Scotland.
Sheep's wool insulation: Insulation made from treated sheep's wool. Vapour-open and pleasant to handle, but more expensive.
Soffit: The board under the eaves. Soffit vents let air into the loft.
Spray foam: Polyurethane foam sprayed onto the underside of a roof. It can cause problems if installed badly.
Thermal conductivity: How well a material conducts heat, measured in W/mK. Lower means better insulation.
Thermal element: A wall, floor or roof that separates heated space from outside or unheated space.
Tile vent: A special roof tile with a vent opening to let air into or out of the loft.
Top-up: An extra layer of insulation laid over existing insulation to reach the recommended depth.
Trussed rafter: A factory-made triangular roof frame. Common in houses built since the 1960s, and designed for light loads.
U-value: How much heat passes through a building element, in W/m²K. Lower is better. The target for an insulated loft in England is 0.16.
Vapour control layer: A layer that slows water vapour moving from the warm side of the insulation to the cold side.
Ventilation path: The route outside air takes through a cold loft, usually in at the eaves and out at the other side or the ridge.
Vermiculite: A loose, pebble-like insulation found in some older lofts. It may contain asbestos and must be tested before it is disturbed.
Warm loft: A loft insulated at roof level so the space stays close to the temperature of the house.
Warm Homes Plan: The UK government's programme, announced in January 2026, to fund energy upgrades, mainly for low-income households in England.
Wood fibre insulation: Insulation made from compressed wood fibres. Vapour-open and dense, suited to older homes.

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