External Wall Insulation Glossary

Adhesive: The mortar-based or foam adhesive that bonds insulation boards to the wall before anchors are added.
Algae: Green or black growth on shaded render; cleared with a biocide wash.
Anchor (mechanical fixing): A plastic or steel-pinned fixing driven through the board into the masonry to hold the system in place.
Approved Document B: Government guidance in England on fire safety, including external wall materials.
Approved Document L: Government guidance in England on energy efficiency, including the 0.30 W/m²K target for renovated walls.
Arris: The sharp edge where two render faces meet at a corner.
Basecoat: The reinforcing render layer applied over the boards, with mesh embedded in it.
BBA certificate: An independent assessment of a construction system by the British Board of Agrément, setting out its use and limits.
Bead: A metal or PVC profile that forms a clean corner, edge or stop in the render.
BISF house: A steel-framed house type built after the Second World War by the British Iron and Steel Federation.
Breathability: Informal term for vapour permeability: how readily moisture vapour passes through a material.
Brick slip: A thin slice of real clay brick bonded to an insulated wall to give a brick finish.
Building warrant: Approval needed in Scotland before most building work, including external insulation cladding systems.
Cavity barrier: A barrier that closes a cavity, such as behind cladding, to limit the spread of fire and smoke.
Cold bridge (thermal bridge): A path where heat escapes faster, such as at floor edges, lintels or reveals.
Condensation: Water forming on a surface colder than the dew point of the air.
Conservation area: An area of special architectural or historic interest where permitted development rights are reduced.
Damp-proof course (DPC): A barrier near the base of a wall to stop rising damp; EWI starts above it.
Dash (dry dash): Chippings thrown into wet render to give a pebbledash finish.
Drip bead: A bead that throws water clear of the wall below it, used at the base of systems and over openings.
Eaves: The lower edge of a roof that overhangs the wall.
ECO4: The fourth Energy Company Obligation scheme, which funded insulation for eligible households and closes on 31 December 2026.
EPC: Energy Performance Certificate: rates a home's energy efficiency from A to G.
EPS: Expanded polystyrene, the most widely used EWI board; available in white and graphite grades.
ETA: European Technical Assessment: a technical assessment of a construction product or kit, such as an EWI system.
EWI: External wall insulation: insulation fixed to the outside of a wall and protected by a finish.
Fire break: A strip of non-combustible insulation within a combustible system at positions set by the system design.
Graphite EPS: Grey polystyrene containing graphite, which insulates better than white EPS of the same thickness.
GBIS: The Great British Insulation Scheme, which closed on 31 March 2026.
Header bond: A brick pattern with bricks laid end-on; a clue that a wall is solid.
Hygroscopic: Able to absorb and release moisture from the air, like wood fibre.
INCA: The Insulated Render and Cladding Association, a UK trade association for the EWI sector.
Insulating lime render: A thick lime render with lightweight insulating aggregate, used on older walls.
Lamella: Mineral wool cut with fibres running at right angles to the wall, giving high strength.
Listed building consent: Permission needed for changes to a listed building's character, inside or out.
Light reflectance value (LRV): How much light a colour reflects; system holders set a minimum LRV to limit heat stress from dark colours.
Mesh: Glass-fibre reinforcing mesh embedded in the basecoat to resist cracking.
Mineral wool: Non-combustible stone wool insulation, typically class A1 or A2.
National Landscape: The new name for an Area of Outstanding Natural Beauty (AONB) in England and Wales.
No-fines concrete: Concrete without fine aggregate, cast in place to form walls of many post-war houses.
Non-traditional house: A house built by a method other than brick or block cavity walls, such as concrete panels or steel frame.
PAS 2030: The standard for installing energy-efficiency measures in existing homes, required for most funded work.
PAS 2035: The framework for whole-house retrofit assessment, design and coordination.
Permitted development: Building work that can be done without applying for planning permission, subject to conditions.
Phenolic foam: A high-performance rigid insulation board, thinner than EPS for the same U-value.
Plinth: The lowest band of a wall, often finished in a darker or tougher render.
PRC house: A precast reinforced concrete house; some types were designated defective under the Housing Defects Act 1984.
Primer: A coat applied over the basecoat to prepare it for the finish render.
Rainscreen cladding: Cladding fixed on battens with a ventilated cavity behind it.
Reaction to fire: How a material contributes to fire; classed A1 (best) to F.
Reveal: The side and top surfaces of a window or door opening.
Roofline extension: Extending the eaves or verge so the roof overhangs the new, thicker wall.
Silicone render: A water-repellent, through-coloured thin-coat render widely used over EWI.
Silicate render: A very vapour-open mineral render bound with potassium silicate.
Sill extension: A longer window sill fitted so water drips clear of the insulated wall.
Solid wall: A wall with no cavity, typically 225mm brick or stone in older homes.
Starter track: A metal or PVC track that supports the first row of boards and closes the base of the system.
Stop bead: A bead that ends the render neatly, for example at a party wall line.
System holder: The manufacturer that designs, tests and certifies an EWI system.
Thermal conductivity (λ): How well a material conducts heat, in W/mK; lower is better for insulation.
Thermal element: A wall, floor or roof that separates heated space from outside; renovating one triggers energy standards.
TrustMark: The government-endorsed quality scheme for work in and around the home, required for much funded work.
U-value: The rate of heat loss through an element, in W/m²K; lower is better.
Vapour-open: Allowing moisture vapour to pass through, so a wall can dry outwards.
Warm Homes: Local Grant: A government scheme in England, run by councils, funding improvements for low-income households.
Wood fibre: A natural, vapour-open insulation board made from compressed softwood fibres.

External wall insulation glossary

Find out what your walls need