Render & Wall Insulation Glossary

Adhesive: The cementitious or polymer mortar used to bond insulation boards to the wall, applied in a perimeter-and-dab pattern or as a full bed.
Airey house: A post-war precast reinforced concrete (PRC) house type built with concrete columns and horizontal concrete planks.
Article 4 direction: A council order that removes certain permitted development rights in a defined area, so planning permission is needed for changes that would otherwise be permitted.
Base coat: The reinforcing render layer applied over insulation or masonry, with glass-fibre mesh embedded in it to resist cracking.
Bead: A metal or PVC profile fixed at corners, edges and the base of render to give a clean line and protect the edge. Types include corner, stop, drip and bell-cast beads.
Bell-cast: A flared detail at the bottom of render that throws water away from the wall below.
BISF house: A steel-framed house type designed by the British Iron and Steel Federation after the Second World War, usually with steel sheeting on the upper storey.
Brick slip: A thin slice of brick, or a brick-effect tile, bonded to a wall or insulation system to give a brick appearance.
Building control: The process of checking that building work meets the Building Regulations, provided by the local authority or a registered building control approver.
Building warrant: In Scotland, the permission from the council's building standards service needed before certain building work starts.
Cavity barrier: A fire-resisting barrier that closes a cavity to stop fire and smoke spreading through it, used at compartment floors and walls.
Cill extension: A longer window cill fitted after external insulation so rainwater still drips clear of the new wall face.
Cold bridge: A point where heat escapes faster than the surrounding wall — for example where a floor meets an external wall. Also called a thermal bridge.
Condensation risk analysis: A calculation, usually to BS EN ISO 13788 or BS 5250 methods, checking whether moisture could condense within a wall build-up.
Conservation area: An area of special architectural or historic interest where extra planning controls apply.
Cornish Unit: A precast reinforced concrete house type developed in Cornwall after the war, recognisable by its concrete panels and often a mansard roof.
Damp-proof course (DPC): A horizontal barrier in a wall, usually around 150mm above ground, that stops rising damp. EWI should start above it.
Dry dash: A finish where small stones are thrown onto wet render. Modern dry-dash finishes are available on some EWI systems.
ECO4: The Energy Company Obligation scheme funding insulation and heating for eligible households, extended to 31 December 2026.
EPS: Expanded polystyrene: a lightweight foam insulation board, white or grey (graphite-enhanced), widely used in EWI on low-rise buildings.
EWI: External wall insulation: insulation fixed to the outside of a wall and protected by render, brick slip or another finish.
Eaves extension: Extending the roof overhang so that it still projects beyond the new, thicker wall after insulation.
Grain size: The size of the aggregate in a thin-coat render, typically 1–2mm, which sets the texture of the finish.
Graphite EPS: Grey EPS containing graphite, which reduces radiant heat transfer and improves insulation per millimetre.
Harling: The Scottish term for roughcast — a render with small stones mixed in, thrown onto the wall.
Higher-risk building: In England, a building at least 18 metres or seven storeys tall with at least two residential units, regulated by the Building Safety Regulator.
Hygroscopic: Able to absorb and release moisture from the air — a property of wood fibre and lime that helps buffer humidity.
IWI: Internal wall insulation: insulation fixed to the inside face of external walls.
Lamella: Mineral wool cut so the fibres run perpendicular to the wall, giving higher strength for render systems.
Lime render: A breathable render based on natural hydraulic lime or lime putty, suited to older solid walls.
Listed building consent: Permission needed for works affecting the character of a listed building, separate from planning permission.
Mechanical fixing: A plastic anchor with a steel or plastic pin, drilled through the insulation into the wall to hold boards in place.
Mesh: Alkali-resistant glass-fibre reinforcement embedded in the base coat to resist cracking.
Mineral render: A thin-coat render based on cement or lime, often painted, used particularly on mineral wool systems.
Mineral wool: Non-combustible insulation made from spun stone or glass fibres, used in EWI where fire performance or breathability matters.
Monocouche: A one-coat, through-coloured cement-based render applied 15–20mm thick, usually on blockwork.
Movement joint: A deliberate joint in render that lets the wall move without cracking the finish.
No-fines concrete: Concrete made without sand, used for poured walls in Wimpey no-fines houses.
Non-traditional house: A house built with a system other than brick or block cavity walls — concrete, steel or timber systems, mostly from the 1940s–1970s.
PAS 2035: The UK specification for whole-house retrofit, setting out assessment, design and coordination for funded energy-efficiency work.
PAS 2030: The installer specification for energy-efficiency measures, used alongside PAS 2035 on funded schemes.
Permitted development: Building work that can be carried out without a planning application because national rules grant permission, subject to limits and conditions.
Phenolic board: A rigid foam insulation with very low thermal conductivity, used where space is tight.
PIR: Polyisocyanurate: a rigid foam insulation often used in insulated plasterboard.
PRC house: A precast reinforced concrete house. Some PRC types were designated defective and need certified repairs for many lenders.
Primer: A coat applied over the base coat to even out suction and help the finish render bond and colour evenly.
Relevant building: In England, a building 18 metres or more with residential or certain other uses, where combustible materials are banned in external walls.
Reveal: The side of a window or door opening. Reveals are insulated where frame position allows, to reduce cold bridging.
Section 20 consultation: The process a landlord must follow in England and Wales before major works charged to leaseholders above the statutory threshold.
Silicone render: A thin-coat, through-coloured render containing silicone resins, which repels water while staying vapour-open.
Solid wall: A wall with no cavity — typically 225mm or more of brick, or stone — common in homes built before about 1920.
Starter track: A metal or PVC base profile fixed above the damp-proof course to support and level the first row of insulation boards.
Thermal element: A wall, floor or roof that separates heated space from outside. In England and Wales, renovating more than half of one, or more than a quarter of the whole building envelope, triggers energy requirements.
Thin-coat render: A render finish applied around 1–3mm thick over a reinforced base coat, such as silicone, acrylic or mineral render.
TrustMark: A government-endorsed quality scheme for trades; registration is required for many funded retrofit schemes.
U-value: The rate of heat loss through a building element, in W/m²K. Lower is better; about 0.30 is a common target for upgraded walls.
Vapour-open: Allowing water vapour to pass through, so a wall can dry out — important for older solid walls.
Warm Homes Plan: The UK government's plan, published in 2026, for funding home energy upgrades through grants and low- or zero-interest loans.
Wimpey no-fines: A non-traditional house type with walls of poured no-fines concrete, built widely from the 1940s to the 1970s.
Wind-driven rain exposure: How much rain a wall receives when driven by wind, which varies by location and affects render specification.
Wood fibre board: A vapour-open insulation board made from compressed wood fibres, suited to older walls and lime finishes.

Glossary

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