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Glossary of Render and EWI Terminology

If you’re new to render and EWI, sometimes getting to grips with the terminology can be a bit of a challenge.


Auto aerated concrete – a lightweight, high performance building block with insulating capabilities and resistance to fire and mold.


The undercoat, used to create a smooth surface before the render/decorative finish is applied.


British Board of Agrement -a leading notified body, offering approval, certification and test services to manufacturers of products and systems supplying the construction industry.


Beading is used in rendering, plastering and external wall insulation systems to reinforce weaker areas of the substrate that are prone to cracking/impacts.


The ability of a material to allow water to pass through its structure and evaporate.

Dew point

The dew point is the temperature at which water vapour condenses.


Damp proof course/damp proof membrane – situated at the base of the wall, the DPC acts as a barrier to prevent moisture rising through the structure from the ground. It is generally made of rubber, plastic or lead, although chemical DPC’s are also available for retrospective installations/repairs.


Expanded polystyrene insulation.


Expanded metal lathing – a type of metal mesh used for reinforcing plaster and render coats.


External wall insulation – insulation that is fitted onto a property (usually a solid wall property) before being covered over with a protective render coating.


Used to mechanically secure insulation boards to a substrate.

First Pass/Second Pass

‘Passes’ refer to applying separate layers of a material, e.g. first pass/layer of the basecoat.

Grain Size

Grains run through certain types thin coat renders. These are available in different sizes, enabling you to choose the level of texture that you prefer.

Go Off

E.g. ‘leave the basecoat to go off’ – means leave the basecoat to dry/set.

Harl coat

A traditional rendering technique, the harl coat is commonly used with lime and aggregates mixed into a slurry consistency before being ‘harled’ at the wall to create a textured finish.


A surface that repels water.


A substance’s ability to absorb water from its surroundings.

Interstitial Condensation

Occurs when moisture reaches its dew point within the fabric of a material.


Insulated concrete forms a modern building material consisting of polystyrene insulation filled with concrete and rebar steel.


Monocouche renders are single coat renders, applied to the outside of a building to create a decorative finish.

Mechanical Key

Creating a mechanical key means creating a surface that is rough/textured so that the next layer can bond to it with greater ease.

Polymer Modified

Polymer modified renders are renders that contain silicone or acrylic additives.

Pebbledash/Dry Dash/Roughcast

A traditional decorative finish. It is created by applying a layer of mortar to the external surface, before throwing pebbles/gravel/shells at the mortar to create a rough finish.


A measure of how well an object (per unit of exposed area) resists conductive flow of heat.


A type of plaster applied to the external walls of a property to protect against the weather.


‘An underlying substance or layer’ – the existing wall to be rendered/insulated.

Scratch coat

A rough coat, scratched before it is completely dry to ensure that the next coat bonds completely.

Sponge finish

Using a sponge to create a smooth surface, either as a final finish or before painting.

Thin coat render

A specific type of render (usually silicone or acrylic) that is applied in a very thin layer.

Thermal bridges/cold bridge/heat bridge

An area within the insulation system that has a higher thermal conductivity than the surrounding materials. I.e. gaps between insulation boards create thermal bridges as heat can escape more quickly from the exposed area.

Thermal mass

Thermal mass is the ability of a material to absorb and store heat.


A measure of heat transmission through a building part or insulation material. The thicker the insulation material the lower the numbers.


Stands for watts per meter-kelvin, a measure of thermal conductivity.


Extruded polystyrene insulation.


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