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External Wall Insulation

External Wall Insulation

What Is It?

An external wall insulation system (or EWI) is a thermally insulated, protective, decorative exterior cladding procedure involving the use of expanded polystyrene, mineral wool, polyurethane foam or phenolic foam, topped off with a reinforced cement based, mineral or synthetic finish and render

External Wall Insulation


External wall insulation systems generally comprise firstly an insulation layer (an element which helps to achieve the requisite thermal performance); and secondly,a protected weatherproof finish (usually a render, although brick slips, tiles, and decorative boards can also be used). Insulating render can also be an advantage in certain locations. Choice of types and sizes will depend on the substrate and design exposure requirements.

Dry finishes are usually fixed to the substrate by means of timber battens independently fixed to the substrate.

How It Works?

Similar to cavity wall insulation, EWI creates a barrier that stops heat escaping through your walls. Unlike loft and cavity insulation however, EWI uses a rigid insulation material – expanded polystyrene slabs. Expanded polystyrene contains thousands of trapped air pockets, which makes it an extremely good insulator. It is also easy to cut to into shape, meaning it can fit neatly around windows, door frames and other architectural features.

The slabs are held in place with fixing anchors which are drilled into the existing walls, and then covered with a layer of reinforcing mortar. This is covered with a glass fibre mesh, which when dry gives the insulation system its structural strength.

Finally, a primer layer and a decorative render go on.

The insulation adds about 9cm to the thickness of your walls, so the shape of your house will not look drastically different.


Essex, UK