What exactly is external wall insulation and how does it work?

External solid wall insulation involves adding a layer of insulating material to the outside of your house. This improves the thermal efficiency of the walls and therefore reduces heat loss out of the property. The layer of insulation also helps to protect the underlying brickwork and often improves the look of the property (especially if it is applied to pebbledash!)

What are the specifics of External Wall Insulation?

Each Solid Wall Insulation system consists of numerous different materials including adhesives, insulation (either EPS or mineral wool), renders, reinforcement mesh and mechanical fixings and each of these different elements need to be installed in a specific order to ensure a long lasting finish.

The systems we sell are designed specifically with the UK housing stock in mind and can be installed as part of retrofit works or new builds. All of the systems we sell will meet building regulations provided they are installed by a certified installer and this is just another reason why EWI Store is one of the leading retailers of solid wall insulation systems in the UK.

Our EWI Pro system is one of the few BBA approved solid wall insulation systems available here in the UK – find out more about BBA here.

What are the layers of External Wall Insulation?

As discussed a solid wall insulation system is made up of lots of different layers. In this section we are going to look at each in turn.


We offer two different types of insulation within our solid wall insulation systems.

Our best selling product is our grey EPS (EWI-410). This is the best quality polystyrene insulation and a u-value of 0.3 w/m2k can be achieved with just 100mm of this product. The EPS comes in packs of 6 slabs (100mm thickness) and each slab is 1000mm x 500mm in size.

This is the easiest insulating product to work with and unlike the white polystyrene insulation boards offers far better thermal efficiency properties.

The other insulating material we offer is Mineral wool. This can form the basis of a fully breathable system provided it is used in conjunction with a mineral or Silicone Silicate render – it is also fire proof, therefore is sometimes preferred to EPS.


EPS & MW Adhesives
The EPS adhesive contains a special mix of aggregates and cementitious components, packed in 25kg bags. After mixing with water to desired consistency, EPS adhesive is applied onto the back of the EPS insulation boards and these are then fixed onto the wall. The mineral wool adhesive is similar but contains white cement and additional fibreglass to provide added strength and flexibility to the system. The adhesives also are used in the reinforcement layer which we discuss a little later.


EPS mechanical Fixing
Our EPS fixings consist of a plastic head for heat loss prevention and a rigid plastic body – so they are simple, yet very effective. The length of the EPS mechanical fixing is calculated by the thickness of EPS insulation and also the wall construction. Simply mark and drill holes where necessary (we recommend 7 fixings per EPS insulation slab), insert the plastic body into the drilled hole and then hammer the metal rod through the EPS board until the head of fixing is within the EPS slab.

At EWI Store we recommend using EPS caps – these are used to fill in the hole produced by the EPS mechanical fixing when it is hammered in. The great thing about the EPS caps is that they completely prevent thermal bridging through the fixings. In addition they perform another really important role – once they are cemented in place with the EPS adhesive and the system dries it means that you won’t be able to see where the mechanical fixings have been used. Trust us, if they aren’t used (and not many installers use them) then as soon as the system finishes and everything contracts (as it dries) you can see hundreds of little circles under the surface of the solid wall insulation system.


In the reinforcement layer we always use fibreglass render mesh. These come as rolls that are 1.0m wide by 50m long. We offer a variety of different densities of mesh that provide different levels of strength within the solid wall insulation system. The weights vary from 150g/m2 to 160g/m2. We also offer an extra strong “Panzer-mesh” 335G/M2 (size 1.0m x 25m).

The mesh, when sunk into the adhesive bonds together and acts as reinforcement to all the EPS insulation boards. It is really important to choose the right mesh for the right circumstances. If you have kids that like playing football (and use you walls as a goal), then a stronger mesh will be more impact resistant and therefore might be more appropriate for example.


Prior to installing the render on the walls, the walls need to be primed to help aid the adhesive properties of the walls and make sure that the render sticks fast to the reinforcement layer.

Different primers are used depending on the different renders chosen (e.g. acrylic render, mineral render, silicone render). Simply use brush or roller to apply to the area and allow to touch-dry.

If your render is pre-coloured (which we do in our warehouse), the render primer will in some instances be pre-coloured as well.


We sell a huge number of different renders at EWI store each with different properties. Our most popular render is our Silicone Silicate render, which although the most expensive, offers unparalleled performance.

We also offer an acrylic render, which is fantastic if you are looking to colour your house with a very vibrant colour and a mineral render which is breathable if mineral wool is used.