If you are considering improving your home’s thermal efficiency, then you can’t do much better than external wall insulation. In fact, installing EWI can reduce your annual heating bills by up to 50%, while also stopping damp and reducing condensation. However, insulation on different properties will need to be different thicknesses in order to properly do the job.

What is external wall insulation?

External wall insulation is a type of layered insulation system attached to the exterior of a property in order to improve its thermal efficiency. EWI is created in layers, with an insulation board fixed to the outside of the property first, followed by a polymer-based render, and then a glass fibre mesh set into the surface.

A thinner layer of render is then added to cover the mesh, followed by a final topcoat render that contains the desired finish and colour of the wall. The insulation board that attaches to the walls first can vary in thickness, depending on the material of the existing wall.


In order to know how thick external wall insulation should be, you first need to understand U-values. U-values, also known as thermal transmittance, indicate how much heat is lost through a certain thickness of material, and a lower U-value means the building is better insulated. You can see the full Knauf UK guideline table for U-values using the Warm Wall System on our website here.

How thick should external wall insulation be?

External wall insulation needs to be the right thickness to achieve 0.3W/m2K or better. This means that your external wall insulation should be approximately 60mm and can be up to 100mm, but this will depend on the material your existing wall is made from.

Employ the services of a professional EWI Installer

If you use a professional external wall insulation installer, you can be sure of achieving the right thickness. You can also be sure the installation will be done to the highest standard and this will give you a choice of a range of great finishes.

Another benefit of having a professional do the work is that there is minimal disruption, as the entirety of the work takes place on the exterior of the property, unlike some other solutions, such as interior wall insulation. By hiring a professional installer, you can carry on with life as normal while your home is being insulated.


External wall insulation is a great improvement to any home’s thermal efficiency, and it also saves you from having to redecorate, as you can have an attractive, maintenance-free finish of your choice. By hiring a professional, you can also be sure that your external wall insulation will be the correct thickness for your property.

With the government’s Green Homes Grant still available until March of next year, now is a great time to have external wall insulation installed, as two-thirds of the cost could be covered by the scheme.