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Solid wall insulation – external

The front of an Edwardian house that has had solid-wall insulation applied.

Nearly half of the heat lost from some solid-walled houses escapes through the walls. Insulating these will keep your home warmer,  save energy and money.

Last updated in February 2023

Solid wall insulation may be suitable for a variety of wall types such as brick, stone, steel-framed and concrete construction. And, depending on the circumstances, the walls can be insulated internally (from the inside) and externally (from the outside); both are significant undertakings in terms of cost and disruption.

This leaflet looks at external solid wall insulation. Click here for our page on internal solid wall insulation.

How do I know if my home has solid walls?

Look at the diagram below. If your home is made of brick, and the bricks have an alternating long-short-long pattern, like on the left, then the walls are likely to be solid. But, if you can see only the long edge of the bricks, like on the right, then the wall is almost certainly a cavity wall.

Diagram showing two types of brick pattern: long-short-long-short, and long-long-long.

If you can’t see the pattern of the brick work then measuring the thickness of the wall at any entrance or window will help determine the construction type. A solid brick wall is usually about 22cm thick, a cavity wall between 27cm and 30cm, and a solid stone wall could be as much as 50cm. The age of your home can also be a good indicator. As a general rule, if it was built before the late 1920s it is likely to have solid walls.

What does external wall insulation involve?

External solid wall insulation involves adding a layer of insulating material to the outside walls of a building and coating this with a protective render or cladding. There are lots of options to create the finish that you want and these may even add value to your home. The total depth of this layer ranges from around 50mm to 130mm, depending on the system used.

External solid wall insulation, rather than internal, may be particularly suitable if you want to avoid any loss of space or the disruption of work going on inside your house, or if you are doing other work to the exterior of your property, such as re-rendering. It is a specialist job that requires an approved contractor who is qualified to a particular insulation system.

Find out what the process was like for one Bristol homeowner in this short video:

Changes to the external appearance

External solid wall insulation won’t affect the size of your rooms (unlike internal solid wall insulation) but you might need planning permission as it could change the appearance of the building. There are a wide range of colours and finishes that can be applied and all the options should be discussed with your chosen contractor. Often the existing finish can be replicated to preserve much of the original appearance of the house.

As solid wall insulation is now constituted as an ‘improvement’ rather than an ‘enlargement’ you are unlikely to need planning permission unless your house is listed or is in a national park, area of outstanding natural beauty, conservation area or world heritage site. If in doubt, check with your local authority before having any work done.

Average costs and savings

Externally insulating your home costs on average £12,000, though this depends on the size of the building and the number of outside walls being insulated. It is likely to be more expensive than internal wall insulation because the cost of material and labour are higher. A typical 3-bedroom semi-detached house using gas heating could save around £398 per year on heating bills by installing external wall insulation and a detached house around £488 per year.


External solid wall insulation is a significant undertaking that inevitably involves a degree of disruption. Your installer should make you aware of any particular issues but things to consider include:

A cut-away model of solid wall insulation

A cut-away model of solid wall insulation layers showing the brickwork, insulation board, a middle section of mesh and a top layer of render. The model also shows how the insulation the window frame and window sill.
1Insulation board (next to brickwork)This is the layer of insulation that will slow the loss of heat from inside the house. It is attached to the external wall, either with an adhesive or mechanical fixings and protects and extends the life of your brickwork.
2Middle section (grey and mesh)This section is composed of a mesh between two thin layers of render and adds strength and rigidity.
3Top layer (white on this diagram)A final coating of cladding or render is applied to give the wall the required appearance.
4Window sillExternal fittings such as pipework and satellite dishes may need to be removed before insulation is applied. It may be necessary to remove and extend window sills so they protrude beyond the cladding.
5Window and frameWindows can change in appearance, as the insulation needs to extend into the window recess.

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