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Solid wall insulation - external

Nearly half of all the heat lost from some solid-walled houses (those without a wall cavity) escapes through the walls. Insulating these walls keeps the warmth at home for longer and makes homes more comfortable and cheaper to heat.

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.

How do I know if my home has solid walls?

If your home is made of brick, and the bricks have an alternating long-short-long pattern like this, then the walls are likely to be solid. If you can see only the long edge of the bricks, like this, then the wall is almost certainly a cavity wall.

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 to determine the construction type. A solid brick wall is usually about 22 cm thick, a cavity wall between 27 cm and 30 cm thick, and a solid stone wall could be as much as 50 cm. 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 unlikely to have a cavity.

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.

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 installer who will specialise in work using a specific 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 is 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. Consent and planning permission are likely to be required if your house is listed or is located in a National Park, area of outstanding natural beauty, conservation area or within a World Heritage Site. If you have any doubts at all then check with your local authority before you have any work done.

Average costs and savings

Externally insulating your home costs on average £13,000, 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 of the cost of material and labour are higher. A typical 3-bedroom semi-detached house using gas heating could save around £300 per year on heating bills by installing external wall insulation and a detached house around £700 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:

  • Garden access may be required and boundary walls and lean-to structures may need to be adapted or removed
  • Scaffolding may need to be erected, and a space found for a skip and storage space for materials
  • External fittings like rainwater pipes, satellite dishes and telephone and power cables may need to be removed and replaced afterwards
  • Contractors will require water and power and the use of a toilet 
  • There will be a lot of noise, including power tools, and the work will generate a lot of dust and dirt

A cut-away model of external solid wall insulation

1 | Insulation board (next to brickwork)external solid wall insulation

This is the layer of insulation that will slow the lost 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.

2 | Middle section (grey and mesh)

This section, composed of a mesh between two thin layers of render, adds strength and rigidity.

3 | Top layer (white on this diagram)

A final coating of cladding or render is applied to give the wall the required appearance. It brings the total thickness of the added material to between 50 and 125 mm.

4 | Window sill

External 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

5 | Window and frame

Windows can change in appearance, as the insulation needs to extend into the window recess.

Frequently asked questions

Aren't all walls, 'solid walls'?

The term 'solid wall' refers to walls constructed as a single solid layer with no cavity section within the wall. This means the wall is literally solid from inside to outside. Solid walls are typically made from brick or stone, and are generally found within houses built before the 1930s.

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How can I insulate solid walls?

Insulation for solid walled properties can be carried out on the inside (internal wall insulation) or the outside (external wall insulation) of the property. To fit internal wall insulation any fixtures and fittings must be removed and refitted afterwards. This includes switches, sockets, skirting boards, kitchen units, etc. To fit external wall insulation you may require scaffolding and fixtures such as drain pipes and satellite dishes may need to be removed and refitted afterwards.

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What is 'thermal mass' and why is it relevant when considering solid wall insulation?

Solid brick or stone walls will absorb heat from both inside and outside of the property during the day, acting as a heat sink. Their relative ability to do this is known as their 'thermal mass'. Some of the heat absorbed by the walls is released back into the house when the temperature drops, typically after dusk. In the winter external wall insulation will allow more of the heat that is produced in the home to be absorbed into the walls and then be released back into the building, rather than passing right through the walls to the air outside. External wall insulation will also reduce the absorbtion of heat from the outside by the walls in summer. This will help stop the home becoming too hot on the warmest days. In the winter internal wall insulation will do a good job of keeping the heat produced by your heating system in your home. However it is not as good at taking advantage of the thermal mass because of the barrier it creates between the heat in your home and the walls.

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