Solid walls

If your home was built before 1930, its external walls are probably solid rather than cavity walls. Cavity walls are made of two layers with a small gap or ‘cavity’ between them and this gap acts like insulation to restrict the flow of heat. Solid walls have no gap, so they let more heat through.


Older buildings have solid walls and the pattern of bricks look like the picture on the left. Notice how the pattern of the bricks differs from the cavity wall picture (right):

Solid walls are harder than cavity walls to insulate. There are two options:

  1. Internal insulation, which keeps the external look of your house the same and may be suitable in conservation areas but makes the room smaller. Staircases against an external wall can cause problems if they are made narrower;
  2. External insulation, which will involve your bricks being covered by the insulation and then either by a render or by other coverings such as tiles or timber panels.

It is possible to have an attractive front internally insulated and the less attractive side or back walls externally insulated. Particular attention needs to be paid with internal solid wall insulation to avoid condensation.

External insulation

If your solid walls are in a poor state, and/or the brick is not very attractive, the simplest solution may be to insulate them from the outside and cover the insulation with any of a range of cladding materials, for example wood, tiles, brick slips or render. External insulation is less disruptive for the occupants than internal insulation but may cost more.

Care must be taken to prevent rising damp by stopping the external insulation a bit above ground level. It is also important that the insulation does not cover up the existing damp proof course.

A wooden frame is constructed over the bricks, and the insulation fixed to the frame. Insulation can be wool, polystyrene, cork, wood fibre, or beads. See this description of some of the options for natural materials, which explains why traditional materials may be better as they reduce the risk of condensation.

There are several national companies that specialise in external insulation. They have their own systems that they use, and the work is carried out by accredited contractors, for more infomation see the INCA website.

It is also possible (and we think likely to be better) to ask an architect experienced in green retrofitting to create a specification, and then a local builder can carry out the work. It is important that the work is done well, to obtain the best results.

It may be possible to keep the external insulation to those parts of the property that are not visible from the street, and internally insulate the rest. If you do this, we advise keeping a particularly close eye on the work: it will be important to ensure that there are no uninsulated patches where the internal and external insulation meet.

Where the walls meet windows, doors and the roof, careful finishing is needed to make sure that the insulation is complete. This can mean changes to window frames, soffits and flashing. If you are planning to replace windows, it makes a lot of sense to do this at the same time as the wall insulation or before. All guttering and downpipes will have to be moved.

Internal insulation

Depending on the existing state of the walls and the weight they need to bear afterwards from shelves or cupboards, you can choose either rigid boards which include insulation (take up less space, but don’t bear as much weight) or a stud wall (bears more weight – useful where wall-mounted cupboards are needed, for example – but takes up quite a lot of space).

Rigid boards are fitted straight on to the existing surface, so require reasonably straight walls and a good standard of existing plaster. Stud walls involve a metal or wooden frame being added to the wall and the insulation is then fixed on top of that (a similar process to external insulation).

As with external insulation, there is a choice of materials: mineral wool, foam, cork. You can search the Energy Saving Trust list of recommended products here (choose ‘internal wall insulation’).

Window reveals will need to be replaced with larger reveals because the thickness of the wall has increased. Curtain rails, radiator pipes, bookcases and dadoes may also need to be extended, moved or replaced, and of course you will need to redecorate.

Geek says

  • It is possible to have an attractive front internally insulated and the less attractive side or back walls externally insulated.
  • Care needs to be taken with solid wall insulation to avoid condensation.
  • Because external insulation changes the look of the property, you may need planning permission.
  • Inevitably, internal insulation reduces the size of the rooms in which it is fitted – but remember that it is only needed on external walls!

Local experiences

If you’ve had any experience with solid wall insulation (external or internal), we’d love to hear about your experiences – both good and bad. Please get in touch!

Useful links

Thermal imaging of solid walls

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