Would you like to cut the amount you spend on heating while keeping your home warm and cosy in winter?

About a third of all the heat that escapes from an uninsulated house goes through its outside walls, according to the Energy Saving Trust. So installing wall insulation could help you save money on your heating bills and help the environment.

There are two main types of wall insulation in the UK: cavity wall insulation and solid wall insulation. If your home was built recently, it may already have cavity wall insulation or be suitable for it. You can find out more in our complete guide to cavity wall insulation.

If you’re not sure what type of walls you have, read on to find out. We’ll tell you all you need to know about solid wall insulation.

Diagram of 1. a solid wall with a long- and short-brick pattern, and 2. a cavity wall with a concrete-block inner wall and regular brick outer one

What’s a solid wall?

  • A solid wall has no cavity. It’s a single, solid wall that's typically two bricks wide or made of stone.
  • A cavity wall is in fact a double wall – two walls with a gap, or cavity, between them. The outside wall is usually brick. The inside one can be made of bricks or concrete blocks.

Have I got cavity walls or solid walls?

If your home has brick walls, there are a number of ways you can tell if they’re cavity or solid:

  • When was your home built? Most houses dating from the 1920s onwards have cavity walls. Houses built before 1920 usually have solid walls.
  • What’s the brickwork like? If the bricks all appear the same length, you’ve probably got cavity walls. An irregular pattern of long and short bricks is a sign of a solid wall.
  • How thick are your walls? Measure them in a window or doorway. Walls over 260mm thick are likely to be cavity walls.

Stone walls are usually solid and may be suitable for solid wall insulation. But they do vary in thickness and structure, and some newer stone walls may have been built with a cavity. So if you’ve got stone walls, it’s best to seek professional advice on what kind of insulation is right for your home.

How does solid wall insulation work?

Solid wall insulation is like a giant scarf wrapped around the exterior walls of your home. It forms an extra barrier between your rooms and the outside world, trapping the heat inside.

You can install solid wall insulation externally or internally:

  • External insulation: A layer of insulating material is applied to the outside brickwork of your home. This is then covered with a special plaster-like coating called render, which can have a smooth, textured or patterned finish. Another option is to install cladding over the insulation – an external layer that comes in various materials, from timber to stone tiles and brick slips. External insulation could change the appearance of your home, so you may need to get planning permission from your local authority first.
  • Internal insulation: Rigid insulation boards are fitted to the walls inside your home (exterior walls only, not the ones between the rooms). The other main method is to build a hollow, stud wall on top of your existing walls and fill it with an insulating material, such as mineral wool fibre.

For more details on the different types of installations and materials used, visit the National Insulation Association.

How much could I save with solid wall insulation?

The money you could save with solid wall insulation depends on the kind of home you live in and the type of installation.

The Energy Saving Trust estimates that you could cut your home fuel bill by around £115 a year if you live in a flat. That figure rises to £415 for a large detached home. These estimates are for gas-heated homes in England, Scotland and Wales, and are based on fuel prices as of April 2018.

Detached house Semi-detached house Mid-terrace house Bungalow Flat
Savings on fuel bill (per year) £415 £245 £155 £165 £115


Costs may vary significantly depending on the level of work required. Source: Energy Saving Trust

Internal wall insulation is usually cheaper than an external installation. But neither option is cheap, so it could take some time to get your money back.

According to the Energy Saving Trust, the typical installation costs for a semi-detached house in Great Britain are:

  • External wall insulation: around £13,000
  • Internal wall insulation: around £7,400

Whatever type you choose, you may be able to keep the cost down by combining it with other home improvement work. You could fit internal insulation when you next decorate your home or have external insulation applied if you need to repair your outside walls.

Can I install solid wall insulation myself?

Like cavity wall insulation, installing solid wall insulation is a job for a professional. You can’t do it yourself unless you know what you’re doing and have the right equipment.

A professional installer will also be able to advise you on what type of insulation is right for your home.

Should I get internal or external solid wall insulation?

External solid wall insulation is generally more expensive than an internal installation, but it does have a number of advantages.

Take a look at the main pros and cons of each option:

Pros Cons
Internal solid wall insulation
  • Usually cheaper than external wall insulation
  • Can be installed room by room
  • Easily combined with internal home improvements (decorating)
  • Installation can be disruptive for your household
  • May reduce floor area of the room
  • Penetrating or rising damp must be sorted out first
External solid wall insulation
  • Installation doesn’t disrupt your household
  • Improves weatherproofing, protects brickwork
  • Helps prevent penetrating damp (but not rising damp) and draughts
  • Reduces external noise
  • Can improve appearance of outside walls
  • Generally more expensive than internal wall insulation
  • May need planning permission
  • Roof may need to be extended to cover top of insulating material

More energy-saving tips

Solid wall insulation is not the only way to cut your energy bills. Find out more about energy saving at home, including cavity wall insulation, loft insulation, draught proofing and exclusion, and hot water tank insulation.