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Solid wall insulation - internal

Around half of all the heat lost from a typical solid-walled home escapes through the walls. Insulating these walls will keep the warmth inside for longer and greatly reduce your heating bills.

Solid walls can be insulated internally (from the inside) and externally (from the outside) - both are significant undertakings in terms of cost and disruption. This lpage is about internal solid wall insulation. You can find our external solid wall insulation page here.

Solid wall insulation may be suitable for homes made from brick, stone or concrete construction, and works by adding a layer of thermal material to the existing inside wall. This will reduce the size of your rooms a bit, though you may find if a cold wall is made warmer you actually increase the amount of usable space in a room.

Internal solid wall insulation is particularly appropriate where you need to maintain the external appearance of the building (e.g. in a heritage context).

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 they are almost certainly cavity walls.

If the brickwork is not visible, 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 22cm thick, a cavity wall between 27cm and 30cm and a solid stone wall as much as 50cm thick. The age of your home can also be a good indicator; if it was built before the late 1920s it is likely to have solid walls.

Types of internal solid wall insulation

There are various ways to insulate a solid walled building from within, but they broadly fall into four categories:

1) Rigid insulation boards

These come in a variety of materials and thicknesses and deliver the highest energy saving. Some have pre-attached plasterboard which makes the installation process more straightforward.

 2) Dry lining

Here, battens are fixed to the walls, insulation is fitted between them and then covered with plasterboard. This is a good option if the wall has a lot of heavy fittings such as book cases or kitchen cupboards, or if the original wall is rough and uneven, as in some stone properties.

3) Flexible thermal lining

This comes in rolls like thick wallpaper and is glued to the wall with a special adhesive. It may not provide the same level of insulation but can be installed by a competent DIYer. Flexible linings tend to be no more than 10mm thick so can be a good option for small rooms.  

4) Insulated plaster

This is a mix of plaster and insulating material, such as cork. It is trowelled or sprayed on. It is a good option for uneven walls and can help achieve good levels of airtightness. 

Managing moisture

Most solid walls were built to be ‘vapour-open’ meaning that moisture can pass through the wall. If we insulate with ‘vapour-closed’ materials (such as foil-faced insulation foam) we risk trapping moisture inside the wall which could lead to damp, mould and damage to the building. If the existing wall is vapour-open you should always use vapour-open insulation materials, plasters and paints. Suitable insulation materials include woodfibre, mineral boards and cork. These should always be plastered with a lime-based finish and painted with vapour-open paints.

Average cost and savings

Internally insulating your home will probably cost between £5,500 and £8,500, depending on the number of rooms are being renovated and the type of material being used. It's generally less expensive than external solid wall insulation because of the lower cost of materials and labour. A typical 3-bedroom semi-detached house using gas heating could save around £260 per year on heating bills by installing internal wall insulation whereas a detached house could save around £430 per year (ref).


Internal 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 skip may be required.
  • Rooms where the work is being done may not be usable whilst the work is being carried out and furniture, kitchen units etc may need to be temporarily removed.
  • Fittings such as radiators, skirting boards, window sills and plug sockets that are on the wall that is being insulated will need to be removed and reattached afterwards.
  • Pipe work and wiring may need to be re-laid.
  • The works will produce dust and could be noisy at times.
  • Contractors will require water and power, and the use of a toilet.
  • The newly insulated walls and adjacent surfaces will need to be re-decorated when the work has finished.

A cut-away model of internal solid wall insulation.

1) Surface coating (green)internal solid wall insulation

This is the new plasterwork that covers the insulation layer. Just like a normal internal wall, it can be painted (green in this case) or papered

2) Insulation (orange)

This is the layer that prevents warmth escaping through the outside walls of the house. In this case, rigid insulation boards have been used.

3) Internal wall (white)

This is the old internal plaster which is now covered by the new insulation board and plasterwork.

4) Windows

Ideally, the insulation board is fitted to the inside of the window recess to prevent cold patches developing where condensation forms. But, as is the case here, this is not always possible.

5) External wall (brickwork)

From the outside, your property will look exactly the same.

Image: Will Anderson 

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.

Need more help?

We can advise you about saving energy, or help you understand what grants and support you're eligible for:

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This information is available as a freely downloadable PDF from this page.

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