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The ultimate guide to solid wall insulation

Solid walls let twice as much heat escape as cavity walls, so insulating them makes a good deal of sense.

However, while cavity walls can be insulated quickly and cheaply, solid wall insulation is an expensive undertaking. You can insulate either the inside or the outside of the wall – both methods have benefits and disadvantages.

How can I tell if my home has solid walls?

If your home was built before 1920, it probably has solid external walls rather than cavity walls, which only came into widespread use in the 1920s.

For brick-built homes, you can also tell by checking the thickness of your external walls. Do this by measuring them in an external doorway or window; if a wall is less than 10 inches thick, it’s probably solid.

Stone-built houses can have much thicker walls, but these are very rarely cavity walls. If you live in a timber- or steel-framed home, these will be different again, and the insulation advice in this article probably won’t be relevant to you. Find out more from the National Insulation Association.

A word about words: just to be clear, we only recommended insulating the external walls of your house – the ones that let in heat from the outside world. However, you can do this internally or externally – i.e. insulate the inside of your external walls or the outside of your external walls. So from now on, the word ‘external’ just means the outside of the external walls of your house, and the word ‘internal’ means the inside of the same external walls.

How much could I save by insulating my home’s solid walls?

These figures are from the Energy Saving Trust’s website.

Type of property Detached Semi detached Mid terrace Bungalow Flat
Fuel bill savings per year £455 £260


£180 £145
Carbon dioxide savings per year 1,900 kg 1,100 kg 740 kg 610 kg 610 kg

These figures are based on insulating an average gas-heated home.

How much does external wall insulation cost?

The cost of insulating walls externally is around £100 per square meter[1], so it can range from £8,000 for a small flat up to £22,000 for a large detached house, and it could take many years to earn back your investment. These costs vary significantly, depending on how much work you’re having done, how easy it is for installers to get at the walls and so on.

For internal walls it could be £3,500 - £14,000, which is obviously considerably less, but you’d still take a while to earn your money back.

How could I reduce the costs of solid wall insulation without compromising on quality?

The costs quoted above are for paying expert installers to come to your home, insulate all your external walls, redecorate afterwards and replace everything just as it was.

However, you might be able to reduce these costs by combining the work with other building or decorating projects. For example, if you choose external insulation, you’ll need scaffolding – but if you combine the outside wall insulation project with work on your roof, or painting the exterior woodwork, you could spread the scaffolding costs across both projects. If your home’s exterior walls need repairing or repointing, you could combine this with the external house insulation work and cut back on costs.

If you choose to insulate the walls internally, you could spread the cost and reduce the disruption by doing it one room at a time.


What is internal wall insulation?

Solid walls can be insulated internally by fitting rigid boards to the wall, or building a separate stud wall and filling the gap with insulating material – for example, mineral wool fibre.

Rigid insulation boards are around 2 ½ - 4 inches thick, and made of plasterboard backed with insulation material, usually foamed plastic. They’re normally fixed on to the wall with ribbons of plastic or adhesive, or fitted to battens if the walls are lumpy and uneven. The installer will then add extra fixings to hold the boards firm, and seal the joints between the boards.

Stud walls are made of metal or wooden frames, which are attached to the wall. Once the insulation material has been inserted into the gap between the studwork frame and the wall, the new wall is plastered and can then be painted or wallpapered. Alternatively, it could be covered with rigid insulation boards, making your insulation even more effective and your running costs lower, but further reducing the size of your room.

What are the advantages of insulating internally?

What are the disadvantages of internal insulation?

What is external wall insulation?

If you decide to insulate your home on the outside, your installer will fix a layer of insulating material to the wall, using mechanical fixings and adhesive, and then cover it with protective layers of render or cladding. Render is generally cheaper than cladding but doesn’t offer you as much choice.

If the surface of your wall is strong enough to support the insulating material, your installer will go ahead and fit it. However, if the existing render is old and cracked it will need to be removed, adding to the cost of the operation.

If you live in a historic building, or want to improve the appearance of your home as well as its heat retention, some architects and installers can offer bespoke insulation solutions for your property. These can be created from a variety of finishes such as oak weatherboarding, stone, glass or terracotta tiles.

Before work starts, it’s absolutely vital to make sure you have no rising or penetrating damp in the walls. Outside wall insulation will cover up damp patches temporarily but the actual problems will get worse.

What are the advantages of insulating outside?

Can I do the solid wall insulation work myself?

If you’re a very experienced, expert DIY-er you might be able to do internal wall insulation, but external solid wall insulation should only be fitted by a specialist installer who’s been trained by an approved system designer.

If you do decide to attempt internal insulation yourself, be sure you keep all your receipts for the materials, and take photographs of the installation at each stage. You’ll need these for an Energy Performance Certificate if you ever want to sell or let your home.

Before you start, make absolutely sure there’s no damp in the walls you’re about to insulate. Lining them with insulation could actually make the problem worse.

How do I choose an insulation installer?

As with most building work, it’s probably best to get a range of quotes from professional installers before you make a decision.

You can search for specialist companies through the main trade association websites, such as the National Insulation Association (NIA) for either internal or external work, or the Insulated Render & Cladding Association (INCA) for external insulation.

Before you commit to a specific installer, ask them:

What sort of guarantee can I expect?

Before you give your installer the go-ahead to start work, ask whether they offer a guarantee from SWIGA (the Solid Wall Insulation Guarantee Agency). This will give you 25 years’ cover for defective materials, design or workmanship. You should also make sure the products and materials they’re using are BBA (British Board of Agrement) certified.

There are other appropriate guarantees, such as a Kinnell ECO Guarantee (which covers a range of natural building technologies).

Will I need to get planning permission, or comply with any building regulations?

You shouldn’t need planning permission, but you may need a building warrant. Your installer should arrange approval from the local Building Control Office. Check that they’re going to do this – if they’re not, make sure you contact the Building Control Office before any work starts.

Ask your local council whether there are any special regulations or permissions that affect your property. For example, if your home is a listed building, you may need Listed Building Consent, and if it’s in a Conservation Area or World Heritage Area there are probably restrictions on what you can do to change its external appearance.

It’s also worth checking with your local council whether there are any solid wall insulation grants available in your area to help you with the cost of the work.

If you live in a flat in a building with several owners, and you want to arrange external insulation, you’ll need to get permission from each of the owners.

Is it okay to use ‘non-breathable’ insulation materials in my home?

In traditional solid-wall buildings, it’s usually best to use ‘breathable’ or ‘vapour permeable’ insulation materials, as they let air move around and stop moisture building up.

However, if for some reason you want to use non-breathable (aka vapour impermeable) materials, we suggest you ask your installer what steps you should take to keep rooms fresh and dry.

*Source and notes for graphs and table

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