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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?

  • It’s usually less expensive than external insulation.
  • It’s easier to install, and there’s no need for scaffolding.
  • It won’t change the outer appearance or character of your home, so it’s more suitable for visually attractive buildings and period properties.
  • It’s also more suitable for flats and maisonettes, as you can hardly use external insulation for just one unit in a block.
  • It can also be acceptable for buildings in Conservation Areas where there are often strict rules about the kind of changes you can make to the outside appearance of your home.

What are the disadvantages of internal insulation?

  • Before you start, you’ll need to sort out any damp problems, as rising or penetrating damp can ruin the effects of your insulation.
  • It will reduce the size of the room, as it will bring the wall in by about 4 inches.
  • It’s quite disruptive, as the installers will be working in your house.
  • You’ll need to carry out a lot of preparatory work, as you’ll have to move furniture, pull away any carpet laid up to the wall, and remove skirting boards, door frames, wall light fittings, radiators, pipework, coving and cornices.
  • And afterwards you’ll have to re-lay the carpet after cutting 4 inches or so from the edge, and replace all the fixtures and fittings you removed.
  • If the wall’s surface is lumpy and uneven (this is often the case in older buildings, particularly in rural areas), it will need to be levelled off with plaster or render before fitting rigid insulation boards.
  • If you use rigid boards, it may mean your walls are no longer suitable to support heavy items like wash basins, kitchen units or radiators. However, you can get special fixings for this, and if you choose stud wall insulation, that should be strong enough to support these fixtures.
  • It could damage or cover up period features.

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?

  • It’s less disruptive, as the installation is carried out externally – although your gas, electricity or water may have to be cut off for a while at some stage, and your garden may take a bit of a battering.
  • It doesn’t reduce the size of any rooms.
  • It can improve the appearance of your home’s outside walls, as you can choose whatever finish you want: smooth, textured, painted, tiled, panelled, pebble-dashed or brick slips. Cladding can be timber panels, shingles, tiles, aluminium panels or brick.
  • It can make your home more soundproof and weather resistant.
  • It fills cracks and gaps in brickwork, which can reduce draughts.
  • There’s less risk of ‘cold spots’ causing condensation and mould on the inside of the walls, because the insulating material is applied continuously without leaving any gaps.
  • What are the disadvantages of external insulation?
  • It’s expensive – so it’s probably best to arrange for it to be done at the same time as other external work on your home, as this could help to reduce the cost.
  • You may need to get planning permission; please check with your local council.
  • The installers need to be able to get access easily to the outer walls of your home – so if you live in a terrace, they may need to carry the scaffolding through your house.
  • It’s not suitable if the exterior walls of your home are dilapidated and can’t be repaired before installing external house insulation.
  • It’s not suitable for period or character properties.
  • All external pipework and other fittings, such as satellite dishes or security lights, will need to be removed and replaced.
  • You may have to extend window sills or roof eaves to accommodate the extra thickness of your exterior wallIt could prevent old walls in a period property from breathing.
  • It’s not suitable for DIY.

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:

  • To show you before and after photos of similar work they’ve done.
  • What ‘U-value’ your insulation will achieve once the work is completed. This is the rate at which heat will pass through the insulated wall. It should be no more than 0.30 watts per square metre kelvin.
  • Which materials they recommend, and why.
  • Whether there’s likely to be any build-up of moisture in the rooms once they’ve been insulated, and what you should do to mitigate this.

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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