The ultimate guide to energy-saving solid wall insulation

03 September 2020 | Matt Mostyn

In case you didn’t realise, solid walls let twice as much heat escape as cavity walls – so insulating them makes a good deal of sense. It’s possible to insulate either the inside or the outside of your walls, and each option has its pros and cons.

savings in energy bills of solid wall insulation

Here's how much you could save on your energy bill (£/year) with solid wall insulation1

While cavity walls can be insulated quickly and cheaply, solid wall insulation is a more expensive undertaking. But it’s worth remembering that while the initial costs are greater, the savings on your heating bills will be bigger, too!

A word about words: just to be clear, we only recommend insulating the external walls of your house – ie. the ones that let in air from the outside world. You can do this internally or externally – for instance, by insulating the inside or the outside of your external walls. With that in mind, when we use the word ‘external’, we just mean the outside of the external walls of your house – while the word ‘internal’ means the inside of the same external walls.

Here’s our handy guide to the ins and outs of solid wall insulation:

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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. Instead, read our guide to cavity wall insulation to find out everything you need to know.

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How much does external wall insulation cost?

The cost of insulating walls externally is around £100 per square meter2 – so it can range from £8,000 for a small flat, up to £22,000 for a large detached house. With that in mind, it could take a fair few years to earn back your investment. But costs can vary significantly, depending on how much work you’re having done, ease of access for installers and so on.

For internal walls, the average cost is around £7,4003 – 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?

If you're hoping to spend less, think about insulating a wall when you’re having other building or decorating work done. Internal insulation is a good add-on when you’re planning to redecorate, or to fit a new kitchen or bathroom. 

You could also spread the cost by tackling one room at a time. And external insulation will also cost less if you do it when you're having other work done outside. For instance, if you're having a new roof, painting windows or getting solar PV panels fitted, you’ll probably already have scaffolding up, which can save a bit on costs. 

Whatever work you’re doing, it’s worth getting a quote that includes insulation – as it’ll probably work out cheaper than doing the two things separately.

Solid wall insulation savings

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

Type of propertyDetachedSemi detachedMid terraceBungalowFlat
Fuel bill savings per year£375£255£145£150£105
Carbon dioxide savings per year1,540 kg930 kg590 kg620 kg425 kg

Figures based on insulating an average gas-heated home in England, Scotland and Wales. Prices based on fuel prices as of April 2019.

Reduce installment costs with a solid wall insulation grant

Introducing the Green Homes Grant scheme – a government-run initiative offering homeowners and landlords in England the opportunity to apply for some very useful money-saving and energy-saving vouchers. 

The scheme is designed to help with the cost of installing everything from insulation to heat pumps and solar thermal. You can choose from a number of categories, including insulation for solid walls, as well as insulation for under-floor, cavity walls, loft, flat roof, room-in-room or roof.

The vouchers are worth up to two-thirds of the cost of making your home more energy efficient  – up to a maximum of £10,000 per household. 

Find out more about the Government’s Green Homes Grant scheme.

If you’re in Scotland, there are various other grants and loans set up to help you with insulation costs. Contact your local authority to see if there’s any in your area, and read more about financial support to improve your home.

External vs internal insulation

Internal solid wall insulation

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 2½ to 4 inches thick, and made of plasterboard backed with insulation material, usually foamed plastic. They’re normally fixed 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. This includes moving furniture, pulling away carpet laid up to the wall and removing skirting boards, door frames, wall light fittings, radiators, pipework, coving and cornices.
  • 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’ll 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. But you can get special fixings, and stud wall insulation 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. They’ll 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. But 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 offer bespoke insulation solutions for your home. They can use a variety of finishes, such as oak weatherboarding, stone, glass or terracotta tiles.

Before work starts, it’s 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 unfortunately 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 – whether 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 reduce the cost.
  • You may need to get planning permission, so check with your local council.
  • The installers need to be able to easily access the outer walls of your home – so if you live in a terrace, they may need to carry 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 wall.
  • It could prevent old walls in a period property from breathing.
  • It’s not suitable for DIY.

Solid wall insulation DIY

solid wall insulation DIY

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 trained installer.

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.
  • The ‘U-value’ of your insulation once the work is completed. This is the rate at which heat passes 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 can do to minimise 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 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.

Thinking about expanding your home? Read our guide to eco-friendly loft conversions.

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.

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

Looking for other ways to reduce energy waste and save money? Check out our most affordable energy plan, Better Smart and start saving on your gas and electricity bills today. 

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More ideas to help you insulate your home

Our user guides give you lots of ideas for ways you can improve your home’s insulation levels for roof, loft, walls, windows and doors. And for some inspiring energy efficient design, see our guide to the amazing 'superinsulated' Passive Houses.

Cavity wall insulation

Insulating your cavity walls could be a very cost-effective way to retain heat in your home and save money on your bills. It’s estimated that cavity insulation could save you up to £160 a year in heating bills. Find out more in our guide to cavity wall insulation.

Roof and loft insulation

We lose much of our body heat through our heads – and the same goes for your roof! As much as a quarter of the heat in uninsulated homes wafts out through the roof. Filling your loft space with insulation could be a very cost-effective way to retain heat in your home and save on your energy bills. Find out more in our guide to roof and loft insulation.

Draught-proofing windows and doors

Draught-proofing your home’s windows and doors could save you between £25 and £50 on your heating bills each year – not to mention the environmental benefits. Find out more in our guide to draught-proofing windows and doors.

1Estimates based on insulating a gas-heated home.Costs may vary significantly depending on level of work required. Above estimates are based on a typical install, ranging from a small flat to a large detached home. Prices are based on fuel prices as of April 2019.

2 The Greenage - External Solid Wall Insulation

3 Energy Saving Trust - Solid Wall Insulation  Based on a typical semi-detached house in Great Britain.

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