Solid wall insulation

On average, around a third of the heat lost from uninsulated homes escapes through the external walls. Solid wall insulation plays an essential role in regulating the temperature of your home efficiency, alongside your heating system.

Most homes built before the 1920s have solid brick walls, which have very poor insulating qualities. That accounts for 8.5 million homes in Great Britain, and more than 90% of those are uninsulated.

If your home has solid walls, you could save £115-360 a year by installing solid wall insulation.

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Internal vs external solid wall insulation?

Solid wall insulation can be complex and disruptive. If you pay an installer to insulate the entire house at once, including full repair and redecoration, it will cost a significant sum. 

You can save money by planning insulation work at the same time as other building or decorating projects, or spread the cost by doing the work in stages. Either way, it's important to understand all the options and carefully weigh up the pros and cons to avoid costly mistakes.

Solid walls can be insulated externally, which changes the outward appearance of the property, or internally, which leaves the exterior unchanged but slightly reduces the internal floor area. 

Prices vary widely depending on your home and the type you choose, and installation can be disruptive, but the bill savings and aesthetic improvements can be significant.

External solid wall insulation (EWI)

A continuous layer of insulating boards is fixed to the outside walls with adhesive and mechanical fixings, then coated with render or another finish such as brick slips, tiles or timber cladding. If your home is in need of a facelift, this can be a good way of improving its kerb appeal as well as its energy efficiency rating.

In the case of period homes with attractive features, this option may be unacceptable. If your home is listed, or in a conservation area, planning rules may not allow external wall insulation.

The most thermally efficient EWI systems typically use plastics such as expanded polystyrene (EPS), polyisocyanurate (PIR) or phenolic boards. 

More sustainable, breathable materials such as wood fibre and cork boards are often recommended for older buildings as they reduce the risk of damp. They are less efficient, though, so you need thicker boards to achieve the same U-value.

EWI installation usually requires scaffolding, and can take around 7-10 days for a semi-detached house. You may need additional work such as extension of the eaves, window and door surrounds, and any pipework and other services that penetrate the walls, to accommodate the extra wall thickness.

The thicker walls can also reduce outside areas such as paths and driveways at the side of the house, so make sure this won't cause a problem.

Internal solid wall insulation (IWI)

IWI consists of either a layer of insulation-backed plasterboard fixed to the inside of the walls, or an additional stud wall filled with insulation material, generally around 100mm thick. This slightly reduces the floor area of each room.

It costs around the same to install as EWI and the energy savings are similar. It can be very disruptive though, as you’ll need to remove radiators, wall sockets, built-in furniture, skirting boards and door frames prior to installation, and replace and redecorate afterwards. IWI can make it more difficult to fix shelves and other fittings to walls; you may need longer fixings to reach through to the solid wall.

It may also be problematic in period homes, where decorative features such as covings, picture rails and dados must be removed during installation, although they can often be replaced afterwards.

If you’re planning to redecorate, or fit a new kitchen or bathroom, it’s an ideal time to add internal insulation as it can be done room by room.

Looking to buy a new bathroom? Read our guide to the best and worst bathroom brands to find out which companies were rated the best.

Is solid wall insulation right for you?

Existing damp or structural problems must be rectified before fitting any solid wall insulation. The choice to insulate internally or externally depends largely on your budget and whether you wish to maintain the external appearance of your home or maximise the internal floor area.

If you want to retain the character and original features of your home at the front, but the rear of the building is less important, you can opt for a hybrid solution. IWI at the front of house allows you to retain a period façade, while EWI at the sides and/or rear can be less disruptive and doesn't require removal of kitchen and bathroom fittings, if they are in that part of the house.

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How much does external solid wall insulation cost? 

We've worked with the Royal Institution of Chartered Surveyors* (RICS), which publishes average building work and repair costs, to bring you the average cost for external solid wall insulation. 

We've split it so you can look at the average costs for a terraced, semi-detached and detached house. Costs will vary depending on where you live in the country. 

Which? members can log in to see our detailed costings and savings for solid wall insulation across a range of typical types of home. 

Not a member? Join Which? for access to our pricing data for insulation and other key home improvement projects.

Solid wall insulation is more expensive than cavity wall insulation, but it should eventually lead to bigger savings on heating bills. 

The type you need is determined by what type of walls you've got. 

If you have cavity walls rather than solid walls, the process is quicker and cheaper. Head to our guide to cavity wall insulation cost and savings to find out more.

Solid wall insulation subsidy

A subsidy called the Energy Company Obligation (ECO) might be available to you. Find out more in our guide to the Energy Company Obligation.

You can find out more information about solid wall insulation, including how to find an installer, from the NIA and the Insulated Render and Cladding Association. 

To find a local, trustworthy installer, go to Which? Trusted Traders. You can also use or Trusted Traders search tool below.

RICS cost calculations

*To arrive at the average prices above, RICS uses cost data from its Building Cost Information Service (BCIS) database, where costs are collated from a variety of sources and analysed. 

Materials costs are based on the best trade prices from a range of suppliers across the UK, which are then benchmarked to reveal the best national average. Labour rates are based on the current Building and Allied Trades Joint Industrial Council wage agreement.  Data copyright RICS 2023, reproduced with permission. Data is current as of November 2023. 

Answering key solid wall insulation questions

What type of walls does my property have?

If your home was built any time from 1920 onwards, it's likely to have cavity walls. The cavity should be at least 50mm wide and the walls need to be in good condition to be suitable for cavity wall insulati. 

If your home was built in the past 20 years, it's likely that the cavity walls have already been insulated.

If your home is more than 100 years old, it's likely to have solid walls. That means you'll need solid wall insulation to stop heat from escaping.

Can I get financial help for solid wall insulation?

There is little financial help available to help you pay for solid wall insulation. Contact your council's energy efficiency officer to find out if something might be available in your area. Certain households might qualify for the Energy Company Obligation.

Do I need planning permission for solid wall insulation?

Planning permission is not normally required for wall insulation.

However, if your property is listed, is in a conservation area or if the insulation will change the appearance of your property, you should consult your local planning authority.

Do I need to comply with Building Regulations?

If you’re adding or replacing internal or external render or cladding it must comply with the relevant building regulations for existing buildings. Currently, this means that solid walls should achieve an improved U-value of 0.3 W/m2K. Where this is not technically or functionally feasible, then the walls should be upgraded to the best U-value possible, but not higher than the ‘threshold’ U-value of 0.7 W/m2K.

Where can I find a solid wall insulation installer?

Make sure you choose an installer registered with one of the following organisations:

• TrustMark
• National Insulation Association (NIA)
• British Board of Agrément (BBA).

Check that your installer has signed up to a code of conduct, such as the NIA's Code of Professional Practice. And check they are registered with the Solid Wall Insulation Guarantee Agency (SWIGA), which provides 25 year guarantees for wall insulation fitted by registered installers in the UK. 

To find a local trustworthy installer, go to Which? Trusted Traders. 

You can also contact Simple Energy Advice with any questions by calling 0800 444202.

What other types of insulation are available?

Insulating your loftdoesn't cost much and could save households as much as £415 a year on their energy bills, according to Energy Saving Trust.

You can buy jackets to fit around your hot water cylinder and piping. These are available from DIY stores and you can fit them yourself, as long as your pipes are easily accessible.

The energy savings are smaller than with wall or loft insulation but, as jackets cost around £15, they pay for themselves in a year or less.

You can also reduce the amount of heat lost in your home by fitting draught excluders around doors, windows and letterboxes. Gaps between skirting boards and floorboards are also worth tackling.

Double glazing also cuts heat loss and reduces noise and condensation problems. Remember to close your curtains at dusk to stop heat escaping.