Internal and external wall insulation for solid walls

In October 2025, the National Audit Office reported on failures with the quality of external and internal wall insulation under government schemes.

In response, the UK Government is offering affected homes free audits to identify any issues. TrustMark is conducting these audits. If they identify any problems, then TrustMark will arrange for them to be fixed at no cost to you.

Find out more about TrustMark’s free Find & Fix audit offer.

Solid wall insulation is an extra layer that can help keep more heat inside your home.

Solid walls can be insulated either by applying insulation:

• on the outside surface (external wall insulation)
• on the inside surface of the walls (internal wall insulation)

Before you can insulate your walls, you need to find out what sort you have. Most houses in the UK have either solid walls, or cavity walls, some have timber frames.

If your home was built before the 1920s, its external walls may be solid. Walls from that era were usually made of brick or stone, but they could also be made of earth.

Solid walls have no gap, so they can’t be filled with cavity wall insulation. It’s also possible that a pre-1920s house could be made of a timber frame with panels between the timbers.

If your house was built after the 1920s, its likely to have cavity walls. Find out more about cavity wall insulation.

If your house was built before 1920 and is made of stone, the walls are likely to be solid. However, some newer stone buildings have modern cavity walls.

If your house is made of brickwork that’s visible on the outside, look at the pattern of the bricks.

All walls have an ability to retain heat in your home. But how well they do this depends on the materials they’re made of. This also affects how much benefit you’ll get from insulating them.

For example, a thin single brick wall or timber framed building will lose heat more rapidly than a modern insulated cavity wall. They’ll also lose heat quicker than a thick stone or earth one. Therefore, single brick wall or timber frame buildings might benefit more from being insulated.

Solid wall insulation might not be suitable for a few reasons if your home is:

• exposed to extreme weather
• exposed to flooding
• in a poor condition

It might also not be suitable if there’s a risk of your rooms overheating.

You should check for these risks should with an experienced installer or buildings professional.

Insulating solid walls internally or externally can:

• reduce heat loss
• lower your energy bills
• reduce risk of condensation forming on your walls
• reduce draughts

Typical installation costs of solid wall insulation can vary depending on the size of your house and its walls.

Based on a typical three-bedroom, semi-detached house in Great Britain:

• External wall insulation can cost around £18,000.
• Internal wall insulation can cost around £12,000.

If you’re already making home improvements, then you might be able to reduce costs by carrying out the work at the same time.

The costs we give for installing solid wall insulation are based on:

• paying a company to come in
• insulating your whole house in one go
• fully redecorating to restore it back to original condition in the case of internal insulation

If you want to spend less, you might want to have it done at the same time as other work. For example, internal insulation can be fitted when you’re planning to redecorate.

External insulation can also cost less if you fit it when you’re having other work done to the outside. This work might be:

• having a new roof
• fitting new windows
• fitting solar panels
• repairing gutters or your roof

With these jobs, you’ll probably have scaffolding already, so you can make the most of it by fitting external wall insulation at the same time.

It’s important to plan ahead and think about potential future work you’ll want to do throughout the whole building.

It’s crucial that you complete any repair and maintenance to the exterior of your building before adding any wall insulation. This includes:

• dealing with damaged brickwork or pointing
• repairing roofs and overhangs
• repairing or renewing gutters and downpipes

Not doing these jobs first might trap damp or cause your insulation to fail.

You might be able to get funding or grants for insulation. Find financial support for energy efficiency in your area.

Here are a few things to think about if you’re getting solid wall insulation.

In properties with solid walls, traditional materials (such as brick, earth, stone, timber and lime) can usually manage moisture well.

This keeps it in balance with the surrounding environment, meaning damp never becomes a problem. This is because the materials are permeable, meaning they readily absorb and release moisture in response to changes around them. It might also be partly because of the high levels of ventilation caused by gaps in poorly fitting door or windows.

Energy efficiency improvements like solid wall insulation can affect this balance. This makes it important to consider moisture when fitting solid wall insulation to make sure you avoid damp problems occurring.

When you insulate an older building, you change the way that moisture behaves, both as liquid and vapour.

• The insulation may create a barrier and trap moisture (depending on materials) or reduce ventilation through the wall.
• Internal wall insulation can make the external wall colder on the outside so moisture may build up inside the wall.

To avoid moisture issues, your installer should carefully consider the materials used. This includes:

• using suitable permeable materials that will help prevent moisture getting trapped in the wall.
• using suitable permeable finishings such as paint, wallpaper or any other wall covering.

You should check with your installer or another experienced professional about selecting suitable materials.

Your installer should also check:

• the condition of the wall and roof, ensuring that any repairs are made as part of the work
• if you have any flood risk or have high driving rain exposure
• if they need to provide you with more targeted ventilation, such as extractor fans

Air needs to flow in and out of your house so it stays fresh, dry and healthy. It also helps prevent the build-up of moisture and damp in your home.

They shouldn’t block or seal any intentional ventilation such as:

• Underfloor grilles or airbricks: these help keep wooden beams and floors dry.
• Wall vents and trickle vents: they let small amounts of fresh air flow into rooms.

A registered installer should also guarantee the work for 25 years. An experienced specialist building professional and/or installer can help you design a proposal tailored to your building.

Yes, it’s often possible to insulate an older or listed building in some way, where it’s appropriate to do so. When considering wall insulation for any property you should seek professional advice.

Insulation influences the building and its internal environment, including possibly increasing the risk of moisture build up or overheating within your home.

It’s particularly important to seek advice if you have a listed building or live in a conservation area. Some insulation options might change a home’s character or appearance, so insulating these properties will require some kind of permission. You should check with your local planning department to find out more.

Historic England has general advice for listed building owners looking to increase the energy efficiency of their home, and in-depth advice on installing insulation.

Historic Environment Scotland also has a guide on energy retrofit of traditional buildings

Welsh Government also has detailed guide on external solid wall insulation

• Internal solid wall insulation should be fitted by a professional installer.
• External solid wall insulation should be fitted by a specialist installer trained by an approved system designer.

Applying external wall insulation involves putting a layer of insulation material on the outside of the wall. This is then covered with a type of render (plasterwork) or cladding.

The finish can be smooth, textured, painted, tiled, panelled, pebble-dashed, or finished with brick slips, depending on what you need.

Benefits of external wall insulation include:

• Can be applied without disruption to the household.
• Doesn’t reduce the floor area of your home.
• Renews the appearance of outer walls.
• Should improve weatherproofing and sound resistance.
• Should increase the lifespan of your walls by protecting the brickwork.
• Can reduce condensation on internal surfaces and can help prevent future damp issues (but cannot be installed before any existing damp issues are resolved).

You’ll also need to think about:

• Costs: external wall insulation is best installed at the same time as other external home improvements work to reduce the cost. Often, it will require works to extend your roof, move rainwater pipes and utility meter boxes.
• Planning permission: check with your local council to see if you need permission.
• Airtightness and ventilation: external wall insulation can reduce the air flow into your rooms as any cracks and gaps in the wall get filled. This means your installer should assess what additional ventilation may be needed.
• Access to the outer walls: you need to be able to get to the walls easily.
• Maintenance and repair: any wall insulation can’t be done before fixing any problems with damp or if the outer walls are not structurally sound.

Applying internal wall insulation to the external walls of your home involves:

• Fitting rigid insulation boards to the internal side of an external wall.
• Building a stud wall on the internal side of an external wall filled in with insulation material.
• Applying insulating plaster to the internal side of the external wall.

Benefits of internal wall insulation include:

• Generally cheaper than applying external wall insulation.
• Doesn’t affect the external appearance of outer walls.
• Should improve sound resistance.
• Can reduce condensation on internal surfaces and can help prevent future damp issues (but cannot be installed before any existing damp issues are resolved).

But there are still a few things you’ll need to consider with internal wall insulation.

• Floor area: it will slightly reduce the floor area of any rooms where it’s installed.
• Disruption: it can be quite disruptive to install as rooms cannot be used while work takes place. It’ll also involve some level of redecorating.
• Extra work: requires skirting boards, door frames and external fittings to be removed and reattached.
• Airtightness and ventilation: internal wall insulation can reduce the air flow into your rooms. This means your installer should assess what additional ventilation may be needed.
• Shelving and other wall fixtures: it can it difficult to fix heavy items to inside walls (although you can get special fixings).
• Maintenance and repair: wall insulation can’t be done before fixing any problems with damp.

TrustMark is a government-endorsed quality assurance scheme for work done in and around your home. Find a TrustMark-certified insulation installer via their find a tradesperson service. You should get at least three quotes to help find the best value.

Any TrustMark solid wall insulation company will offer a 25-year guarantee. This will either be backed up by an insurance company or provided through the Solid Wall Insulation Guarantee Agency (SWIGA).

Yes, through TrustMark’s free ‘Find & Fix’ audit offer.

98% of homes fitted with external wall insulation under ECO4 and the Great British Insulation Scheme (GBIS) have problems that need fixing.

In response, the UK Government is offering affected homes free audits to identify any issues. TrustMark is conducting these audits. If they identify any problems, then TrustMark will arrange for them to be fixed at no cost to you.

What action do I need to take?

If you’ve had external wall insulation installed under with ECO4 or GBIS, you should’ve had a letter from Ofgem in early 2026. This letter includes a Unique Measure Reference Number.

TrustMark will contact you to offer a free audit check of your home. They’ll get in touch by email, phone or by post. You can also contact TrustMark yourself:

• by phone: 0333 555 1234 and select Option 1
• by email: [email protected]

Important: only legitimate parties (like Ofgem and TrustMark) will know your Unique Measure Reference Number.

If someone offers you a home audit check but can’t provide this number, don’t share any personal details with them or let them into your home. Find out what to do if you suspect someone is trying to scam you.

Watch TrustMark’s video to find out more.

Solid wall insulation is an effective way to cut heat loss from your home and save money on energy bills. You can also insulate other areas of your home to save even more.

Find out about other types of insulation and other energy efficiency measures.

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