Cavity Wall vs External Wall Insulation

Uninsulated walls let a lot of heat escape. Whether you can have cavity wall insulation or need external (or internal) wall insulation depends mostly on how your walls were built. This guide compares the two, with typical costs and savings from the Energy Saving Trust (EST), and explains which funding schemes are still open.

Understanding Your Wall Type

Before comparing insulation methods, you need to know how your walls are built. This decides which options are open to you.

Cavity Walls

If your house was built after the 1920s, its external walls are probably cavity walls: two masonry walls with a gap between them. Houses built from the 1990s onwards usually already have wall insulation.

How to identify cavity walls:

• Built after the 1920s (most likely)
• Bricks laid in an even pattern, all lengthways
• A brick wall between 260mm and 350mm thick, measured at a door or window opening

Solid Walls

Homes built before 1920 are more likely to have solid walls with no cavity, usually brick or stone. Some older houses do have cavities, but they are often too narrow to insulate and should be treated as solid. Solid walls need external or internal wall insulation.

How to identify solid walls:

• Built before 1920 (most likely)
• Bricks in an alternating pattern, with some short ends showing
• A brick wall narrower than 260mm; stone walls tend to be solid at any thickness

If your walls are rendered or clad, or you are unsure, ask a building professional. Source: Energy Saving Trust.

Cavity Wall Insulation: The Overview

Cavity wall insulation fills the gap between the two layers of your external wall with insulating material. For suitable homes it is quick and causes little disruption.

How Cavity Wall Insulation Works

After a survey, installers drill small holes in the outside wall and inject insulation into the cavity, then fill the holes. EST says that for an average home with easy-to-reach walls this usually takes about two hours. It is not a DIY job.

Cavity Wall Insulation Materials

• Polystyrene beads: EST says these are the most suitable type for most homes
• Mineral wool fibre: blown into the cavity
• Foam: occasionally used where standard cavity insulation is not suitable; your installer should explain why

Cavity Wall Insulation Costs

Some households can get help with the cost through ECO4 or Warm Homes: Local Grant. ECO4 work may be fully or partly funded, and you may be asked to contribute. See the funding section below.

Energy Savings from Cavity Wall Insulation

EST estimates these yearly bill savings for a gas-heated home (July 2026 fuel prices):

• Semi-detached house: around £200 a year
• Detached house: around £380 a year
• Mid-terrace house: around £110 a year
• Detached bungalow: around £160 a year
• Mid-floor flat: around £110 a year

At those figures, a semi-detached house paying the full £2,200 and saving about £200 a year would take roughly 11 years to recover the cost from bill savings alone. Savings are estimates, not guarantees.

External Wall Insulation: The Overview

External wall insulation (EWI) means fixing a layer of insulation to the outside of your walls and covering it with render or cladding. It is one of the two ways to insulate solid walls.

How External Wall Insulation Works

Installers fix insulation boards to the outside walls, then apply a base coat, reinforcement mesh and a weather-resistant finish. Finishes include smooth or textured render, pebble-dash, panels or brick slips. EST says EWI should be fitted by a specialist installer trained by an approved system designer.

External Wall Insulation Materials

• EPS (expanded polystyrene): widely used and relatively affordable
• Mineral wool: better fire resistance, usually more expensive
• Phenolic foam: thinner for the same insulation value

On solid walls built from traditional materials, your installer should use materials that let moisture escape. Extra wall depth also affects windowsills, gutters, pipes, meter boxes and roof overhangs.

Solid Wall Insulation Costs

External wall insulation costs far more than cavity wall insulation because of materials, labour and scaffolding. EST says it can cost less if done alongside other outside work, such as a new roof or windows, when scaffolding is already up.

Energy Savings from Solid Wall Insulation

EST estimates these yearly savings for solid wall insulation (internal or external) in a gas-heated home:

• Semi-detached house: around £280 a year
• Detached house: around £500 a year
• Mid-terrace house: around £150 a year
• Detached bungalow: around £220 a year

If you pay the full cost yourself, bill savings alone take many years to repay it. Most people choose solid wall insulation for comfort and lower bills rather than a quick payback, or do it when funding is available.

Head-to-Head Comparison

Detailed Pros and Cons

Cavity Wall Insulation Advantages

• Quick installation: often done in a few hours with little mess
• Affordable: the cheapest way to insulate walls
• Invisible: no change to your home's appearance
• No planning permission: not usually needed
• Funding may help: ECO4 and Warm Homes: Local Grant can include it
• Proven: installed in UK homes for decades

Cavity Wall Insulation Disadvantages

• Not for solid walls: only works where there is a suitable cavity
• Damp risk if unsuitable: EST says it should not cause damp if installed correctly, but narrow or uneven cavities can let water in (Cavity Wall Advice explains the signs that a cavity fill has gone wrong)
• Not for exposed or flood-risk homes: driving rain and flooding can cause problems
• Limited by the cavity: cavities under 50mm are not suitable
• Hard to remove: removal needs a specialist company

External Wall Insulation Advantages

• No floor space lost: work is done outside
• Little disruption indoors: you can stay in your home
• Weatherproofing: should improve weather and sound resistance
• Protects brickwork: should increase the lifespan of your walls
• Renews appearance: a new finish to tired outside walls
• Less condensation: can reduce condensation on inside surfaces

External Wall Insulation Disadvantages

• High cost: around £15,000 for a typical semi (EST)
• Longer job: scaffolding stays up while work is done
• Planning permission: may be needed, especially in conservation areas or for listed buildings
• Changes appearance: which may not suit every home
• Extra work: roof overhangs, rainwater pipes and meter boxes may need moving
• Quality matters: poor installation under past schemes has left many homes needing repairs
• Ventilation: your installer should check whether extra ventilation is needed

When to Choose Cavity Wall Insulation

• Your home has cavity walls that are not already filled (a borescope check or your EPC can tell you)
• The cavity is at least 50mm wide and clear of rubble
• Brickwork is in good condition, and the walls are not exposed to driving rain or flood risk
• You want a quick, low-cost improvement that doesn't change how your home looks

When to Choose External Wall Insulation

• Your home has solid walls
• You don't want to lose floor space inside
• Outside walls need re-rendering or repair anyway, or scaffolding is already planned
• Planning rules allow a change to the outside of your home
• You have funding, or you are prepared for a long payback

What About Internal Wall Insulation?

Internal wall insulation (IWI) is the other option for solid walls: rigid insulation boards, an insulated stud wall or insulating plaster on the inside of the outside walls.

• When it suits: conservation areas, brickwork you want to keep, or rooms you are renovating anyway
• Cost: around £12,000 for a typical three-bedroom semi, including redecoration (EST)
• Savings: the same EST estimates as external wall insulation above
• Downsides: slightly smaller rooms, disruption while rooms are out of use, and skirting, door frames and fittings need refitting

Funding for Wall Insulation

Funding depends on your income, your home's EPC rating and where you live. It is never guaranteed.

• ECO4 (Great Britain, ends 31 December 2026): for households on certain benefits, or with income under £31,000 through ECO4 Flex Route 1, and other council routes. Owner-occupied homes need an EPC of D to G, private rented homes E to G. It is not a grant with a set amount; work may be fully or partly funded. Apply through an energy supplier or your council.
• Warm Homes: Local Grant (England, to 31 March 2028): run by councils for homes rated EPC D to G, usually with household income of £36,000 or less (or certain benefits or postcodes). Owner-occupiers pay nothing; landlords may contribute. Apply on GOV.UK.
• Great British Insulation Scheme: closed on 31 March 2026 and was not extended.
• Scotland: Home Energy Scotland offers grants and interest-free loans for insulation. Wales has the Warm Homes Nest scheme.

UK Energy Partnership is run by Harber Glaves Trading Ltd. We are not a government body and cannot decide who gets funding — check eligibility with the official scheme.

How to Get Started

1. See which schemes may apply using our grant finder tool
2. Contact an energy supplier or your council about ECO4, or apply for Warm Homes: Local Grant
3. Have a survey to confirm your wall type and whether insulation is suitable
4. Get the guarantee details in writing before work starts

Installation Quality: What to Look For

Whichever type you choose, installation quality matters. Poor installation can reduce performance or cause damp.

Installer Checks

• TrustMark: check the installer on the TrustMark register
• CIGA: a 25-year Cavity Insulation Guarantee Agency guarantee for cavity work (CIGA installers)
• SWIGA or insurance-backed guarantee: 25 years for solid wall insulation
• Several quotes: EST suggests getting at least three
• No cold calls: don't agree to work from unsolicited callers

Pre-Installation Surveys

• Cavity check: borescope inspection of cavity width, rubble and existing insulation
• Damp check: existing damp must be fixed before insulating
• Repairs first: brickwork, pointing, roofs, gutters and downpipes
• Exposure and flood risk: driving rain and flooding can rule insulation out
• Ventilation: airbricks, vents and trickle vents must not be blocked

Making Your Decision

• Cavity walls: cavity wall insulation, if a survey finds it suitable
• Solid walls: external wall insulation, or internal where outside changes are not possible

Cavity wall insulation is usually far cheaper and quicker. Solid wall insulation costs much more and needs careful design, but it is the main way to cut heat loss through solid walls. Where you qualify, funding may cover some or all of the cost.