Is solid wall insulation worth it?

Solid wall insulation is one of the most expensive retrofit measures, but for pre-1920s homes with uninsulated solid walls, it can make a big difference to comfort and running costs. This guide walks through the two approaches (external and internal), what they cost, and when they make financial sense.

Do I have solid walls?

Most UK homes built before 1920 have solid walls. Most homes built after 1920 have cavity walls. To check:

• Measure the wall thickness at a door or window reveal (the bit where the wall is exposed). Solid walls are usually 225mm (9 inches) thick. Cavity walls are usually 260mm to 300mm thick.
• Look at the brickwork pattern. Solid walls often have a Flemish bond (alternating long and short bricks in each row) or English bond (alternating rows of long and short bricks). Cavity walls usually have a stretcher bond (all bricks laid lengthwise, no short ends visible).
• Check your EPC certificate. It usually states whether you have solid or cavity walls.

If you have solid walls, they're probably uninsulated. Very few solid wall homes were built with insulation, and retrofitting it is expensive, so most homeowners haven't done it.

External vs internal: which should I choose?

There are two ways to insulate a solid wall:

External wall insulation (EWI)

Insulation boards (usually 90mm to 150mm thick) are fixed to the outside of the house, then covered with render (smooth or textured) or cladding (timber, brick slips, etc). The whole external appearance of the house changes.²

Pros:

• No loss of internal floor area.
• Better thermal performance (fewer cold bridges).
• Protects the brickwork from weather.
• Can improve the look of the house (new render, fresh appearance).

Cons:

• More expensive (£8,000 to £15,000 for a semi).
• Changes the external appearance (may need planning permission in conservation areas or for listed buildings).
• Requires scaffolding.
• Can be tricky around windows, porches, and bay features.

Internal wall insulation (IWI)

Insulation boards (usually 60mm to 100mm thick) are fixed to the inside of the external walls, then covered with plasterboard. You lose 100mm to 150mm of room width on each external wall.⁵

Pros:

• Cheaper (£5,000 to £10,000 for a semi).
• No change to external appearance (no planning issues).
• Can be done room by room (spread the cost).
• No scaffolding needed.

Cons:

• Reduces room sizes (particularly noticeable in small rooms).
• Disrupts the rooms (you have to move furniture, strip walls, re-decorate).
• Harder to get right (risk of condensation and mould if not vapor-controlled correctly).
• Sockets, light switches, skirting, and architraves need moving out.

Which to choose?

• External if you can afford it, you don't mind changing the appearance, and you don't have planning restrictions (conservation area, listed building).
• Internal if external would be refused planning permission, you want to keep the original brickwork visible, or you can't afford external and need to do it room by room.
• Combination (internal on some walls, external on others) if you have planning restrictions on the front but not the rear.

How much does solid wall insulation cost in 2026?

• External wall insulation (EWI): £8,000 to £15,000 for a 3-bed semi-detached house. Detached houses cost £12,000 to £22,000. Terraced houses cost £5,000 to £10,000 (only the front and rear walls need insulating). Includes 100mm to 140mm insulation boards, mesh reinforcement, base coat, and silicone or acrylic render finish.
• Internal wall insulation (IWI): £5,000 to £10,000 for a semi-detached house. Detached houses cost £7,000 to £14,000. Includes 60mm to 100mm insulation boards (rigid PIR or phenolic), vapour control layer, and plasterboard finish. You also need to re-decorate and move sockets/switches (electrician cost: £300 to £800).

The work takes 2 to 4 weeks for external insulation (depending on weather and drying time for render). Internal insulation takes 1 to 2 weeks per floor of the house.

Can I get a grant?

Yes. Solid wall insulation is one of the few measures expensive enough that grants make a big difference:

• ECO4: Covers solid wall insulation if you receive certain benefits (Pension Credit, Universal Credit, Child Benefit with household income under £31,000, or others) or if your home has an EPC rating of D, E, F, or G. The grant can cover up to £10,000 of the cost. If your quote is higher, you may need to contribute the difference.³
• Home Upgrade Grant (HUG): Available to low-income households in England with off-gas-grid heating (oil, LPG, electric). Can cover up to £10,000 for solid wall insulation as part of a whole-house retrofit. Apply through your local authority.⁴
• Great British Insulation Scheme (GBIS): Similar to ECO4 but with slightly different eligibility. Check with an installer.⁶
• Scottish and Welsh schemes: Home Energy Scotland and Nest offer grants and interest-free loans for solid wall insulation.⁷

To apply, contact an ECO4-registered installer. They will check your eligibility and handle the grant application. The gov.uk Simple Energy Advice service lists approved installers.⁸

How much will I save?

The Energy Saving Trust estimates solid wall insulation saves:

• £260 per year for a semi-detached house
• £460 per year for a detached house
• £160 per year for a mid-terrace house

These figures assume gas central heating and 2026 energy prices (6.24p/kWh for gas).^1,2 Electric heating savings are higher in pounds (about 4x) but the bills stay high.

Your actual savings depend on:

• How you heat. If you heat the whole house to 21°C all day, savings are high. If you only heat occupied rooms, savings are lower.
• Wall area. Detached houses have more external wall area than semi-detached or terraced houses, so they lose more heat and save more from insulation.
• Existing draughtproofing. Solid wall insulation saves more if your windows are double-glazed and draught-proofed. If you have single-glazed sash windows with big gaps, fix those first (cheaper and faster payback).

Should I insulate my solid walls or prioritize something else?

Solid wall insulation should come after loft insulation and double-glazing, but before more exotic measures like heat pumps or solar. Here's the priority order for a solid-wall home:

1. Loft insulation (270mm): £300 to £500, saves £200+ per year. Do this first. Often free via ECO4.
2. Double-glazing or secondary glazing: £3,000 to £8,000, saves £80 to £120 per year. Makes a big difference to comfort (reduces draughts and cold spots near windows).
3. Solid wall insulation: £5,000 to £15,000, saves £160 to £460 per year. Do this third, especially if you can get a grant.
4. Floor insulation: £700 to £1,500 per room, saves £40 to £60 per year. Lower priority.

Skip solid wall insulation if:

• Your loft or windows are still uninsulated (do those first).
• You're planning to move within 10 years (payback is too long).
• You live in a listed building or conservation area and can't get planning permission for external insulation (internal may still be possible, check with your conservation officer).

What about condensation and damp?

Solid wall insulation can cause condensation problems if not done correctly:

• Internal wall insulation makes the wall colder on the outside (behind the insulation). If the insulation doesn't have a proper vapour control layer, warm moist air from the room can reach the cold surface and condense. This causes mould and can rot the wall structure.⁵
• External wall insulation has fewer condensation risks because the wall stays warm. However, you must use breathable render (not cement render) on older solid walls to let moisture escape. Trapping moisture in a Victorian brick wall can cause frost damage.⁹

A proper survey before install checks for existing damp (rising damp, penetrating damp, condensation). Fix any damp problems before insulating. Any reputable installer will refuse to insulate a damp wall.

What about listed buildings and conservation areas?

If your house is listed or in a conservation area, you may need permission for external wall insulation (because it changes the appearance). Internal wall insulation usually doesn't need permission because it doesn't affect the exterior.⁹

Check with your local planning authority before starting work. Historic England recommends:

• Prioritize loft, floor, and window upgrades first (less intrusive).
• If you do insulate solid walls, use breathable materials (wood fibre, lime-based renders) rather than cement-based systems.
• Avoid covering decorative brickwork or architectural features.

Summary

Solid wall insulation is worth it if you have a pre-1920s home with uninsulated solid walls, you've already insulated the loft and upgraded the windows, and you can get a grant to cover most of the cost. External insulation costs £8,000 to £15,000 and saves £260 to £460 per year. Internal insulation is cheaper but reduces room sizes.

Without a grant, payback is 20 to 35 years, so only worth it if you're planning to stay long-term. With a grant, payback drops to 7 to 15 years, which is much more reasonable.