Solid Wall Insulation

If your home was built before the 1920s, it almost certainly has solid walls with no cavity to fill. Solid wall insulation — applied internally or externally — can dramatically reduce heat loss, but it is a bigger job than cavity wall insulation and typically more expensive.

Grant funding available

Eligible households can get solid wall insulation funded through the Great British Insulation Scheme (GBIS) and the Warm Homes: Local Grant, significantly reducing or eliminating the cost.

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~45%
Heat lost through uninsulated solid walls
According to the Energy Saving Trust

Independent information

This is an independent information service. Cost estimates are compiled from Energy Saving Trust publications and industry data. Actual costs depend on your property's size, condition, and location. Planning permission requirements vary by local authority. Always consult a qualified surveyor and your local planning department before proceeding.

What Is Solid Wall Insulation?

According to the Energy Saving Trust, solid walls let through almost twice as much heat as unfilled cavity walls. Because there is no gap to fill, insulation must be applied to either the inside or outside surface of the wall.

Which Properties Have Solid Walls?

Most homes built before the 1920s have solid walls. This includes Victorian terraces, Edwardian semis, Georgian townhouses, and many older cottages and farmhouses. You can identify solid walls by the brickwork pattern — alternating headers (short ends) and stretchers (long sides) indicate solid construction.

Your EPC report will confirm your wall type. If it lists "solid walls" with no insulation, your property could benefit significantly from solid wall insulation.

Why It Matters

According to the Energy Saving Trust, insulating solid walls in a detached home could save a significant amount on annual heating bills. It also makes your home more comfortable by reducing draughts and cold spots near external walls, and can raise your EPC rating by two or more bands.

Key facts

At a glance

  • IWI cost: £4,000–£14,000 according to EST and industry data
  • EWI cost: £8,000–£22,000 according to EST and industry data
  • Duration: 1–3 weeks depending on method and property size
  • EPC impact: Can raise your rating by 2–3 bands
  • Funding: GBIS and Warm Homes funding for eligible households

Internal vs External: Comparing Your Options

IWI

Internal Wall Insulation

Insulation boards or a stud wall with insulation material are fitted to the inside face of external walls. According to the Energy Saving Trust and industry data, this typically costs between £4,000 and £14,000 depending on property size.

Advantages

  • Lower cost than external insulation
  • No change to external appearance
  • No planning permission usually required
  • Can be done room by room

Considerations

  • Reduces internal room size (typically 60–100mm per wall)
  • Disruption during installation — rooms must be cleared
  • Skirting boards, coving, and electrics may need repositioning
  • Potential for condensation if not installed correctly

EWI

External Wall Insulation

Insulation boards are fixed to the outside of the building and covered with a protective render or cladding. According to the Energy Saving Trust and industry data, this typically costs between £8,000 and £22,000 depending on property size and finish chosen.

Advantages

  • More thermally effective — wraps the building completely
  • No loss of internal floor space
  • No internal disruption during installation
  • Protects existing brickwork from weathering
  • Can improve the appearance of tired-looking walls

Considerations

  • Changes the external appearance of the building
  • May require planning permission (see below)
  • Not suitable for listed buildings in most cases
  • Higher cost than internal insulation
  • Requires scaffolding during installation

Planning permission considerations

Do you need permission?

According to gov.uk, internal wall insulation does not usually require planning permission. External wall insulation often falls under permitted development rights, but there are important exceptions:

  • Conservation areas: EWI may require permission if it alters the character of the building or streetscape
  • Listed buildings: Listed building consent is almost always required for any external changes — and often for internal alterations too
  • Flats and maisonettes: Permission is usually needed for external changes to shared buildings
  • Boundary considerations: EWI increases the building footprint and could encroach on neighbouring land or public highways
  • Article 4 directions: Some areas have additional restrictions that remove permitted development rights

Always check with your local planning authority before commissioning external wall insulation. A reputable installer will advise you on this as part of their assessment.

Which properties benefit most?

Suitable

Good candidates

  • Pre-1920s properties with solid brick or stone walls
  • Victorian and Edwardian terraces and semis
  • Properties with an EPC rating of D, E, F, or G
  • Homes with high heating bills and cold internal walls
  • Properties where cavity wall insulation is not possible

Caveat

May need special consideration

  • Listed buildings (specialist advice essential)
  • Properties with existing damp or structural issues
  • Homes with decorative brickwork worth preserving (EWI would cover it)
  • Very exposed locations with persistent driving rain
  • Properties with unusual construction methods

These properties may still benefit, but need a specialist survey to determine the right approach.

Grant scheme overview

Check If You Qualify for Funded Solid Wall Insulation

Solid wall insulation is one of the most expensive energy improvements — but GBIS and Warm Homes funding can cover much of the cost for eligible households. Check your eligibility in minutes.

historical — replaced by GBIS & BUS
Grant available

Key eligibility factors:

  • ECO4 closed March 2026
  • Insulation routes now via GBIS
  • Heat pump routes now via BUS

Check your property against the live MHCLG EPC register to see your current band and every grant you qualify for — free, in under a minute.

Check your property

Get a free solid wall assessment

A qualified surveyor will assess your property, recommend whether internal or external insulation is more suitable, and confirm your eligibility for GBIS and Warm Homes funding.

No obligation. We'll connect you with certified installers in your area.

How to get solid wall insulation

1

Check eligibility

Use our eligibility checker or contact us. GBIS and Warm Homes funding can cover much of the cost for qualifying households.

2

Professional survey

A surveyor assesses your walls, checks for damp, and recommends internal or external insulation based on your property and circumstances.

3

Installation

Certified installers carry out the work. IWI is typically done room by room over several days. EWI involves scaffolding and rendering over 1–3 weeks.

4

Certification

You receive a guarantee and building control sign-off. Your EPC can be updated to reflect the improvement in your energy rating.

Related guides

Grant scheme

Great British Insulation Scheme

Fund solid wall insulation through GBIS and the Warm Homes: Local Grant — covering both internal and external options.

Read guide →

Heat pumps

Heat pumps for older homes

Solid-walled properties can still benefit from heat pumps. Insulate first, then upgrade your heating.

Read guide →

EPC guide

Improve your EPC rating

Solid wall insulation delivers the biggest EPC improvement of any single measure. See the full list.

Read guide →

Ready to insulate your solid walls?

Solid wall insulation is a significant improvement — and GBIS and Warm Homes funding mean eligible households can get it done at little or no cost.

Check grant eligibility → Improve your EPC rating