Important Notice
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
- 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: ECO4 covers eligible households in full
Internal vs External: Comparing Your Options
Internal Wall Insulation (IWI)
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
External Wall Insulation (EWI)
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?
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
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.
Check If You Qualify for Funded Solid Wall Insulation
Solid wall insulation is one of the most expensive energy improvements — but ECO4 funding can cover the full cost for eligible households. Check your eligibility in minutes.
Key Eligibility Factors:
- Low-income household
- Property EPC rating D-G
- Owner-occupier or private tenant
Get comprehensive property analysis with grant recommendations, savings estimates, and installer connections.
Full Property AnalysisGet 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 ECO4 funding.
No obligation. We'll connect you with certified installers in your area.
How to Get Solid Wall Insulation
Check Eligibility
Use our eligibility checker or contact us. ECO4 funding can cover the full cost for qualifying households.
Professional Survey
A surveyor assesses your walls, checks for damp, and recommends internal or external insulation based on your property and circumstances.
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.
Certification
You receive a guarantee and building control sign-off. Your EPC can be updated to reflect the improvement in your energy rating.
Related Insulation Guides
Cavity Wall Insulation
For 1920s-1990s properties with cavity walls. Quick to install, minimal disruption, and often completely free through ECO4.
Read guideLoft Insulation
The most cost-effective energy improvement. Quick to install and often free through government schemes.
Read guideFree Insulation Schemes
Full guide to ECO4, Warm Homes Plan, and LA Flex — all the routes to funded insulation in the UK.
Read guideReady to Insulate Your Solid Walls?
Solid wall insulation is a significant improvement — and ECO4 funding means eligible households can get it done at no cost.