Solid Wall Insulation: The High-Cost, High-Impact EPC Improvement

Solid wall insulation is the most impactful single improvement you can make to a property’s EPC rating — but it is also one of the most expensive and disruptive. For landlords with pre-1930s properties that have solid brick or stone walls (no cavity), it is often the only realistic route to Band C because these properties lose so much heat through their walls that cheaper fabric measures alone cannot close the gap.

This guide covers the two types of solid wall insulation, realistic SAP point gains, costs, when it makes financial sense versus cheaper alternatives, available grants, and the practical issues landlords need to consider — from tenant disruption to planning permission.

Internal vs external: the two approaches

Solid wall insulation comes in two forms, each with distinct advantages and drawbacks.

External wall insulation (EWI)

EWI involves fixing insulation boards to the outside of the building and finishing with a render or cladding system. The insulation layer is typically 50–100mm thick.

Pros:

• No disruption to the interior — tenants can remain in the property during installation
• No loss of internal floor area
• Eliminates thermal bridges at junctions between walls and floors
• Can improve the appearance of a tired exterior
• Typically achieves better thermal performance than internal insulation

Cons:

• Changes the external appearance of the building
• May require planning permission (especially in conservation areas)
• Not suitable for listed buildings or buildings with decorative brickwork worth preserving
• More expensive than internal insulation
• Requires scaffolding, which adds cost and restricts access during installation

Internal wall insulation (IWI)

IWI involves fixing insulation to the inside face of external walls, either as rigid boards or as a stud-wall system filled with mineral wool. The insulation layer is typically 50–80mm thick.

Pros:

• Does not alter the external appearance — suitable for listed buildings and conservation areas
• No scaffolding required
• Lower cost than external insulation
• Can be done room by room, spreading cost and disruption

Cons:

• Reduces internal floor area (50–80mm per insulated wall)
• Significant disruption — rooms must be cleared, skirting boards removed, electrics repositioned
• Tenants may need to move out temporarily if the whole property is done at once
• Risk of moisture issues if not installed correctly (vapour barrier essential)
• Can affect original features (cornicing, dado rails, picture rails)

SAP point gains from solid wall insulation

Solid wall insulation delivers the highest SAP point gain of any single measure. Typical gains are:

The exact gain depends on the property’s starting condition. A property with completely uninsulated solid walls gains more than one that already has some partial insulation. The EPC assessor’s recommendations will include an estimated SAP gain for your specific property.

Cost: what to budget

Solid wall insulation is significantly more expensive than cavity wall insulation (£800–£1,600) or loft insulation (£300–£600). Realistic costs for a typical three-bedroom semi-detached house:

• External wall insulation: £8,000–£15,000 (including scaffolding, render finish, window reveals and making good)
• Internal wall insulation: £5,000–£10,000 (including board installation, repositioning electrics, re-plastering and decoration)

For a mid-terrace property with only front and rear walls exposed (party walls on both sides), costs are lower because there is less wall area to insulate. For a detached property with all four walls exposed, costs are higher.

These figures often push solid wall properties towards the £10,000 cost cap threshold. If you spend £10,000 or more on qualifying improvements and still cannot reach Band C, you can register a cost cap exemption. See our guide on the £10,000 EPC cost cap for details.

When it is worth it vs cheaper alternatives

Solid wall insulation is not always the right answer. Before committing to a £5,000–£15,000 project, consider whether cheaper measures could get you to Band C:

• If your SAP gap to Band C is under 10 points: Loft insulation, draught proofing, LED lighting and heating controls upgrades might close the gap for under £2,000. Solid wall insulation is overkill if cheaper measures can get you there.
• If your property has unfilled cavity walls: Cavity wall insulation (£800–£1,600) delivers similar SAP gains (5–8 points) at a fraction of the cost. Check your EPC — if it says “cavity wall, unfilled” you have a much cheaper option. See our guide on cavity wall insulation.
• If your SAP gap is over 20 points: Solid wall insulation alone will not bridge a 20+ point gap. You will need a package of measures including wall insulation, heating system upgrade, loft insulation and more. In this case, solid wall insulation is necessary but not sufficient.
• If you can reach Band C without touching the walls: A heating system upgrade (replacing an old gas boiler with a condensing model, or installing a heat pump) combined with loft insulation and lighting improvements may be enough. Check your property’s specific recommendations.

Grants available for solid wall insulation

The high cost of solid wall insulation means grants can make a substantial difference. Two main schemes are relevant:

ECO4 (Energy Company Obligation)

ECO4 runs until December 2026 and funds solid wall insulation for qualifying households. Eligibility is based on the tenant’s circumstances (income, benefits received) rather than the landlord’s. If your tenant qualifies, the scheme can fund solid wall insulation at no cost to you — or at a heavily subsidised cost. Contact your local authority or an ECO4-registered installer to check eligibility.

Warm Homes: Local Grant (formerly HUG)

The Warm Homes: Local Grant provides funding for energy efficiency improvements including solid wall insulation in off-gas-grid properties. Eligibility and availability vary by local authority. Check your council’s website or contact them directly to find out if the scheme is operating in your area and whether your property qualifies.

Grants reduce your net cost, which affects cost cap calculations. If solid wall insulation costs £12,000 and you receive a £8,000 ECO4 contribution, only the £4,000 you pay out of pocket counts towards the £10,000 cap. For a broader overview of all available grants, see our guide on home energy grants in 2026.

Disruption and planning considerations

Both types of solid wall insulation involve significant disruption, but the nature of that disruption differs:

• External insulation: Scaffolding around the building for 2–4 weeks. Noise from drilling and rendering. Restricted access to external areas. However, the interior is unaffected — tenants can continue living in the property.
• Internal insulation: Each room being insulated must be cleared of furniture. Electrics (sockets, light switches) on external walls must be repositioned. Skirting boards removed and re-fitted. Re-plastering and redecoration required. Typically takes 1–2 weeks per room.

Planning permission: External wall insulation on most properties falls under permitted development rights and does not require planning permission. However, planning permission is required in the following cases:

• Listed buildings (listed building consent required)
• Conservation areas (if fronting a highway or public space)
• Properties where the insulation would project beyond the existing building line
• Flats and maisonettes (permitted development rights are more restricted)

Internal wall insulation does not generally require planning permission, though it may need Building Regulations approval depending on the scope of works.

Landlord-specific issues

Solid wall insulation raises several practical issues specific to rental properties:

• Tenant disruption: Internal insulation may require the tenant to vacate rooms or even the entire property temporarily. You may need to provide alternative accommodation or agree a rent reduction during the works. Good communication and reasonable notice are essential.
• Void periods: Some landlords choose to carry out solid wall insulation during a void period between tenancies, avoiding tenant disruption entirely. This is the ideal scenario if timing allows.
• Reduced room sizes: Internal insulation reduces the usable floor area of each room. In already small rooms, this can push the room below the minimum sizes specified in the Housing Act 1985 (6.51m² for a single bedroom). Check room dimensions before proceeding.
• Moisture management: Incorrectly installed internal wall insulation can trap moisture behind the insulation, leading to damp and mould. Insist on a proper vapour control layer and use an installer experienced with solid-wall properties.
• Insurance: Notify your buildings insurance provider before external wall insulation is installed, as it changes the building’s construction type and external finish.

For a comprehensive guide to all improvement options and their SAP gains, see how to improve your rental property’s EPC rating.