How to insulate solid walls

Video by Real Life Architecture. This guide draws on the "Solid Wall Insulation" guide from the Real Life Architecture YouTube channel. Real Life Architecture is a well-regarded UK architecture and building practice channel that covers renovation and construction topics with a practical, technically grounded approach - especially useful for older UK properties.

1. Understand whether you have solid or cavity walls

Solid walls are typically found in UK homes built before 1920, though some interwar properties also have them. A quick way to check: measure the thickness of the wall at a window or door reveal. A solid brick wall is usually around 220 mm thick (one brick width). A cavity wall is typically 270 mm or more, with a gap between the two leaves.

You can also check from outside: on a solid brick wall, the bricks alternate between stretchers (long face showing) and headers (short face showing) in the same course. On a cavity wall, you will generally only see stretchers. If you are unsure, your architect or a surveyor can confirm.

2. Decide between internal and external insulation

Internal wall insulation (IWI) is fitted on the inside surface of the external walls. It is less disruptive externally and cheaper per square metre, but it reduces the floor area of every room it is applied to and requires skirting boards, window boards, and electrical sockets to be moved. It also means the thermal mass of the wall sits on the cold side of the insulation, which affects how the house heats and cools.

External wall insulation (EWI) is fixed to the outside of the building and then rendered or clad. It does not affect the internal room size, keeps the thermal mass on the warm side, and can dramatically improve the appearance of a rendered property. However, it requires planning permission in conservation areas and for listed buildings, and it changes the external appearance of the house, which may or may not be desirable.

3. Understand the condensation risk before you insulate

Solid wall insulation done poorly is one of the most reliable ways to create a damp and mould problem in an old house. When you insulate a wall, you move the dew point - the point at which water vapour condenses - to a different position within the wall structure. If that position ends up in the wrong place, condensation forms inside the wall fabric.

A condensation risk analysis (sometimes called an interstitial condensation check) should be done before specifying the insulation type and thickness. This is not guesswork - it uses the actual thermal properties of your wall construction and the proposed insulation to predict where condensation will form. Many insulation installers skip this step. The consequences - black mould, wet plaster, rotting timber - can take two to three years to become obvious.

4. For internal insulation: prepare the existing wall surface

Before fitting internal insulation, the existing wall surface needs to be clean, structurally sound, and free of any active damp. Treat any signs of rising damp or penetrating damp before insulating - trapping moisture behind an insulation board is far worse than leaving the wall uninsulated.

Remove skirting boards, architraves, and any fittings that will need to be repositioned. Strip any loose plaster back to sound material. A breathable lime plaster on an older property performs very differently from modern gypsum plaster - your insulation choice needs to be compatible with it.

5. Fit insulated plasterboard or rigid insulation boards

The most common approach for internal insulation in domestic properties is insulated plasterboard (PIR or EPS foam bonded to a plasterboard facing) fixed directly to the wall with adhesive dabs and mechanical fixings. Typical thicknesses range from 37.5 mm to 72.5 mm depending on how much U-value improvement you are targeting.

Boards are cut to size, dot-and-dabbed to the wall, and then skimmed or tape-and-jointed at the seams. The key detail is continuity - cold bridges at window reveals, floor and ceiling junctions, and internal corners are where heat loss continues and condensation occurs. These junctions need to be treated carefully, typically with thinner insulation returns.

6. For external insulation: fix the carrier system and boards

External wall insulation systems consist of insulation boards (usually mineral wool or EPS) fixed mechanically and with adhesive to the existing wall surface, followed by a reinforcing mesh embedded in basecoat render, and a finish coat of through-coloured render or specialist cladding. The entire system must come from a single manufacturer to ensure compatibility and warranty.

Scaffolding is required for anything above ground-floor level. The work includes new window and door reveals, new flashings, new sills, and integration with the existing roof verge and eaves. It is a significant building project, not a weekend DIY job. Costs typically run from £8,000 to £20,000 or more for an average semi, depending on the system and the amount of detailing required.

7. Check insulation and grant eligibility

Solid wall insulation is one of the most heavily subsidised home improvements in the UK. The Government’s Great British Insulation Scheme and ECO4 scheme both target solid wall properties, and eligible households (low income, certain benefits) may qualify for fully funded installation. Even without full funding, the scheme can significantly reduce the cost.

Check eligibility through the Energy Saving Trust or your energy supplier before spending any money. If your home is in a conservation area, check with your local planning authority before specifying an external system - permitted development rights do not always apply.

When to call a handyman

Solid wall insulation is specialist work - particularly the condensation risk analysis and the detailing around junctions. Richard can help with the preparation work: stripping plaster, treating damp, repositioning sockets and skirting boards before or after the insulation is fitted. For the insulation installation itself, use a contractor registered with the Insulation Assurance Authority (IAA) or the National Insulation Association (NIA).