Why solid walls are the weak point in older homes

If your home was built before the 1920s, there is a good chance it has solid walls: two layers of brick with no gap between them. There is nowhere for insulation to go, so heat simply conducts straight through the brickwork and out into the street. A typical uninsulated solid brick wall has a U-value of around 2.0 W/m²K, compared with 0.3 or better for a well-insulated cavity wall. For many period properties, the walls account for roughly a third of all heat lost.

You feel that as cold internal wall surfaces, rooms that never quite warm up, condensation in the corners and higher bills than a similar-sized modern home. External wall insulation (EWI) tackles the problem from the outside. A layer of insulation is fixed to the wall and protected by a new weatherproof finish, which means no loss of floor area, no rooms turned upside down, and — if the detailing is right — a genuinely comfortable house.

The main insulation materials

• Expanded polystyrene (EPS) — the most common and best value option. Rigid boards, usually 60–100mm thick, are fixed to the wall and covered with a rendered system. Light, predictable and easy to fit on regular brickwork.
• Mineral wool — slightly lower performance per millimetre, so boards are thicker, but non-combustible and good at accommodating movement in older masonry. A sensible choice on uneven substrates.
• Phenolic foam — the best thermal performance for the thickness, which matters where space is tight: at a boundary with a neighbour, or under a shallow eaves overhang. It costs more per square metre.
• Wood fibre and cork — breathable, plant-based boards with a lower embodied impact. They pair naturally with lime renders and suit older, moisture-sensitive walls, though they are pricier and less widely available.

On top of the insulation goes a finish. Most homeowners choose a silicone or acrylic render in a colour of their choosing. In conservation areas, a brick-slip system that mimics the original masonry often satisfies planners. Ventilated cladding in timber or fibre cement is another route, particularly on gable ends or extensions.

What the installation actually involves

The work is carried out from scaffolding and usually takes one to three weeks on a typical semi. A competent installer will:

• Survey the walls and deal with any existing damp, failed pointing or blown render before starting.
• Fix the boards with adhesive and mechanical fixings, cutting neatly around windows, doors and services.
• Insulate the reveals at window and door openings and extend sills so water drips clear of the new face.
• Re-route or extend pipes, vents, alarm boxes and cables so nothing ends up buried in the insulation.
• Apply a base coat with reinforcing mesh, then the decorative finish.

Details matter far more than the headline material. Short sills, uninsulated reveals and cold bridges at the base of the wall will undo much of the benefit, and can create new damp patches where there were none before.

Costs, savings and support

Expect to pay roughly £90 to £160 per square metre for a full system, or somewhere between £8,000 and £18,000 for a typical three-bedroom semi, depending on material, finish and how much detailing is required. A mid-terrace house will be less; a detached or listed property considerably more.

In return, wall U-values typically drop to around 0.30 W/m²K or better. Households that make the change often report savings in the region of £300 to £500 a year on heating, with larger figures where the previous heating was electric or oil. You should also see a meaningful uplift in your EPC rating, warmer surfaces, less condensation and a quieter house.

Grant funding is worth investigating before you commit. The Energy Company Obligation and the Great British Insulation Scheme both support solid-wall insulation for eligible households, and there are separate schemes in Scotland, Wales and Northern Ireland. Energy-saving materials also benefit from a reduced rate of VAT, so always ask for a quote that reflects it.

Permissions, breathability and pitfalls

• Planning: many homes have permitted development rights for external insulation, but conservation areas, Article 4 directions and listed buildings usually need consent. Check before ordering materials.
• Boundaries: in a terrace, the insulation normally stops at the party wall line, which can leave a visible step and a cold bridge. Discuss it with neighbours early.
• Breathability: traditional solid walls manage moisture by letting it evaporate. Cement render and impermeable boards can trap that moisture and cause problems. On exposed or damp-prone walls, a breathable system is usually the safer bet.
• Ventilation: once a house is sealed up, it needs deliberate ventilation. Trickle vents, effective extract fans or a mechanical system with heat recovery will keep indoor air healthy.
• Guarantees: choose a certificated system installed by an approved contractor, with an insurance-backed guarantee — typically 25 years — covering both materials and workmanship.

Take your time over the specification and the installer, and external wall insulation is one of the most effective improvements you can make to an older home. Get it right and the difference is immediate: rooms that hold their heat, walls that feel warm to the touch, and a house that finally behaves the way you always wanted it to.