External wall insulation: pros and cons

External wall insulation, usually shortened to EWI, is a layer of insulation board fixed to the outside face of a wall and then protected with a reinforced render or another finish such as brick slips. A typical system starts with a base rail just above the damp proof course, followed by insulation boards that are bonded with adhesive and secured with mechanical fixings. A base coat with an embedded glass fibre mesh goes over the boards, then a primer and a thin coat of decorative render, often silicone or acrylic. The whole build-up is designed and tested as one system, which is why mixing components from different manufacturers is a bad idea.

The common insulation materials are expanded polystyrene (EPS), mineral wool, phenolic foam and wood fibre. Each behaves differently when it comes to thermal performance, fire behaviour and the way water vapour passes through it. That choice matters a great deal on older houses, as the sections below explain.

The most obvious drawback is appearance. EWI covers the existing brick, stone or flint, along with features such as decorative bands, quoins and window surrounds, and it makes the walls noticeably thicker. On a terrace or a matching pair of semis, insulating one house can leave an awkward step at the boundary and a front that no longer matches the street. Some homeowners welcome the cleaner look, but others find it a loss of character.

Cost and complexity are the other big issues. The job normally needs scaffolding, and items fixed to the walls such as downpipes, gas meter boxes, satellite dishes, lights and boiler flues have to be taken off and refitted further out. Windowsills usually need extending, and the roof edge may need altering so it still overhangs the thicker wall. Checkatrade puts EWI at £150 to £200 per m² in a table headed "Cost+VAT", while the Federation of Master Builders gives a range of £85 to £130 per m². Our external wall insulation cost guide goes into the figures in more detail.

Most external wall insulation failure comes down to poor specification or poor workmanship rather than the idea itself. The government's Warm Homes Plan notes that solid wall insulation "has also been found to be much more likely to be non-compliant" than other measures, which shows how much depends on getting the details right. The biggest risk is moisture. Older solid walls made with soft bricks and lime mortar rely on drying out through both faces, and covering them with a non-breathable system can trap water in the masonry, leading to damp patches, mould, decay in timber joist ends and frost damage.

A sound specification starts with a survey of the existing wall. Any rising or penetrating damp, failed pointing, leaking gutters or defective render must be put right first, and on older homes a vapour-open system such as mineral wool or wood fibre with a breathable render is often the safer choice. A condensation risk assessment helps show whether the proposed build-up will stay dry. Better airtightness also means the house needs adequate ventilation afterwards, so check that extractor fans and trickle vents are working.

• Gaps and cold bridges at window and door reveals where insulation is left out
• Water getting in behind the system at sills, eaves, verges and service penetrations
• Cracking caused by missing mesh, poor board joints or render applied too thinly
• Boards taken below ground level or too close to paths without suitable materials
• Impact damage at low level where no extra reinforcement was used
• Algae and staining on shaded or north-facing walls

Detailing at windows, eaves and junctions is where good and bad installers differ most. Reveals need thin insulation returned into them, sills need to project well clear of the new face, the roof line must overhang and shed water clear of the render, and junctions with neighbouring walls, porches and extensions need proper trims and sealants. Ask any installer to explain how they will deal with each of these on your house before you accept a quote.

Insulating from the inside is often the better route where the outside of the building should stay as it is, for example on a listed building, in a sensitive conservation area or on a terrace where the front elevation matters to the street. It also suits flats, where you control only your own walls, and homes that are being renovated room by room. Insulated plasterboard or a stud wall with insulation can be fitted one room at a time, which spreads the cost and disruption, and our guide to what dry lining is explains the methods.

Internal insulation has drawbacks of its own. It reduces room sizes, sockets, radiators and skirting have to be moved, and it is harder to deal with cold bridges where floors and internal walls meet the outer wall. The same moisture cautions apply to older solid walls, so breathable materials and careful vapour control are just as important inside. For a mix of the two, some owners insulate the rear and side walls externally and the front internally.