External wall insulation is one of the bigger home improvement jobs you can take on, and the quotes you receive can vary wildly. As a rendering firm working across Radlett and Hertfordshire, we fit and finish these systems regularly, so here is an honest breakdown of what you should expect to pay and why.
Most homeowners in the UK pay somewhere between £90 and £140 per square metre of wall for a full external wall insulation system, supplied and fitted. That figure includes the insulation boards, fixings, basecoat, mesh, and a decorative render finish, but scaffolding and any preparation work usually sit on top.
In practical terms, a typical three bed semi with around 80 to 100 square metres of external wall tends to land between £10,000 and £15,000 all in. A mid terrace with only front and rear elevations might come in at £7,000 to £10,000, while a large detached property can reach £18,000 to £22,000 or more. Around Radlett and south Hertfordshire, prices sit towards the upper half of national ranges because of labour costs in the area, so treat any quote well under £80 per square metre with caution.
Two houses of the same size can get very different quotes, and there are usually good reasons. The insulation material is the first: expanded polystyrene (EPS) is the most common and the cheapest, while mineral wool costs roughly 15 to 25 per cent more but offers better fire performance and breathability, which matters on older solid wall homes.
The condition of your existing walls matters just as much. Blown render that needs hacking off, deep repointing, or repairs to cracked brickwork all add labour before a single board goes on. Then there are the fiddly details that separate a proper job from a quick one: extending window sills, moving downpipes and soil stacks, adjusting the roofline or verge to cover the new wall thickness, and forming neat reveals around every window and door.
For solid wall properties, which include most homes built before the 1930s, external wall insulation is usually the only realistic way to insulate the walls without losing internal space. The Energy Saving Trust estimates savings of roughly £400 to £500 a year on heating for a typical semi, though your actual saving depends on how you heat your home and how leaky it was to begin with.
The honest answer is that payback on energy savings alone is slow, often 15 to 25 years. But EWI also gives you a fresh, low maintenance rendered exterior, deals with penetrating damp, and evens out cold internal walls, so many owners treat it as re-rendering and insulating in one job rather than a pure energy investment. If your render needs replacing anyway, the extra cost of adding insulation at the same time is much easier to justify.
Government schemes such as the Great British Insulation Scheme and ECO4 can cover some or all of the cost for eligible households, generally those on certain benefits or in homes with low EPC ratings. Eligibility rules change often, so check the current criteria on GOV.UK before assuming you qualify or ruling yourself out.
When comparing quotes, make sure each one specifies the insulation type and thickness, the render system brand, whether scaffolding and detail work are included, and the length of the system guarantee. Ask whether the installer is certified for the specific system they are fitting, since manufacturer warranties usually depend on it, and remember that planning permission can be needed if the work changes the appearance of the property, particularly in conservation areas.
A typical semi detached house takes two to three weeks from scaffolding going up to the final render coat, weather permitting. Cold or wet spells can stretch this, as render needs the right conditions to cure properly.
Often not, as it usually falls under permitted development, but you will need permission if your home is listed, in a conservation area, or if the work significantly changes its appearance. Building regulations approval is required either way, and your installer should handle the notification.
A correctly designed and installed system actually reduces damp by keeping the wall warmer and shedding rain. Problems only tend to arise when details like sills, verges, and the base of the wall are done badly, which is why installation quality matters more than the brand of board.
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