External wall insulation (EWI) is one of the biggest single upgrades you can make to an older home, and one of the most expensive. Whether it is worth it depends heavily on the type of walls you have, the condition of your existing render, and how long you plan to stay put. Here is a straight answer from a trade that fits it week in, week out.
EWI is a layer of insulation board, usually expanded polystyrene (EPS), mineral wool or wood fibre, fixed to the outside of your walls and finished with a reinforced base coat and a decorative render, typically silicone or acrylic. Board thicknesses of 90mm to 110mm are common, which is usually enough to bring a solid brick wall from a U-value of around 2.0 down to about 0.30, roughly the standard expected of a new build.
It is mainly aimed at homes with solid walls, which means most properties built before the 1930s. Across Merseyside's Victorian terraces, older stone cottages in North Wales and pre-war semis in Cheshire, there is no cavity to fill, so insulating externally or internally are the only real options. If your home has cavity walls, cavity insulation is far cheaper and usually the better first step.
For a typical three-bed semi you should budget somewhere in the region of £8,000 to £15,000, or roughly £90 to £130 per square metre, depending on the insulation type, scaffolding access, and how much detailing there is around windows, soffits and pipework. A mid-terrace with only one exposed wall costs far less than a detached bungalow with four.
Energy savings on a solid-walled gas-heated semi are commonly in the £400 to £600 a year range at current prices, more if the house was poorly insulated to begin with. That puts the simple payback at fifteen to twenty five years, which is why we tell people not to buy EWI on fuel savings alone. The stronger case is comfort, condensation control and the fact that you get a completely renewed, low-maintenance exterior at the same time.
It stacks up best when your walls need work anyway. If your existing render is blown, cracked or constantly repainted, you are already facing a re-render bill of several thousand pounds. The extra cost of adding insulation at that point is much smaller than starting from scratch, and you end up with a warmer house rather than just a tidier one.
It also earns its keep on exposed sites. Homes facing the Irish Sea along the North Wales coast or on open ground in Cheshire take a battering from wind-driven rain, and solid walls can stay cold and damp for months. A well-detailed EWI system with a silicone topcoat sheds that rain, keeps the masonry dry, and noticeably reduces condensation and mould on cold internal walls. Rooms hold their heat in the evening instead of going cold the moment the heating clicks off.
EWI changes the look of your house, and that matters legally as well as visually. In conservation areas, on listed buildings, and on many properties in Snowdonia and other designated areas, you will need planning permission and may not get it. It also does not suit every wall: on period brickwork or dressed stone that gives the street its character, covering it is often the wrong call, and internal insulation may be the better route.
Detailing is where these systems live or die. Window sills need extending, roof verges and soffits often need adapting, and the boards must be kept clear of ground level with a proper starter track. Rushed jobs that skip these steps are where the horror stories about trapped damp come from. Fix any existing damp problems, leaking gutters or failed pointing first, because insulating over a wet wall locks the problem in.
If you have solid walls, plan to stay in the house for the medium term, and your exterior is due a re-render regardless, then yes, in our experience it is one of the better value improvements you can make, because you are combining two jobs into one. If your walls are sound, attractive and dry, and you are chasing fuel savings alone, the payback is slow and your money may work harder on loft insulation, draughtproofing or heating controls first.
The best starting point is a proper look at the walls you actually have. Any decent local installer should be willing to tell you if EWI is the wrong answer for your house before quoting for it.
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