External wall insulation (EWI) is one of the few home improvements that tackles heat loss and the look of a tired house in a single job. It is also a serious spend, so it pays to know when it makes sense and when it does not. Here is what we tell homeowners around Radlett, Borehamwood and St Albans when they ask us this exact question.
EWI fixes rigid insulation boards, usually 90mm to 120mm of expanded polystyrene or mineral wool, to the outside of your walls. A reinforced basecoat with mesh goes over the boards, then a thin coat silicone or acrylic render finishes the job. The whole system adds roughly 100mm to 140mm to the thickness of each treated wall.
Around a third of the heat in an uninsulated home leaves through the walls. Solid brick walls, common in the older parts of Radlett, Bushey and Watford, sit at a U-value of about 2.0 W/m2K. A properly installed EWI system brings that down to around 0.30, which is the Building Regulations target for upgraded walls.
For a typical three bed semi in south Hertfordshire, expect a fully rendered EWI system to land somewhere between £9,000 and £16,000, depending on wall area, how many windows and reveals need detailing, access, and the render finish chosen. A detached house with four elevations and scaffolding on all sides will usually run higher. Those are realistic ranges rather than quotes, as every property is different.
On savings, the Energy Saving Trust puts the figure at roughly £300 to £500 a year for a semi with solid walls on gas heating, and more for a detached house or one on electric heating. That gives a simple payback of 20 to 35 years on energy alone, which is why we are honest that EWI rarely stacks up purely as a money saver.
EWI becomes worth it when you were going to re-render anyway. Stripping back a cracked, blown or pebbledashed finish and re-rendering a semi already costs £6,000 to £10,000. At that point the extra for insulation is a much smaller step, and you get the warmth, the quieter rooms and the reduced condensation thrown in.
It also makes strong sense on solid walled houses where internal insulation would eat floor space, disturb kitchens and bathrooms, and mean redecorating every room. EWI is done from the outside, so you can carry on living in the house while the work happens. Adding a heat pump later is easier too, because a better insulated home can run it at lower flow temperatures.
Planning is the first hurdle. In conservation areas such as parts of Radlett, Aldenham and St Albans city centre, changing the outside appearance of a house normally needs consent, and listed buildings are a different conversation altogether. On a standard house EWI usually falls under permitted development, but a quick call to Hertsmere or St Albans planning is worth the ten minutes.
Detailing is where EWI succeeds or fails. Window sills have to be extended, soffits and gutters sometimes need lifting, and the damp proof course must stay clear with a proper starter track above it. Done badly, you trap moisture and end up with damp inside. Done properly, with a breathable silicone finish, the wall stays dry and the render should need little more than an occasional wash for 25 years or more.
If your walls are solid, the render is already on its way out, and you plan to stay in the house for a good few years, EWI is very much worth it. You get a warmer home, a lower EPC rating number in your favour when you sell, and a fresh exterior in one go. If you have filled cavity walls and sound brickwork, the money is usually better spent on the loft, windows or the boiler first.
Yes, the original brick or render is covered and the walls become slightly thicker, so sills and reveals are remodelled. Most people choose a smooth silicone render in an off white or stone shade, though brick slip finishes are available if you want to keep a brick appearance.
A three bed semi usually takes two to three weeks including scaffolding, allowing for drying time between coats and the weather. Detached houses or properties with lots of detailing can take four weeks or more.
Support is limited and changes often, but households on certain benefits or in lower EPC band homes may qualify under schemes such as the Great British Insulation Scheme or ECO4. It is worth checking the current government guidance before assuming you will pay the full cost.
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