You can have the look of a traditional sash window with modern insulation. There are three main ways to do it, and the right one depends on the condition of your windows and whether your home is listed or in a conservation area.
Option 1: new double-glazed timber sash windows
A complete new window, box frame and sashes, made to match the originals. Good joiners copy the mouldings, horns and glazing bar layout, and use weights and pulleys or spiral balances. This is the most thermally efficient option, but it replaces all the original timber.
Typical cost: £1,300 to £3,000+ per window, depending on size and timber.
Option 2: new double-glazed sashes in your existing frames
If the box frames are sound, a specialist can make new double-glazed sashes and fit them into your existing frames, with heavier weights to balance the extra glass. You keep the original frame, architraves and internal joinery, and the work is quicker and less disruptive.
Option 3: slim or vacuum glazing in your existing sashes
Slim double glazing units are thin enough to fit into many original sashes after the rebates are carefully deepened. Vacuum glazing is thinner still, often around 6 to 9mm, and close to the thickness of single glass, so it suits fine Georgian glazing bars. Both keep your original sashes and are increasingly accepted on listed buildings.
uPVC sash windows
uPVC sash windows are cheaper (around £600 to £1,200 per window) and low-maintenance, but they look different from timber up close, with thicker frames and no real weights and cords. They are usually refused on listed buildings and often in conservation areas. See our uPVC sash window companies if they suit your property.
Rules to check first
- Listed buildings. You need listed building consent to replace windows or change the glazing. Historic England’s 2024 advice treats slim and vacuum glazing in historic frames as generally acceptable where the original glazing bars are kept, but each case is decided by the council.
- Conservation areas. Houses can usually replace windows like-for-like without planning permission, but an Article 4 direction may remove that right. Flats have no permitted development rights, so any change to how the windows look needs planning permission. Ask your council.
- Building regulations. In England, replacement windows must meet energy standards (a U-value of 1.4 W/m²K, or Window Energy Rating band B), unless an exemption applies to a listed or historic building. A FENSA or CERTASS registered installer can self-certify the work.
Is double glazing always the answer?
Not always. A full overhaul with draught-proofing cuts heat loss significantly, and secondary glazing can match double glazing for warmth and beat it for noise. If your windows are original and in decent condition, compare those options too. See our secondary glazing guide and cost guide.
