In short: if your timber sash windows let in a draught, you don’t necessarily need to replace them. Professional draught-proofing with brush or compression seals typically costs £5–£12 per window (around £150–£400 for a full house), reduces air leakage by up to 65%, and can cut heating costs by 10–20% according to the Energy Saving Trust. It’s the single best-value improvement you can make to old sash windows — and it doesn’t change how they look.
Why Sash Windows Draught in the First Place
Traditional timber sash windows work on a sliding mechanism, with the sashes running in grooves either side of the frame. That sliding action is exactly what lets air through: over decades, the gaps between sash and frame widen as timber shrinks, cords wear, and paint builds up unevenly. It’s a design trade-off, not a fault — and it’s fully fixable without touching the glass or the frame profile.
Draught-Proofing Methods That Actually Work
- Brush pile seals: a dense brush strip fitted into a routed groove around the sash, the most common and cost-effective method, lasts 3–5 years, roughly £5–£12 per window professionally fitted.
- Compression seals: a silicone or rubber strip that compresses when the window closes, slightly more effective than brush seals and often used on higher-spec restorations.
- Sash stops and staff bead adjustment: tightening the running gaps between sash and frame, usually done alongside brush or compression sealing rather than on its own.
All three methods are typically fitted during a routine sash window service and don’t require any alteration to the external appearance of the window, which also makes them acceptable in conservation areas and on listed buildings where planning restrictions on replacement windows often apply.
How Much Difference Does It Actually Make?
According to the Energy Saving Trust, draught-proofing a typical home can reduce heating costs by 10–20%, saving in the region of £60 a year on energy bills, alongside a noticeably warmer, less draughty feel in the rooms affected. For a period property with 8–12 original sash windows, that’s a modest upfront cost against a saving that repeats every winter for years.
Brush and compression seals typically last 3–5 years before they need renewing, so it’s worth treating draught-proofing as light maintenance rather than a one-off fix — check the seals whenever you’re having any other work done to the windows.
Draught-Proofing vs Replacement: When Each Makes Sense
Draught-proofing is the right call when the sashes, frame and glazing are still sound and the draught is the only real complaint — which covers most Victorian and Edwardian sash windows we’re asked to look at. Replacement becomes the better option when the timber itself has rotted, the sash cords or weights have failed, the glazing is single-pane and you want the thermal benefit of double glazing, or the window has been poorly repaired in the past and no longer closes properly.
If you’re weighing up the full cost of replacement against ongoing maintenance, our guide to timber sash window costs in London sets out current pricing, and our piece on double vs triple glazing covers the thermal performance side in more detail.
Looking After Restored Sash Windows
If you’ve had sash windows professionally restored, draught-proofing is usually included as standard, since it’s fitted at the same time as re-cording and re-hanging. For more on what a full restoration involves, see our guide to Victorian sash window restoration, and our timber sash windows service page for bespoke replacement options where restoration isn’t viable.
Need Your Sash Windows Draught-Proofed?
We service and draught-proof timber sash windows across London, whether they were fitted by us or not. Get in touch for a free assessment and a straightforward quote — often the fix is simpler, and cheaper, than homeowners expect.