How to Draught-Proof Old Sash Windows

Original sash windows are one of the most beautiful features of a British period home — and one of the draughtiest. That icy blast around the edges on a windy night isn’t “character”, it’s money escaping. But ripping them out for uPVC destroys the soul of the house (and in conservation areas, it’s not even allowed). So how can I draught proof old sash windows? Professionally, for £200–£400 per window — or DIY for £30–£60. Either way, it’s one of the best-value improvements you can make to a period property. Here’s how.

Why sash windows draught (and why replacement isn’t the answer)

A sash window is two sliding frames in a box — and by design, there are gaps: between the sashes, between sashes and frame, and around the staff beads. Over a century, timber shrinks, paint builds up unevenly, cords stretch, and those designed clearances become draughty chasms. A typical unsealed sash window leaks as much air as a 6-inch square hole in your wall. Multiply by every window in the house.

But here’s the thing: a draught-proofed, overhauled sash window performs remarkably well — close to modern double glazing for draughts (though not for heat loss through the glass itself). And it keeps the original timber, the original glass (with its lovely wavy imperfections), and the character that uPVC can never replicate. For listed buildings and conservation areas, it’s often the only legal option — which is fine, because it’s the right one anyway.

How can I draught proof old sash windows? The professional route

A specialist sash window company will:

1. Remove the sashes from the box frame
2. Rout grooves into the meeting rails, parting beads and staff beads
3. Fit brush pile seals (tiny brush strips in a carrier) into the grooves — these compress to seal gaps while still allowing the sashes to slide
4. Replace staff and parting beads with new ones incorporating seals
5. Overhaul the sashes: re-cord (new sash cords, £30–£50 per window in materials), re-balance weights, ease sticking sashes, replace broken glass, fill and sand
6. Rehang everything so the windows slide smoothly and seal tightly

Cost: £200–£400 per window depending on size, condition and region (London: £300–£500). A full house of 8 windows: £1,600–£3,200. Compare that to replacement timber sashes at £900–£1,800 per window — draught-proofing is a quarter of the price.

The result: draughts reduced by 80–90%, rattling eliminated, windows that actually open and close properly. Most companies guarantee the seals for 5–10 years.

The DIY route (£30–£60 per window)

You won’t match the routed brush-seal system, but you can get 60–70% of the benefit:

• Self-adhesive brush strips (£8–£15 per window): Stick to the parting beads and staff beads where the sashes meet the frame. The brushes compress to fill gaps. Lasts 3–5 years; replace when flattened.
• Silicone sealant on fixed joints: Where the box frame meets the wall, and any non-moving joints — seal with flexible decorators’ caulk. Don’t seal the moving parts.
• Meeting rail lock: A sash lock (£5–£10) pulls the two sashes tightly together at the meeting rail, closing the biggest single gap. Also improves security.
• Temporary secondary glazing film (£10–£15 per window): Heat-shrink film applied with double-sided tape and a hairdryer. Nearly invisible, adds an insulating air layer, and dramatically cuts draughts. Lasts one winter per application — brilliant as a stopgap.
• Heavy lined curtains (£40–£100 per window): Floor-length, interlined curtains are the traditional solution and still one of the best — they create a still air layer that cuts heat loss enormously. Close them at dusk religiously.

What about secondary glazing?

For the ultimate upgrade without replacing windows: secondary glazing — a discreet second pane fitted inside the existing window. Costs £300–£600 per window professionally fitted (or £100–£200 DIY with magnetic acrylic panels). It roughly halves heat loss through the window and dramatically cuts noise — often better than replacement double glazing for sound. Combined with draught-proofing, it’s the gold standard for period windows. In listed buildings, it’s usually the approved solution.

Maintenance: keeping them working

• Repaint every 5–8 years. Paint isn’t cosmetic on timber windows — it’s the waterproofing. Flaking paint = water ingress = rot. (See our guide to paint finishes — exterior needs are different, but the prep principles apply.)
• Check cords annually. Fraying sash cords snap without warning, dropping a heavy sash. Re-cording is a £30 DIY job or £80–£120 professional.
• Don’t paint them shut. The classic landlord special. If a sash is painted shut, cut the paint line with a utility knife and ease it open — a window that doesn’t open is a fire safety issue.
• Lubricate the pulleys with a silicone spray annually — keeps them running smoothly and quietly.
• Check for rot: Probe the bottom rails and sill with a screwdriver annually. Soft wood = rot. Small areas can be repaired with epoxy wood filler (£15–£25); extensive rot needs a joiner to splice in new timber (£150–£300 per window — still far cheaper than replacement).

Draught-proofing vs double glazing: the honest comparison

• Draught-proofing (£200–£400/window): Eliminates draughts and rattles, keeps original windows, allowed everywhere. Doesn’t significantly improve the glass’s insulation.
• Secondary glazing (£300–£600/window): As above, plus halves heat loss and slashes noise. The best performance without replacement.
• Replacement double-glazed sashes (£900–£1,800/window): Best thermal performance, but expensive, loses original glass, and may need permission. If you go this route, use a FENSA-registered installer so the work is certified as building-regulations compliant.
• Full replacement with uPVC (£500–£900/window): Cheap, but destroys character, devalues period properties, and is banned in conservation areas.

For most period homeowners, draught-proofing + heavy curtains (or secondary glazing for noise/thermal) is the sweet spot — a fraction of replacement cost, all of the character preserved. If you’re also dealing with condensation on the glass, note that draught-proofing can slightly increase it (less ventilation) — keep trickle vents open or ventilate daily.

The practical takeaway

Get your sashes professionally draught-proofed and overhauled (£200–£400 per window) — it’s the best value in period home improvement, eliminating 80–90% of draughts while preserving the windows that make your house special. DIY brush strips and a meeting-rail lock get you most of the way there for £30–£60. Add secondary glazing if noise or heat loss is the priority. And maintain them — paint, cords, rot checks — because original sash windows, properly cared for, will outlast any replacement. They’ve already done a century; give them another.