Hold a lit candle next to the meeting rail of an old sash window on a windy January evening and watch the flame lean sideways. That single gap, where the upper and lower sashes pass each other, is often the worst offender in a Victorian or Edwardian house — and replacing the whole window for £900 to £1,400 a unit is rarely the answer. A draught-proofing job done well costs a fraction of that and keeps the original glass, the wavy reflections and the slim glazing bars that no uPVC copy ever gets right.
The instinct in a cold house is to reach for self-adhesive foam strip from the local Wickes and stuff it round the frame. Don't. Foam tape on the sliding faces of a sash gets shredded within a fortnight, jams the window half-open, and leaves a sticky residue that takes a scraper and a tin of white spirit to remove. Sash windows move past each other vertically, so they need seals designed for sliding contact, not the squashy stuff made for a hinged door that closes once and stays shut.
Find the gaps before you buy anything
There are five places a sash leaks, and they don't all leak equally. The meeting rail in the middle is usually the biggest, because decades of paint and shrinkage stop the two sashes meeting cleanly. Next come the sides, where the sash slides up and down in its channel — the bigger the gap, the more the window rattles in the wind. Then the bottom rail against the cill, the top of the upper sash against the frame head, and finally the gaps around the staff bead. A cheap stick of incense held near each one on a breezy day shows you exactly where the smoke gets pulled through.
Once you know which gaps actually matter, you can stop wasting product on the ones that don't. A window that rattles badly at the sides but seals fine at the meeting rail needs brush pile in the channels and almost nothing elsewhere.
Brush pile is the right material for the sliding faces
For the side channels and the meeting rail, use brush pile seal — the same fuzzy strip the trade fits, sold by Mighton and Exitex in 5mm and 7mm depths. It sits in a routed groove and lets the sash glide while blocking the air. Retrofitting it properly means removing the staff beads and parting beads, routing a slot, and seating the carrier strip, which is a Saturday's work with a router if you're confident, or a job for a sash specialist at roughly £180 to £320 per window.
If routing is beyond you, there's a cheaper honest middle ground: self-adhesive brush strip that sticks into the existing rebate without cutting a groove. It's not as neat or as durable, but on the meeting rail and the top and bottom rails — the faces that compress rather than slide hard — it does a genuine job for under £15 a window.
The one place compression seals belong
The bottom rail meeting the cill, and the top rail meeting the head, both close with a compression action rather than a sliding one. Here a low-profile E-section or P-section rubber seal works well, exactly the kind you'd put on a casement. Keep it off the vertical sliding channels though — put it there and the window won't move.
The cords, the rattle and the things people get wrong
Two mistakes show up again and again. The first is sealing the window so tightly that condensation, which used to escape through the draughts, now sits on the glass and rots the timber from the inside. A sealed sash needs the rest of the room to breathe — trickle ventilation, an extract fan in a nearby bathroom, or simply opening the window for ten minutes a day. The second is painting over the brush pile or the new seals, which clogs them solid and undoes the whole job.
While the beads are off, check the sash cords. If one has frayed to a few strands, replace both cords on that sash now — waxed cotton sash cord costs about £6 for enough to do a window, and a cord that snaps with the sash up can drop a heavy pane onto your fingers. It's the kind of small repair that's trivial with the window already apart and miserable to come back to.
Done properly, draught-proofing a single sash takes a sheet of 4mm draught from a 30mph gust down to almost nothing, and the room stops having that cold pool of air under the window in winter. The glass stays original, the house keeps its character, and you've spent £15 to £300 instead of four figures. Get the meeting rail right first — it's where most of the cold actually comes in.