Short answer. Sash windows rattle because they must be slightly smaller than the frame they run in — a clearance of roughly 3mm, and more on a warped or over-painted window. The rattle stops when that clearance is filled with brush pile and the two sashes are pulled tightly together by a screw-action fastener. Where the noise comes from the glass rather than the sash, the cause is failed putty and the fix is re-puttying, not sealing.
Why sash windows rattle
There is rarely one cause. A window that has started rattling this winter usually has two or three of the following at once, and the reason cheap fixes disappoint is that they address one of them.
The running clearance is built in
A box sash is two timber frames sliding inside a hollow cased frame, hung on cords over axle pulleys to counterweights in the weight pockets. For the sashes to move, they have to be marginally smaller than the space between the pulley stiles. The figure quoted in the trade is about 3mm of running clearance; on a window that has been repainted a dozen times, or has twisted, the tolerance can be nearer 5mm in places. Wind pressure moves the sash across that gap and it knocks. Nothing has gone wrong — this is how the window was made, and it is also why sash windows are draughty by design.
Worn, shrunken or painted-over beads
Two thin mouldings do all the work of holding the sashes against their runs. The staff bead is the innermost bead on the room side, retaining the lower sash. The parting bead sits in a groove machined into the pulley stile and separates the upper sash from the lower. Both are soft, both are a century or more old, and both shrink, wear hollow where the sash rubs, or get pulled about by previous repairs. Historic England's schedule of defects in traditional windows lists worn or damaged beads as a standard cause of rattles and draughts. Beads are consumable parts; replacing them is normal maintenance rather than a repair.
The sashes have dropped
Sash cords stretch and eventually perish. When a cord lengthens, the sash it carries sits lower than it should, and the meeting rails — the top rail of the lower sash and the bottom rail of the upper — no longer wedge against each other on their splay. The joint that is supposed to be tight has a gap in it, and the two sashes drum against one another. The same thing happens when the wrong weights are in the box, which is common after someone has fitted heavier glass or replaced a sash. A window that has been re-glazed and never re-balanced will rattle whatever else you do to it.
A broken cord also throws the balance onto the remaining three, so they tend to fail shortly afterwards. That is why cords are renewed as a set of four rather than one at a time.
The fastener only holds, it does not pull
This is the single most overlooked cause. A fitch catch is a cam that swings over and locates the two meeting rails against each other. It stops the window opening; it does not apply any load. A Brighton fastener is a screw-action fastener: turning it draws the meeting rails together and holds them under tension. Several specialists prefer a Brighton fastener on a draught-proofed window for exactly this reason, and it improves security at the same time. Swapping the fastener alone will often quieten a window that is otherwise sound.
The glass is loose in the rebate
People miss this one because it sounds identical from across the room. Putty becomes brittle with age, cracks and falls away, particularly once the paint no longer covers the putty line and water has got into the joint. Historic England lists cracked, loose or missing putty among the core defects of a traditional window. Once the putty has gone, the pane sits loose in its rebate and vibrates against the timber. No amount of brush pile around the sash will touch it, and a firm who seals the window without noticing has taken your money for nothing. Loose glazing is also a real air path in its own right.
Working out which cause you have
You can narrow this down in about two minutes with the window closed and, ideally, on a windy day.
- Press the lower sash firmly against the staff bead. If the rattle stops, the sash is moving in its clearance — beads, pile and fastener are the answer.
- Press on the middle of a pane. If the noise stops only when you press the glass, the putty has failed and the pane is loose.
- Look at the meeting rails from the side. They should line up. If one sash sits proud of the other, or you can see daylight through the joint, the sashes have dropped and need re-cording and re-balancing before anything is sealed.
- Work the fastener. If it swings over with no resistance and the sashes still move afterwards, it is a fitch catch doing a Brighton fastener's job.
- Run a fingernail along the staff and parting beads. Hollowed, split or loose beads have done their service.
The fixes, cheapest first
Work down this list rather than up it. There is no point routing pile into a sash that is hanging low on a stretched cord, and Historic England is explicit that windows should be assessed for repairs before any draught proofing is attempted — noting that straightforward repairs on their own can reduce air infiltration and heat loss by up to a third.
| Fix | Addresses | Notes |
|---|---|---|
| Replace a fitch catch with a Brighton fastener | Meeting rails drumming against each other | Priced by some London firms at around £40 a set as an add-on; often bundled into a full job. |
| Re-putty loose panes | Glass rattling in the rebate | Made good as standard by some firms, charged separately by others. Paint must cover the putty line. |
| Replace staff and parting beads | Worn, shrunken or missing beads | Replacement beads are bought in 3m lengths, usually with a pile carrier already fitted. |
| Re-cord and re-balance | Dropped sashes, misaligned meeting rails | All four cords together. Done as a standalone callout this is expensive; done alongside other work it is marginal. |
| Full draught proofing | All of the above, permanently | Sashes out, eased, re-corded, pile routed into the meeting rails, new beads carrying pile. |
What the full job does about rattle specifically
Two things, worth separating. First, easing: the sash stiles and rails are planed and adjusted so the sash runs without binding and, critically, so the meeting rails come back into close alignment when the window is rebuilt. Second, de-rattling: brush pile is fitted at all five sealing positions — the head, both parting beads, the meeting rails, both staff beads and the cill — so the clearance the sash was moving in is occupied by a flexible seal the sash can still slide past. Add the screw-action fastener and the movement has nowhere left to go. The full step-by-step is here.
Brush pile rather than foam matters on the sliding faces. Historic England is unambiguous that wiper seals are the only way of sealing the sides and meeting rails of sliding sash windows, and the Energy Saving Trust states directly that foam strips do not work well on sliding sashes.
Temporary fixes and how long they last
There is nothing wrong with a stopgap if you know it is one.
- A foam or card wedge pushed between the sash and the bead. Free, instant and silent, and it locks the window shut. Fine for a noisy bedroom in January; it does nothing for the draught and it is not a permanent arrangement.
- Self-adhesive V-strip. Historic England's guidance notes that V-strip wiper seals can be used between the stiles and boxes of sliding sashes, so this is a legitimate product rather than a bodge. It takes up some of the clearance and reduces the knock. It will not survive being slid past for many years, and it cannot bridge a gap that varies in width along its length — which most old sashes do.
- Self-adhesive foam. The one to avoid on a sliding face. Historic England notes foam strips are cheaper still but have a short life; damp compressed foam is slow to spring back. Foam belongs on new joinery with tight, consistent tolerances, not on a hundred-year-old sash.
- Self-adhesive brush pile. The best of the stick-on options because the filaments adapt to an uneven gap. It will quieten a sound window with good beads. It cannot align dropped meeting rails and it cannot replace a worn bead.
Full details, prices and tools are in the DIY draught proofing guide.
Why rattle is the one thing draught proofing always fixes
Most claims made for draught proofing are estimates that depend on your house, your exposure and how you heat the place. Rattle is not one of them. The noise is caused by a measurable gap; pile and a screw fastener remove that gap. There is no weather dependency and no modelling involved.
It is worth being equally clear about what will not change. Draught proofing does not warm the glass — measured testing of a sash window before and after found the U-value moved only from 4.5 to 4.2 W/m²K, a difference the researchers described as not statistically significant. If cold glass or traffic noise is the real complaint, the comparison you want is secondary glazing against draught proofing: Historic England's guidance notes that secondary glazing, with its larger gap between the panes, is a better sound insulator. One manufacturer publishes noise reduction of up to 6–10 dB(A) for their own sealing system, which is their figure for their product rather than an industry rule.
One further caution: if you feel cold air around the outside of the frame rather than around the sash, that is usually failed pointing between the box frame and the masonry, and no seal fitted to the sash will touch it. See draughty sash windows.
What it costs and how long it takes
A specialist typically spends one to two hours per window on draught proofing alone, and firms that do nothing else report three to five windows in a day. A full renovation, including repairs and finishing, is more often quoted at around a window a day.
Prices divide sharply according to what is included. A stick-on draught strip fitted to a window already in working order is quoted from around £80 a box frame by firms who explicitly exclude overhaul and balancing. A full overhaul-and-draught-proof clusters around £250–£400 a window nationally and £330–£590 in London for a standard size. Both are called "draught proofing" and they are not the same job — our cost page explains the difference. On durability, the pile is a consumable and the carrier is permanent: specialists commonly guarantee the pile for around five years and expect roughly ten years in practice, after which fresh pile slides into the existing carrier for pence.