Sash window draught proofing
Original box sash windows are draughty by design — they need a running clearance to slide at all. Draught proofing bridges that gap with a brush-pile seal, so the window still works but the air stops moving through it.
- Draughts, rattle, dust and street noise reduced
- Sashes eased, re-corded and re-balanced as part of the job
- Your original windows kept — nothing replaced
- Top rail against the head of the box. The upper sash closes up against the head — a compression joint, so either a bulb seal or pile works here.
- Parting beads, both sides, full height. The bead that separates the two sashes is replaced with one carrying pile in a carrier. This is the biggest single gap.
- The meeting rails. Pile is routed into the rail so the two sashes seal against each other. This is also where most rattle is cured.
- Staff beads, both sides. The room-side bead retaining the lower sash, likewise replaced with a pile-carrying version.
- Bottom rail onto the cill. Another compression joint, sealed once the sash has been eased so it sits down square.
What is sash window draught proofing? It is the process of fitting flexible seals into every gap a sliding sash needs in order to move — the meeting rails, both parting beads, both staff beads, the head and the cill. On a proper job the sashes come out, the cords are renewed, the sashes are eased so they close square, and the beads are replaced with versions carrying brush pile in a carrier. It reduces air leakage substantially. It does not change the U-value of the glass.
Why sash windows are draughty in the first place
A vertical sliding sash is two timber frames running inside a hollow box, counterweighted on cords over pulleys. For the sashes to slide, they must be slightly smaller than the space they run in — a clearance of roughly 3mm, and often nearer 5mm on a window that has been repainted many times or has warped. That clearance runs the full perimeter of both sashes, which is why the draught is felt everywhere at once rather than at one spot.
This matters more than the glass does. Historic England's guidance on draught proofing windows and doors states that less than a quarter of the heat lost through a typical traditional window escapes by conduction through the glass — the rest goes as draughts. Draughts also make a room feel colder than it is, so people turn the heating up and run it longer.
The five places a sash window has to be sealed
Historic England divides seals into two families, and the distinction is the single most abused technical point in this trade. Compression seals work where a moving part closes against something. Wiper seals work where parts slide past each other — and Historic England is unambiguous that wiper seals are the only way of sealing the sides and meeting rails of sliding sash windows. That is why brush pile, not foam, is the correct material on the sliding faces.
| Position | Seal type | Why |
|---|---|---|
| Sash stiles against the pulley stiles | Wiper — pile in the parting and staff beads | Sliding contact. A compression seal would be abraded away and would bind the sash. |
| Meeting rails | Wiper — pile routed into the rail | The rails pass each other as the sashes move. |
| Bottom rail onto the cill | Compression, or pile | The sash closes down onto the cill rather than sliding along it. |
| Top rail against the head | Compression, or pile | Same reasoning as the cill. |
What the job actually involves
Draught proofing done properly is a strip-down, not an add-on. The staff beads come off, both sashes come out, the parting beads are levered out of their grooves, the weight pockets are opened and all four cords renewed. The sashes are eased so they run freely and so the meeting rails come back into alignment. Only then is the pile carrier routed in and the new beads fitted.
Sequence matters: Historic England is explicit that windows should be assessed for repairs before any draught proofing, and notes that straightforward repairs on their own can cut air infiltration and heat loss by up to a third. Sealing a window that has a rotten cill or a dropped sash just traps the underlying fault behind new beads. See the full step-by-step.
What it will and will not do
We would rather set the expectation correctly than win a job on a claim that does not survive the first cold night.
What it does well
- Stops rattle. Mechanical, not marginal — the clearance is filled and the fastener pulls the rails together.
- Cuts draughts substantially. 33–50% less air leakage from windows generally; up to 86% on a well-executed single sash in measured testing.
- Cuts dust and some noise. One manufacturer publishes 6–10 dB(A) for their own system; treat that as their figure, not an industry rule.
- Makes the window work again. Re-cording and easing usually leaves a sash easier to open than before.
What it does not do
- It does not insulate the glass. Measured before and after, a sash window's U-value went 4.5 → 4.2 W/m²K — statistically indistinguishable. Around 72% of the heat lost through the window still goes through the glazing.
- It does not cure condensation. Reducing air change can make surface condensation worse, not better. Any company promising otherwise is guessing.
- It does not pay for itself quickly on energy alone. The Energy Saving Trust's figure of about £55 a year is for draught proofing a whole house's windows, doors and floors — not for a full sash overhaul at several hundred pounds a window.
Sash draught proofing is bought for comfort, quiet and keeping original windows working. The energy saving is a contribution, not the business case.
Should you do it yourself?
For a sound window with good beads, a self-adhesive brush-pile strip is a genuine option and costs a few pounds a window. For a window that is painted shut, has dropped, or has worn beads, it is a waste of an afternoon — the strip cannot bridge a gap that varies along its length, and it cannot bring misaligned meeting rails together. Our DIY draught proofing guide gives the real prices, the tools, and the point at which it stops being worth it.
Where we work
We cover London, Surrey, Kent, Sussex, Essex, Hertfordshire, Berkshire, Buckinghamshire and Oxfordshire. Most of our work is on Georgian, Victorian and Edwardian terraces where the whole house is done in one visit — it is markedly cheaper per window that way, and most firms have a minimum charge that makes single-window jobs poor value. See the areas we cover.
Sash window draught proofing: common questions
Does draught proofing sash windows work?
Yes, for the thing it is designed to do. Historic England reports that draught proofing reduces air leakage from windows by 33–50%, and measured testing of a single professionally sealed sash window recorded an 86% reduction in air infiltration. What it does not do is insulate: the same testing found the window's U-value moved only from 4.5 to 4.2 W/m²K, a difference inside the margin of error. Draught proofing stops moving air, it does not warm the glass.
How much does it cost to draught proof sash windows?
A full overhaul-and-draught-proof — sashes out, new cords, new beads carrying pile, pile routed into the meeting rails — runs roughly £250–£400 per window nationally and £330–£590 per window in London for a standard size. Larger, curved, Venetian and spiral-balance sashes cost more. Beware quotes of £80–£150 a window: that is a stick-on strip, not an overhaul. Most firms also have a minimum callout charge. See our full price breakdown.
How can I draught proof my sash windows myself?
You can, and for some windows it is the sensible choice. Self-adhesive brush-pile strip is the only stick-on product worth using on a sliding sash — the Energy Saving Trust is explicit that foam strips do not work well on sliding sash windows, because the sash abrades them and they perish. DIY will not fix sashes that are painted shut, have dropped out of alignment, or have rotten beads. Our DIY guide walks through every option honestly, including what each one costs and what it will not fix.
Are sash windows airtight?
No, and they cannot be. A vertical sliding sash needs a running clearance of around 3mm to move at all; on a warped or over-painted window it can be 5mm in places. That clearance is a permanent air path around the whole perimeter of both sashes. Draught proofing does not remove the clearance — it bridges it with a flexible wiper seal that the sash can still slide past.
How long does sash window draught proofing last?
Specialists typically guarantee the pile for around five years and expect roughly ten years of service in practice, at which point the pile can be slid out of its carrier and renewed without touching the timber. The carrier itself is permanent. The commonest cause of early failure is a decorator painting over the pile — if that happens, fresh pile is a cheap fix.
Will draught proofing stop my sash windows rattling?
This is the thing it is most reliably good at. Rattle is caused by the running clearance letting the sashes vibrate. Filling every sliding gap with pile, easing the sashes so the meeting rails align, and fitting a screw-action Brighton fastener that pulls the rails together removes the movement mechanically. See rattling sash windows.
Do I need listed building consent to draught proof sash windows?
Possibly — and this is widely got wrong. Historic England states that adding simple self-adhesive strips does not require listed building consent, but that more invasive methods such as grooving may affect special interest and may require consent. Routing grooves into original sashes and beads is exactly what professional draught proofing does. If your building is listed, speak to your local authority's conservation officer first and get the answer in writing. See listed buildings and conservation areas.
Is draught proofing or secondary glazing better?
They solve different problems. Draught proofing stops air movement and rattle for a few hundred pounds a window. Secondary glazing cuts heat loss through the glazing by around 63% and is a markedly better sound insulator, because of the wide air gap. If your complaint is draughts, rattle and dust, draught proof. If it is cold glass or traffic noise, secondary glazing is the measure that actually moves the number. See the full comparison.
Get a price for your windows
Send us the number of sashes and your postcode and we will come back with a written guide price, then confirm it on survey.