How to Draught Proof a Sash Window: The DIY Route

The foam tape is on, the room is still cold, and now the bottom sash sticks halfway up. That sequence is common enough to be a rule, and it happens because the tape went where the draught was felt rather than where the air actually gets in. On a box sash those two places are rarely the same.

The measurement that decides the whole job

Before you buy anything, lever the staff bead off one window and measure the gap between the sash stile and the pulley stile. A feeler gauge is ideal. Failing that, fold a strip of card until it just grips and measure the fold. On a window built in 1890 and eased every couple of decades since, that gap is usually 3 to 5mm. On a tight window it can be under 2mm. Measure the meeting rails the same way, window shut and fastener thrown.

Those numbers are the specification. Every seal on the market is rated for a range, and outside that range it either does nothing or stops the window working. Brush pile is sold by height, commonly from about 4.5mm up to 12mm, set in a carrier that sits in a routed slot. Compression gaskets are sold by gap: Aquamac 63, the profile most London sash joiners keep on the van, is made for a 3.4 to 5.4mm gap and wants a groove around 2.7 to 3.0mm wide and at least 6mm deep.

Measure every window. In the same terrace, front and back sashes are routinely a millimetre apart: one elevation has had more paint and more easing.

Where the air comes in, in order of size

A box sash is built loose on purpose. The sashes have to slide, so there must be clearance at the parting bead, at the staff bead and at the sides of the box. That clearance is the price of a window that works, and no seal should remove it entirely.

The meeting rails are usually the largest single leak. They should close on a slight bevel that pulls tight as the fastener is thrown, but where the cords have stretched or been replaced badly the top sash sits a few millimetres low, leaving an unobstructed slot the full width of the window. Nothing interrupts it, which is why it passes so much air. It is also why a sash that rattles is nearly always a sash that leaks: both come from slack between the rails, and the fastener side of that is covered in our piece on stopping sash windows rattling for good.

The pulley slots are the leak almost everyone misses. Behind each pulley stile is a hollow box holding the weights, open to the outside through the cord slots and often through gaps behind the outer lining. Cold air pours into the cavity and warm room air is drawn out of it. Seal the sliding faces and leave the slots open and you have closed perhaps half the leak.

The sliding faces come next, then the bottom rail to the cill, then the top rail to the head. Last, and outside a DIY job, is the gap between the box frame and the masonry behind the architrave, which on a house with cracked plaster at the reveal can pass more air than the window.

Which of these bites hardest depends on where the window sits. Warm air rises and leaves through the upper storeys while cold air is drawn in below, so ground-floor windows feel draughty and top-floor windows often feel fine while quietly pushing heated air out. Do not judge a window by how cold it feels beside it.

The ten-minute test, window by window

Do this on a windy day with the heating on and any trickle vents closed.

1. Light an incense stick and move it slowly round the perimeter, about 50mm off the timber. Watch the smoke, not the flame. Sides first, then the meeting rails, then the bottom rail, then the head.
2. Run the back of your hand down the gap between the sash and the staff bead. Skin reads moving air better than a thermometer reads temperature.
3. Look along the meeting rails with the window shut. Daylight, or paper that slides through, means the full-width slot is open.
4. Cup a hand over each axle pulley. Air moving there means the weight box is behaving like a flue.

Write down the worst fault on each window. That single line is what tells you whether you need a £20 roll of pile, a tube of sealant or a joiner with a router.

What lasts, and what peels off by February

Two rules come out of that table. Adhesive is not a fixing on a surface that slides, and the more paint layers under it the sooner it lets go. And the seal has to match the number you measured, not the packet's picture. There is more detail on the products in our comparison of which draught excluders are worth buying, and on pile heights and routing depths in the piece on brush seals for sash windows.

The DIY sequence that keeps the sash sliding

Allow half a day for the first window, less after that.

1. Score the paint line along the staff bead before levering, or you will tear paint off the architrave and split the bead. Old staff beads are soft, roughly 25mm by 19mm, and some will break. Have replacement stock ready.
2. Lever the beads from the middle, not the ends, and take the lower sash out. Unhook the cords and nail the loops off, or they go over the pulley and vanish down the box.
3. Clean the running faces. Scrape the pulley stile back to sound, smooth paint. Adhesive-backed pile will not hold on flaky gloss, and this is why most DIY jobs fail in the third month.
4. Fit side seals to the sash edge, not the frame. The seal travels with the sash and wipes the stile as it goes, instead of collecting grit at one fixed point.
5. Deal with the meeting rails. Brush strip under the top sash meeting rail, or a thin compression seal on the closing face. Check the fastener still pulls up. If it will not, the seal is too thick.
6. Decide about the top sash honestly. If it has been painted shut for forty years and the room has another window that opens, sealing it permanently with a flexible sealant is a legitimate and cheap fix. If it is the room's only ventilation, leave it.
7. Seal the pulley slots. A short length of pile above and below each pulley, or a proprietary pulley seal, closes the weight box. Nearly every DIY job skips this, and it is worth more than the sides.
8. Refit the beads and test the run. The sash should move with one hand and stay where you leave it. If you have to push, go down a pile size. Never plane the sash to suit a seal.

Done professionally, the seals sit in grooves routed into the staff bead and parting bead, or the parting bead is swapped for one with the pile moulded in. That is why it lasts fifteen years rather than three: no adhesive in the system, and nothing for a paintbrush to clog.

Three ways this job turns into a joinery bill

Painting over the seals. Once a decorator drags gloss across brush pile the bristles set solid and stop sealing, and getting them out means lifting the bead again. Mask the pile, or fit the seals after decorating.

Choosing a seal thicker than the clearance. The sash binds, somebody forces it, and either a cord snaps or the bottom rail joint opens. This is the commonest way a well-meant afternoon creates a bench job.

Sealing the house without thinking about the air. Old houses were ventilated by accident, and that accidental leakage carried moisture out. Seal every window in a flat with no working extractor fans and the moisture from cooking, washing and breathing lands on the coldest surfaces, which are still the windows, and by February you have black mould in the reveals. Keep the extractors working and leave the trickle vents or one openable window in service.

Ventilation and consents: what applies and what does not

Draught-proofing an existing window is a repair, so it does not trigger the controlled-fitting rules that apply when a whole window is replaced. The ventilation thinking behind those rules is still worth borrowing. Where windows are replaced, Approved Document F expects the provision afterwards to be no worse than before, and where the old windows had no background ventilators it sets minimum equivalent areas for the new ones, typically 8,000mm² in a habitable room and 4,000mm² in a bathroom. Those numbers do not bind you when you are only fitting seals. The principle does.

Consents are a separate question. In a conservation area an Article 4 direction bites on what the building looks like from the street and on replacing windows, not on seals fitted inside an existing sash that change nothing visible. Listed buildings differ, and the distinction is the method rather than the aim. Historic England's advice treats simple reversible draught strips as low-impact work that does not need listed building consent, while more invasive methods, including cutting grooves into original joinery, can affect special interest and may require it. If your sashes are original and you want the routed system, ring the conservation officer before anyone starts with a router.

What it costs, both ways

DIY materials run roughly £15 to £35 a window. Add a router and cutters if you intend to groove the beads yourself and the first window costs more than having it done.

A specialist draught-proofing a box sash in London, sashes out, pile routed into new beads, pulley slots sealed and the sashes eased and re-balanced, typically runs between £180 and £350 a window. Cords, weights and small timber repairs added to the same visit cost far less than they would separately, because the labour of getting the sashes out is already paid for. Access, borough and condition all move the figure, and anybody quoting from a photograph is guessing.

Set against that, the savings are real but modest. Historic England's guidance on draught-proofing windows and doors cites research showing air leakage through a window cut by between a third and a half, and air infiltration through a sash window in good condition reduced by as much as 86%. The Energy Saving Trust puts draught-proofing around windows, floors and doors at about £55 a year off a typical Great Britain household bill, and around £80 in Northern Ireland. The comfort gain is what people actually notice, and it is larger than the money.

What a draught-proofing quote should tell you

Get two or three prices and read them for what they include, not what they total. A quote worth comparing says, in writing:

• Whether the sashes come out, or whether the seals are being stuck on in situ
• Whether the pile is routed into a carrier or self-adhesive, and what height
• Whether the pulley slots are sealed, or only the sliding faces
• Whether new parting beads and staff beads are in the price
• What happens if a cord goes during the work, and whether the sashes are re-balanced afterwards
• Who makes good the paintwork, and when the seals go in relative to decorating
• The VAT position: professionally installed draught stripping is a qualifying energy-saving material, zero-rated in Great Britain until 31 March 2027 and 5% after that. Materials bought over the counter are not.

That last point quietly closes much of the gap between doing it yourself and having it done. The draught-proofing we fit is the routed and compression-seal version with new beads, and where a cord has gone we fold sash cord replacement into the same visit rather than charge twice to take one sash out. The rest of our work on box sash windows follows the same principle: the expensive part is access, so everything a window needs should happen while it is open. If a quote will not answer those seven questions on paper, its number is not comparable with anyone else's.