How do I draught-proof a sash window?

A sash window has to slide, which is why ordinary compression draught strip fails on it — the strip either stops the window moving or is torn off by the first opening. Brush seals let the sash slide and still close the gap.

What you'll need

• Brush pile seal in the correct pile height
• New parting bead and staff bead, pre-routed if buying a kit
• A router or a slot cutter, if routing yourself
• A knife and pins
• Silicone sealant

Step by step

1. Find where the air comes in.A lit incense stick or a draught detector on a windy day. In most sash windows the meeting rail is the biggest single leak, followed by the stiles.
2. Check the sash is not simply loose.A sash that rattles in its runs has too much clearance, often after decades of paint stripping. Fitting seals is the fix, but knowing how loose it is decides the pile height you need.
3. Choose the system.The proper answer is a pile brush seal set into a groove in the parting bead, the staff bead and the meeting rail. Kits come with pre-routed beads, which saves buying a router.
4. Deal with the meeting rail first.The joint where the two sashes cross is often the single largest gap. A brush seal or a rebated weatherstrip here makes the biggest difference of any step.
5. Fit new parting beads with the pile set in.Remove the old beads, cut new pre-routed ones to length, and tap them into the groove. The pile bears on the sash stile as it slides.
6. Do the staff bead the same way.Pinned rather than wedged, so it can be removed for future cord replacement without destroying the seal.
7. Seal the head and cill.The top of the upper sash and the bottom of the lower sash close against fixed timber. A compression seal is fine here because those surfaces do not slide.
8. Seal the static joints too.Between the frame and the wall, between the cill and the frame, and around the pulley faces. These are not part of the sliding mechanism and can be sealed conventionally.
9. Consider the pulley openings.The pulley slots are direct routes into the weight box, which is often connected to the wall cavity. Proprietary pulley covers close them without stopping the cord.
10. Do not seal the window shut.A room with an openable window is relying on it for ventilation. If the window becomes airtight, make sure something else provides background air.
11. Test on a windy day.Same incense stick, same positions. It is very satisfying and it finds the one gap you missed.

Tips

• Draught-proofing a sash window typically reduces its air leakage very substantially and often costs less than a tenth of replacing it — and it keeps the original glass.
• Get the pile height right. Too short and it does not touch; too long and the sash becomes hard to move and the pile flattens within a year.
• Do the job in summer. The window has to come apart, and you will want it open while you work.

Common mistakes

• Using self-adhesive foam strip on the sliding faces — The sash slides against those surfaces, so the strip is scraped off within days, and while it lasts it makes the window very hard to open. It only belongs on the head and cill, which do not slide.
• Sealing every gap including the ventilation — Older houses relied on their windows for background ventilation. Making a bedroom airtight without adding a trickle vent or an extractor moves the moisture problem to the walls and produces mould in the corners.

If it doesn't work

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