Brush Seals for Sash Windows: Pile Heights and Common Mistakes

Three pile heights cover almost every box sash in London: 5.5mm, 8.5mm and 11.5mm. The gaps each one is made to close barely overlap, so choosing between them is a measurement rather than a preference. Most draught-proofing that disappoints was decided before anyone arrived on site, when a coil was ordered off a list.

The pile itself rarely fails. It is polypropylene or nylon monofilament woven into a backing strip, and it will outlast the paint around it. What fails is the fit between the pile and the gap it was bought to close.

Two numbers on the coil

Every coil carries two dimensions: the height of the bristle, and the width of the backing that holds it. Everything else follows from how that backing is fixed.

Fin-backed pile is the version that goes into timber. The base is a rigid polypropylene strip, nominally 4.8mm wide, pushed into a routed groove that grips it by compression. That base stays the same across the range, so pile height can change without changing the cutter. Better grades run a thin plastic fin up the centre of the bristles, which seals more reliably and lowers the drag on the sash.

Self-adhesive pile sits on the surface instead of in a groove, with pile at 4mm, 6mm or 8mm. It is the honest fallback where routing is not on the table, and it lifts the first time a decorator catches an edge.

There is a British Standard for the quality of these products, BS 7386, covering draught strips for existing doors and windows in housing. Historic England's guidance names it and says buying to that benchmark helps ensure minimum standards are met.

Matching the pile to the gap

Manufacturers publish a gap range for each pile height, and the ranges are narrower than most people assume.

Published ranges differ a little between suppliers, so read the one printed for the product being fitted.

Two millimetres out either way produces a complaint. Too short leaves a slot, air accelerates through it, and that is the whistle. Too tall folds the bristles over; they pack down, take a set, and the sash gets heavier every year.

So the gaps get measured first. Ten minutes a window:

1. Close the window and fasten it, so the gaps are the ones that exist when it is shut.
2. Feeler gauges are ideal, folded card will do. Take the sash-to-parting-bead gap at the bottom, middle and top of the travel, then the staff bead on both stiles. Finding 2mm one side and 4mm the other is normal.
3. Measure at the head, and under the bottom rail at the cill. Take the cill at both ends and the middle, because it is a wedge rather than a parallel slot and nearly always wants the tallest pile on the window.
4. Pencil the numbers on the sash stile where the bead will cover them, or they will be gone by the time the beads are off.

Do it on every window: two sashes made by the same joiner in the same month will not share gaps after 130 years of paint and movement.

Wipers slide, compressions squash

Historic England's guidance draws the line the way a joiner would. Wiper seals, it says, are the only way to seal the sides and meeting rails of sliding sash windows; compression seals can also be used along the bottom and top rails. Plenty of draught-proofing gets this backwards.

A brush is a wiper: bristles drag across a painted face indefinitely without wearing out or gripping, and they adapt to a gap that changes width up the window.

A compression seal is static, a hollow or foam-cored gasket squashed in a small groove as the window closes, and squashed it seals better than any brush. Schlegel's Aquamac range is what most joiners reach for: Aquamac 21 closes a 4mm to 7.5mm gap in a groove 2.7mm by 6mm, and Aquamac 63 a tighter 3.4mm to 5.4mm in the same groove. That narrow band is the catch, because on a window that has moved one gasket can be crushed solid at one end and standing off at the other.

A sash only compresses in two places: the top rail travelling up against the head, and the bottom rail coming down onto the cill. Everywhere else the timbers slide, meeting rails included. Put a compression gasket in that junction and you have jammed the window. Our comparison of the seal types covers the rest of the family.

Where the seals actually go

A properly draught-proofed box sash carries seals in six positions: four wipers and two compressions. Counted as lengths of material it comes to more, because the staff bead seal runs up two stiles and across the head.

• The outer face of the parting bead, against the inner face of the top sash. Wiper, in contact for the full travel.
• The inner face of the parting bead, against the outer face of the bottom sash. Wiper, and the reason this bead is the awkward one.
• The inner face of the staff bead, up both stiles and across the head, against the bottom sash. Wiper.
• The meeting rails, where the two sashes cross. Wiper, running the full width of the window, and where most of the whistling comes from.
• The head of the frame, against the top rail of the upper sash. Compression.
• The cill, against the bottom rail of the lower sash. Compression, and the one that catches people out because of the fall.

A seventh exists, worth asking about rather than assuming: the pulley mortice, the slot cut through the stile into the weight box. Some systems fit brush-sealed pulleys there, closing a real route for cold air, though it cannot bind the cord. If you are unsure which timber is which, our note on the small timbers that keep a sash working names them.

Why the parting bead comes out

Fin-backed pile wants a clean, parallel groove sized so the base compresses as it goes in. Use the cutter the seal maker specifies: half a millimetre over and the base creeps out under load, under size and the timber splits.

Then the constraint that decides how the job is done at all. Stock timber parting bead is around 25mm deep and only 7 or 8mm thick, and that thickness is what a groove has to bite into. Two carrier grooves, one in each face, would meet in the middle of an 8mm bead. No cutter solves that.

So on most jobs the original parting beads come out and new ones go in, which is not corner-cutting. Replacing missing or worn beads sits on Historic England's own list of routine overhaul work, and their guidance describes one commercial system as replacing the existing staff and parting beads with modern equivalents that incorporate brush seals of woven polypropylene pile. Suppliers sell them primed with the carrier inserted, in Accoya or in solid plastic, and on most the pile pulls out so the bead can be decorated and the pile pushed back.

That last detail matters, because paint is what kills brush seal. Historic England is blunt: painting the flexible part of a seal is not recommended, as it changes the characteristic of the product. Their checklist asks whether the strip will be renewed at every redecoration, and if not, it has to be capable of removal and reinstatement afterwards. Paint wicks up the bristles and sets hard, and a strip of pile becomes a strip of plastic touching nothing. Our guide to painting sash windows covers where the cut lines belong.

Staff beads are more forgiving: the section is far heavier, commonly 15 to 20mm thick, so there is timber to rout into, and the bead comes off and gets repositioned to suit the sash first.

What sealing does to the balance

A box sash is balanced, not held. Each weight carries half its sash, and the only thing stopping a balanced sash drifting is a modest amount of friction against the beads. Four runs of wiper seal at the right height add a little of it. Four runs 3mm too tall add a great deal.

The symptom is a window that needs a shove to start and then runs away once moving. The remedy is not more force but the correct pile height, or weights adjusted to suit the new friction, which means opening the pockets and adding lead. Our piece on working out the right sash weight sets out the arithmetic.

The sashes come out to rout the beads, so that is the cheapest moment in the window's life to deal with the cords. A cord gone grey and hairy where it passes over the pulley has a year or two left at best, and sealing around it means paying twice for the same disassembly. That is why sash cord replacement and draught-proofing are usually one visit on the box sashes we open up around London, and replacing the cords is the job to price alongside.

What the seals are worth

Heat losses from a typical traditional window are predominantly through the gaps around it, which is why draught-proofing beats a glazing upgrade on cost per pound saved in most period houses. Historic England adds a qualifier that tends to get dropped in sales patter: with larger windows, the proportion lost by conduction through the glass tends to be greater. A Victorian 2-over-2 with big panes is a different case from a Georgian six-over-six.

Their research numbers are worth taking in order. Simple repair, mending cracks and eliminating gaps, cut air infiltration by more than a third on the sash tested. Draught-proofing reduced air exchange through the sash by as much as 86%, and their general guidance puts the reduction in window air leakage at between 33 and 50%. For comparison, heavy curtains or well-fitted roller blinds cut heat loss by around 40%, and well-fitted shutters cut conduction losses alone by 60%. Seals cost less than either, and nobody has to remember them at dusk.

Professionally fitted, routed draught-proofing generally runs between £250 and £450 a window in London, with bays and upper floors above that. Materials are a small fraction of it: what you are buying is the order of work, and most of a day for a first window. Our step-by-step on the DIY route and where it goes wrong sets the sequence out.

Ventilation: what the gaps were doing

Before any seals went in, the gaps around the sashes were the room's background ventilation. Nobody chose that arrangement, but in a flat with no extract fan it is often the only thing moving air, and closing it changes the room rather than only the window.

Historic England puts the risk plainly: significantly reducing the ventilation of a room can create moisture problems, particularly in rooms with high moisture content such as a kitchen or bathroom. Their answer is controllable ventilation, an extractor or a trickle ventilator, rather than leaving an uncontrolled leak in place. In a listed building that comes with a caveat, because vents can be visually intrusive and where they go becomes a conservation question.

Sealing is a half-day; putting a fan into a bathroom is another trade and a hole through a wall. Decide the fan first, because a sealed wet room with nowhere for the moisture to go puts it on the coldest surface available.

Nothing in the process will prompt you about any of this. Draught-proofing an existing sash provides no controlled fitting under the Building Regulations, so unlike a replacement window it triggers no ventilation assessment and no certificate. If the room has an open flue, a stove or a gas fire drawing combustion air from the room, that check falls to whoever services the appliance, and to you to arrange.

Three questions before you accept a price

All three are askable on the phone, and a firm that answers them plainly has usually measured the gaps.

• Does the price include new parting beads, or is somebody planning to rout the originals?
• Are the meeting rails, the head and the cill being sealed as well as the sliding faces, where the work is easy?
• Will the cords and the balance be checked while the sashes are out, with a price for putting them right if they are not?

When the whistle comes back

A resealed sash that is quiet in March and audible again by January has one of three things wrong with it, and they can be told apart without taking anything off.

• A note that rises and falls with the wind, from the middle of the window. The meeting rail seal. Either the pile was short for that gap from the start, or the bottom sash has dropped a few millimetres on a stretched cord and opened it. Check whether the meeting rails still line up across the width.
• A window that has become stiff since the work. Pile too tall and taking a set, or paint driven into the bristles at the last redecoration. Bristles you can flatten with a fingernail and that spring back are fine; bristles that stay flat are finished.
• A whistle that shifts as you slide the sash. The pile has come out of its carrier, usually where a bead was refitted with the strip snagged.

Only the first is really a seal problem; the others are the window reporting on its cords or its paint. Which is the argument for having the pile heights written on the quote: it tells you whether anyone has been near the window with a feeler gauge.