Sash Window Seals: Brush, Wool Pile and Compression Compared

A quote lands and it says something like "wool pile to stiles, compression seal to meeting rails and cill, new PVC parting bead throughout". Another firm quotes half the price for "brush draught excluder to all sashes". Both are describing draught-proofing. Only one of them is describing a system.

The difference between the two is not marketing. There are four genuinely different families of seal, they work by different physical means, and each one fails in its own way if it is put in the wrong place. Knowing which is which lets you read a quote properly and lets you argue with a fitter who is about to make your sashes stiff.

Ask three questions before you look at any product

Every seal on a sash window is answering one of two problems, and the questions sort them.

Does the gap close, or does it stay open? The gap between a sash stile and the parting bead exists all the time, whether the window is shut or open. The gap between the two meeting rails only exists when the window is shut, and it disappears the moment the fastener is thrown. A gap that closes can be sealed by squashing something. A gap that never closes can only be sealed by filling it.

Does the seal have to slide? Anything fitted to the running faces is dragged over painted timber every time the window is opened, potentially thousands of times. Anything at the meeting rails, the head or the cill is only ever compressed and released.

How big is the gap, in millimetres? Not roughly. Measure it, at both ends and the middle, with a feeler gauge or a folded strip of card. On a hundred-year-old sash the clearance up one stile is very often not the clearance up the other.

Get those three answers and the product almost picks itself.

Wool pile, and why hardly any of it is wool

Woven pile is the oldest of the four in window use. The original article really was wool, woven into a backing and used as a weatherstrip on sliding sashes and doors. The name has outlived the material. What is sold today as wool pile in the window trade is almost always woven polypropylene monofilament, frequently treated with silicone to shed water, and it is a great deal better than wool ever was: it does not rot, it does not absorb moisture, moths have no interest in it, and it recovers its height after being crushed.

Woven pile is a filling seal. It does not close the gap so much as make the gap into a thicket that air cannot get through cheaply. That is exactly what you want on a sliding face, because the bristles bend rather than grip.

The better grades carry a thin polypropylene membrane running up the middle of the bristles, which the trade calls a centre fin or fin seal. On the same gap, fin-seal pile measurably outperforms plain pile, because the air meets a solid barrier rather than only a dense brush. If a quote specifies pile, it is worth asking whether it is fin seal.

Mechanically it is held either by a rigid backing fin, usually 4.8mm across, pressed into a groove routed into the timber, or by a plastic or aluminium carrier that is pinned or stuck to the surface. The routed version is the one that lasts, because nothing is relying on adhesive.

Brush pile is the same idea with stiffer bristles

In practice "brush pile" and "wool pile" are used interchangeably on quotes, and the products overlap heavily. Where there is a real distinction, brush tends to mean a coarser, stiffer monofilament in a denser row, often in taller heights, and woven pile tends to mean a finer, softer weave in shorter heights.

That matters at the extremes. On the cill, where the gap under the bottom rail is a wedge because the cill has a fall on it, you may need 8.5mm or 11.5mm of pile and you want it stiff enough not to fold flat. Up the parting bead, where the sash is in contact for its full travel, you want the shortest pile that will close the gap, because every extra millimetre is friction. The full set of heights, groove sizes and the reasons the parting bead almost always ends up being replaced are covered in our piece on brush seals, pile heights and routing depths.

Compression gaskets are the ones that actually seal

A compression seal is a hollow or foam-cored gasket that gets squashed flat between two closing faces. Squashed, it seals far better than any brush, because there is no path left at all.

The material is usually EPDM rubber, which is the standard synthetic for weatherseals because it takes ultraviolet light and ozone without hardening. Silicone versions cost more and stay flexible at lower temperatures. There are also foam-cored polyurethane types in a rigid carrier, the sort most people meet on a front door, which have superb recovery but are too bulky and too grippy for anything that slides on a sash window.

The range most London joiners reach for is Schlegel's Aquamac. Aquamac 21 sits in a groove 2.7mm wide by 6.0mm deep and suits a gap of 4mm to 7.5mm; Aquamac 63 uses the same groove but is designed for a tighter 3.4mm to 5.4mm. Those numbers are the entire point of the product. Fit a gasket rated for 5mm into a 2mm gap and the sashes will not close; fit a 2mm gasket into a 5mm gap and you have spent money on nothing.

On a box sash, compression seals belong at the meeting rails first and foremost, because that is the biggest single leak on the window and it is a gap that genuinely closes. They also work at the head against the top rail, and sometimes under the bottom rail against the cill. They do not belong anywhere the sash slides.

Wiper and blade seals

The fourth family is a flexible fin or blade, usually silicone or a soft TPE, moulded into a plastic section and bending over to wipe against the sash face. The commonest form on a sash window is a PVC parting bead with an integral flipper, which replaces the timber parting bead entirely and arrives with the seal already in it.

The appeal is speed. There is no routing, no separate pile to buy and no chance of the groove being cut a half-millimetre oversize. A flipper also seals slightly better than short pile because it is a continuous surface rather than a row of bristles. Against that: it is plastic, visible as plastic on close inspection, and once the blade takes a set it is not adjustable. In a listed building or where the joinery is on show, a timber parting bead with pile in it is the more sympathetic answer. On a rear elevation in a tenanted flat, the PVC bead is a perfectly honest choice.

The four families side by side

Two conclusions come straight off that table. Adhesive is not a fixing on a surface that moves, so every self-adhesive product on a running face is a temporary measure and the more paint layers underneath it, the sooner it lets go. And nothing in the list is a general-purpose seal. A firm quoting one product for the whole window is quoting for the wrong window. Our survey of which draught excluders are worth buying goes through the DIY end of the market in more detail.

How seals actually die

Understanding the failure modes tells you what to look for when someone says your existing draught-proofing has "gone".

Pile packs down. Bristles that spend twenty years being dragged in the same direction lose height at the points of hardest contact, usually just above the meeting rail on the parting bead where the sash sits for most of its life. The seal still looks fine. It has simply become 1mm shorter than the gap.

Gaskets take a compression set. Rubber squashed for long enough stops springing back. A gasket that has gone hard and flat in its groove is doing nothing, and no amount of tightening the fastener will restore it. Press it with a fingernail: it should feel springy and mark, not solid.

Adhesive lets go. Almost always because it was applied to flaking or greasy paint. The fix is not more tape.

Paint kills everything. A decorator running gloss along a pile seal sets the bristles solid and turns a working seal into a plastic strip. Mask the pile, or fit the seals after the last coat has properly hardened.

Somebody forced a stiff sash. Which is the expensive one. A seal that is too thick makes a sash bind, the sash is then shoved, and either a cord parts or the joint at the bottom rail opens up. That single mistake turns a draught-proofing job into a joinery job.

What a proper job includes, and what to ask

The materials for one window come to a modest sum: pile in coils, a length of gasket, new parting beads and staff beads are tens of pounds, not hundreds. Nearly everything you pay for is labour and knowledge, because the sashes have to come out, the paint lines have to be cut, the grooves have to be routed straight and the window has to go back together and still slide.

Professionally fitted, routed draught-proofing on a standard Victorian sash typically runs somewhere between £250 and £450 per window in London, with bays, large ground-floor windows and anything above first floor above that. Access, condition and borough all move the figure, so treat any price quoted without a site visit with suspicion.

Four things worth asking any firm before you sign:

1. Are new parting beads included? They almost always have to be, because you cannot rout a 4.8mm fin groove into a 7mm or 8mm bead and expect it to survive being pulled out.
2. What goes at the meeting rails? The answer should be a compression seal, not brush. If it is brush, they have not thought about it.
3. Will the cords be checked while the sashes are out? Seals add friction, and a window that was marginally out of balance beforehand will be a nuisance afterwards. The disassembly is identical, so doing the cords at the same visit costs a fraction of doing them separately later.
4. Which pile heights, and were the gaps measured? A fitter who has not measured is guessing, and the guess is what makes a sash stiff.

Where to start

Pick the worst room in the house and go round one window with the back of your hand on a windy day. Movement across the middle of the window is the meeting rails, and that is a compression seal missing. Movement up the sides is the parting bead. Movement across the bottom is the cill gap, and it will want the tallest pile in the job. Air coming out of the pulleys means the weight box is open to the outside and needs sealing at the pulley itself, which is the step nearly every job skips.

Then test the balance before you spend anything. Push each sash halfway and let go. If it will not hold, seals are the wrong thing to buy first: the cords and weights need sorting and the seals should go in at the same time, in the same visit, while the sashes are already out. If you would rather do it yourself, our step-by-step on the DIY route and where it goes wrong sets out the order of work.

Our draught-proofing service fits routed pile and compression seals with new beads and a cord check as standard, and the wider sash window repair work covers the joinery it sometimes turns up across the London boroughs.