Sash window draught excluder: a buyer's guide

Why most draught excluders fail on a sash window

A box sash window works because both sashes are slightly smaller than the space they run in. The trade figure is a running clearance of around 3mm between sash and frame, and on a window that has been repainted a dozen times or has twisted with age it can be nearer 5mm in places. That clearance runs the full perimeter of both sashes. It is not a fault to be removed — take it away and the window will not slide — which is why a draughty sash window cannot simply be sealed shut the way a casement can.

Historic England divides draught-proofing seals into two families, and getting the distinction right is the whole of this subject. Compression seals are used where a moving part closes against the frame — the seal is squashed and springs back. Wiper seals are used where moving parts slide past each other; the seal is dragged across a surface every time the window is operated. Historic England's own wording is unambiguous: wiper seals are the only way of sealing the sides and meeting rails of sliding sash windows and sliding doors.

A compression seal on a sliding face fails in two ways. Mechanically, the sash shears past it, so the adhesive peels and the material is abraded. Practically, an old sash gap is not parallel — it may be 2mm at the head and 5mm at the cill — and a strip sized for the narrow end binds the sash while one sized for the wide end seals nothing. Brush pile copes because the filaments simply deflect further where the gap is tighter; Historic England describes brush pile seals as capable of sealing a range of gap sizes and adapting to fill uneven gaps well.

The independent advice lines up. The Energy Saving Trust's draught-proofing guidance, last updated in May 2026, says that for sliding sash windows foam strips do not work well and that it is best to fit brush strips or consult a professional. Historic England notes that foam strips are cheaper still but have a short life. Historic Environment Scotland puts it as brushes being generally thought to work better and last longer.

Where each type of seal belongs

A sash window has five sealing positions: three sliding, two closing. Any product recommendation that does not say which position it is for should be ignored.

Historic England explicitly permits compression seals along the bottom and top rails of a sash window, so a firm using a compressible seal in those two positions is not doing anything wrong; many specialists still use pile everywhere for consistency and low drag. Both are defensible, which makes it a fair question to put to an installer rather than a right-or-wrong test. What is not defensible is a compression strip down the stiles.

Every product sold for this job, one by one

Self-adhesive foam strip

Open-cell or closed-cell foam tape on a peel-off backing, usually sold in plain rectangular section rather than the E and P profiles used for rubber. Typical sizes are 6mm wide by 3mm deep rated for gaps of 1–3mm, and 9mm by 5mm for gaps of 3–5mm. At the trade counters in July 2026 the narrow version was about £6.19 for a 7.5m twin pack and the wider one about £9.99. The DIY sheds sold a 10m soft foam seal for around £8, and an own-brand self-adhesive draught seal at about £7 for 6m of 9mm by 5mm tape.

Where it belongs on a sash: the bottom rail onto the cill, and the top rail against the head — nowhere else. Lifespan: short. Historic England says so directly, and in a sliding position it can fail in a single season.

Self-adhesive EPDM and silicone strip (E and P profile)

Synthetic rubber extruded into a hollow profile so it collapses under load and springs back. The E profile has a shallow double-lobed section for narrow gaps; the P profile has a rounded bulb on a flat leg for wider ones. A white EPDM E strip in 20m was about £14.59 at the trade counters in July 2026, rated for gaps of 1.5–3mm at 9mm wide by 4mm deep. The matching P strip, also about £14.59 for 20m, is 9mm by 5.5mm for gaps of 3–5mm; the same P profile in black, 15m, was around £9.10. Historic England notes that silicone is taking over from EPDM as the material of choice for compression strips because it comes in a wider range of colours, including white.

Where it belongs: the same two closing positions as foam, but it will last considerably longer there. Lifespan: years rather than seasons in a compression position. In a sliding position the adhesive peels and the bulb tears.

Self-adhesive brush pile strip

Woven polypropylene or nylon filament set into a narrow backing strip with adhesive behind it. This is the only stick-on product that is technically correct for the sliding faces of a sash. Black brush pile in 10m was about £8.19 at the trade counters in July 2026: a 6mm pile on a 5mm base, rated for gaps of 3–5mm, and it can also be pinned through the profile directly onto timber. The DIY sheds sold a 5m white pile tape at around £7.50, also for 3–5mm gaps. Trade self-adhesive pile on a high-bond tape backing, in 25m coils, ran about £22.26 for the 4mm pile (gaps 2.5–3.75mm) and £24.68 for the 8mm pile (gaps 6–7.75mm), excluding VAT.

Where it belongs: anywhere on the window, though on the stiles you are sticking it to a surface never machined to receive it, so the strip sits proud and the sash has to be eased to suit. Lifespan: good, provided the adhesive holds — pin or staple the ends, because a peeling end is the usual failure.

Brush pile in a carrier

The professional system, and a fundamentally different product. The pile is a loose extrusion with a plastic root that slides into a channel — the pile carrier — which is itself glued into a rebate or routed into a groove in the timber. The carrier is permanent; the pile is a consumable that slides out and is replaced. One major supplier sells the pile in 25m and 100m coils: a 25m coil of 5.5mm pile was £8.93 ex VAT in July 2026, 8.5mm was £9.98 and 11.5mm was £11.13. Their pile carries a single polypropylene fin down the centre — a fin pile, which Historic England describes as a feature of better brush pile seals. Carriers in 2.2m lengths ran from £1.37 bare to £1.61–£1.81 flat or T-shaped with pile already fitted.

The T-shaped or centre-leg carrier sits in a groove so its top is flush with the timber, which is what the router is for. Specialists use an 8mm straight flute cutter, bond the carrier with a silicone-based adhesive, then insert the pile and retain it with a single pin — pull the pin and the pile slides out. A TCT cutter matched to a T-shaped carrier was about £27.30 ex VAT and needs a router with a ½in collet.

Where it belongs: every position, and it is the only sensible answer at the meeting rails. Lifespan: the carrier is effectively permanent; specialists guarantee the pile for around five years and expect roughly ten in practice.

Retrofit parting bead and staff bead kits

Rather than machine a groove into the original beads, most of the trade throws them away and fits new ones with the carrier and pile already built in. One major supplier supplys all their timber staff and parting beads in 3m lengths. In July 2026, a primed timber parting bead with a plastic brush carrier and 5.5mm brushpile was £7.09–£7.35 ex VAT depending on section; the Heritage version, which takes the brushpile straight into the timber with no plastic carrier at all, was £6.29 for the common 7 × 25mm and 8 × 25mm sections. Staff beads with carrier and brush ran £7.35 for 20 × 15mm up to £8.19 for 25 × 15mm.

Where it belongs: the two stile positions, which is where most of the leakage is. This is the highest-value component in the whole exercise and it deserves its own explanation — see parting bead and staff bead.

V-strip

A folded plastic or metal strip that acts as a leaf spring across the gap. Historic England lists V-strip among the wiper seals and states that it can be used between the stiles and boxes of sliding sashes, and separately notes that V-shaped silicone and EPDM seals can bridge a greater range of gap sizes than a plain compression strip. It is far more common in North America than in the UK, and it is not what British sash specialists fit — pile is quieter, kinder to paintwork and easier to renew. Treat V-strip as a legitimate but minority option for the stiles.

Comparison table

Prices are the shelf price at the retailer named, checked on 28 July 2026, and include VAT except where marked. Trade prices from a major supplier are quoted ex VAT because that is how they are sold.

Pile height and gap sizing — the commonest DIY mistake

More self-fitted draught proofing fails on sizing than on anything else. People buy one roll of strip, fit it all the way round, and find the sash will not shut at the head and still whistles at the cill. The gap on an original sash is not one number.

Brush pile is specified by the height of the filament above its base, and each height covers a band of gap widths. One major supplier's trade sizing is the clearest published example:

• 5.5mm pile — gaps of 1.5mm to 2.5mm
• 8.5mm pile — gaps of 3.0mm to 5.5mm
• 11.5mm pile — gaps of 6.0mm to 8.5mm

The pile is always taller than the gap it seals, because it works by deflecting. Retail brush strip is described the same way even when the pile height is not on the label: a 6mm pile is rated for 3–5mm gaps. Foam and rubber are the other way round — a 3mm-deep foam tape suits a 1–3mm gap, because compression seals are squashed rather than bent.

How to measure properly

1. Close the sash fully and lock it.
2. At the top, middle and bottom of the stile, work a feeler gauge or a folded card shim into the gap between the sash stile and the pulley stile until it just grips.
3. Repeat on the other stile. Old box frames are rarely symmetrical.
4. Repeat at the meeting rails, at the cill under the bottom rail, and at the head above the top rail. Five positions, and they will not all be the same.
5. Size the pile to the largest reading on each run, then ease the sash if it binds. It is easier to take a shaving off a sash stile than to make a short pile reach.

If the readings vary by more than about 2mm along a single stile, the sash has dropped or twisted, or the pulley stile is worn, and no strip of any kind will fix it. That window needs easing and re-balancing first.

Draught proofing kits

A kit gathers the beads, pile, cord and fixings for one window, which saves ordering six items from four suppliers. Contents vary enormously and so does the price. Three UK examples, July 2026:

• Entry level, around £39.95. A pin-on system with adhesive brush pile and P-strip, 25 metres of seal in total. No beads, no cord. This is materials, not a rebuild.
• Mid-range, around £60. Staff beads and parting beads with brush strip already fitted, meeting-rail draught strip, and 6m of waxed sash cord. Sized for windows up to 1.5m wide by 3m high.
• Full kit, from about £164.50 plus VAT. Timber staff and parting beads primed or unprimed, a pile carrier with 22 metres of pile, 14 metres of 6mm waxed sash cord, sash lifts, fasteners and locks, plus screws, pins, mastic and sash weights. Covers one sash up to 2950 × 1450mm.

One VAT point before you order. HMRC's VAT Notice 708/6 puts draught stripping on the list of energy-saving materials attracting a zero rate when installed in residential accommodation, and that relief runs to 31 March 2027 before reverting to 5%. It applies to the installation service and to materials supplied by the installer. Materials you buy yourself are standard-rated at 20%, so the saving from doing it yourself is smaller than the labour difference suggests. Our cost breakdown sets the two routes side by side.

What to buy for your window

Work backwards from the condition of the window rather than from the product.

Sound window, good beads, quick improvement wanted

Buy self-adhesive brush pile strip sized to your measured gap, plus a short length of EPDM P profile for the cill. Expect £15 to £25 in materials per window and an afternoon fitting it, and pin or staple the ends of the pile. Our DIY guide covers the method and the traps.

Beads worn, painted over or missing

Replace them with beads carrying integral pile. This is the highest-return part of the job, because the two stiles are the longest air paths on the window and a new bead restores the fit as well as adding the seal. Budget roughly £14 to £16 ex VAT in beads per window, plus pins and paint.

Sashes that rattle, stick, or have dropped out of alignment

Seals will not fix any of that, and fitting them first locks the fault in behind new timber. Historic England is explicit that windows should be assessed for repairs before any draught proofing, and notes that straightforward repairs alone can cut air infiltration and heat loss by up to a third. Ease the sashes, renew the cords, re-balance, then seal — which is the order a professional job follows.

A listed building

Historic England states that adding simple self-adhesive draught-proofing strips does not require listed building consent, but that more invasive methods such as grooving may affect special interest and may require it. Routing a carrier into an original sash is grooving. Speak to your local authority's conservation officer before ordering anything, and get the answer in writing.

Whatever you buy, buy it for a named position on the window. A single roll of foam applied everywhere is the one approach certain to disappoint.

Last reviewed: 28 July 2026.