Sash Window Draught Excluders

“Draught excluder” covers a dozen very different products, and most of the disappointment people have with them comes from putting the wrong one in the wrong place. A sash window is an unusual case: parts of it slide, parts of it press together, and a seal that is right for one is useless or actively harmful on the other.

This page goes through the types, what each one is for, and where it belongs on a box sash window.

The one rule that decides everything

This is not a rule of thumb we invented. Historic England put it flatly: “wiper seals are the only way of sealing the sides and meeting rails of sliding sash windows and sliding doors”. Compression seals, in their guidance, suit narrow, even gaps — which on a sash means the rails, not the runs.

On a sash window, the sliding surfaces are the sides of both sashes, where they run against the staff bead and the parting bead. The meeting surfaces are the top of the upper sash against the head, the bottom rail against the sill, and the two meeting rails against each other.

Brush pile — the standard for the sliding runs

A dense strip of fine polypropylene filaments set into a slim plastic carrier, which sits in a groove routed into the sash edge or the bead. The pile touches the surface it seals against and wipes along it as the sash moves.

This is what the trade fits, and it is what we fit. It is the only type that will seal a moving joint for decades without either failing or seizing the window. Pile is specified by height, and the right height depends on the actual gap in your window — too short and it does not touch, too tall and the sash is stiff and the pile crushes flat. Some carriers have a central fin to improve the seal against wind-driven rain, which is worth having on an exposed elevation.

The trade-off is that fitting it properly needs the sashes out and a router. There are self-adhesive versions for DIY, covered on the DIY page, which work on the same principle but rely on tape rather than a groove and so do not last nearly as long.

The sizes, and why the gap has to be measured

Brush pile is specified by the height of the brush, and each height covers a narrow band of gap. The trade supplier's published mapping:

Those bands are narrow, and they are the whole reason a competent fitter measures the window before ordering rather than carrying one size. An 8.5 mm pile in a 7 mm gap does not reach; an 11.5 mm pile forced into a 3 mm gap makes the sash stiff and is crushed flat within a season. If someone quotes a draught proofing job without ever measuring the gaps, that is worth asking about.

Materially: the pile is polypropylene, silicone-treated to resist water, mould and mildew, and heat-set so it recovers after being compressed. Better versions carry a thin central fin to improve the seal against wind-driven rain. The carrier comes in three geometries — flat and self-adhesive (the DIY option), T-shaped to sit in a routed groove, and offset-leg. For listed and conservation work there are also timber-only heritage beads with no plastic carrier at all, where the brush sits directly in the moulding.

Silicone and thermoplastic wiper seals, and V-strip

A thin flexible blade or folded strip rather than a brush, again set in a carrier. These are still wiper seals, so they are legitimate on the sliding parts of a sash — Historic England note that V-strip wiper seals can be used between the stiles and the boxes of sliding sashes, and that silicone or thermoplastic strips are used where a heavy-duty seal is wanted. They seal better against driving rain than brush pile because they are a continuous membrane rather than filaments; the trade-off is a little more friction.

Do not confuse this V-strip — a wiper — with the self-adhesive EPDM compression profiles below. They are different jobs.

EPDM rubber profiles — E, P and D section

Self-adhesive synthetic rubber strip, sold in profiles named after the shape of their cross-section. They are hollow bulbs that squash when the joint closes. (You will also see “V” or tension profile sold as a draught excluder — that is a folded plastic leaf seal, not an EPDM rubber profile, and it is a door product.)

These are compression seals, so on a sash window they belong on the meeting surfaces only: the top rail of the upper sash, the bottom rail where it sits on the sill, and the meeting rails. Used there they are genuinely good value — EPDM is weather-resistant and lasts far longer than foam. Used in the sliding runs they will wreck the window's operation.

Each profile is rated for a gap range, so measure before buying. The manufacturer's published ranges:

Ranges as published by Stormguard. A coin is a rough gauge: if you cannot slide a 10p into the gap, you are in E-profile territory.

Foam tape — the one to be wary of

The cheapest product on the shelf, and the one most often bought for sash windows by mistake. Open-cell foam compresses easily, which is why it works on a hinged door, but it has no resilience worth speaking of: it takes a set, absorbs water, perishes in sunlight and comes apart when rubbed. On the sliding runs of a sash window it lasts weeks. On the meeting surfaces it is a short-term measure at best — the same money on EPDM buys something that will still be there in five years.

This is not just our view. The Energy Saving Trust's own advice is that “for sliding sash windows, foam strips do not work well. It's best to fit brush strips or consult a professional” (Energy Saving Trust, Draught proofing).

Sealed parting beads and staff beads

Rather than routing the existing beads, the usual professional approach is to replace them with new ones that have the seal built into the section. A new parting bead carries pile on both faces, sealing the top sash on one side and the bottom sash on the other; the new staff beads carry pile on the inner face. It is neater, it is more durable than retro-fitting into an old worn bead, and it renews a component that is usually past its best anyway.

Meeting rail fasteners

Not sold as a draught excluder, but on many windows it is the most effective one available. A proper fitch or Brighton fastener does not merely latch the sashes — the cam action pulls the two meeting rails tightly together and lifts the bottom sash into its rebate. A worn or missing fastener leaves an open slot across the full width of the window. Fitting a sound one is cheap and frequently makes more difference than any strip.

Secondary glazing

A second pane inside the existing window. Strictly it is glazing rather than draught exclusion, but because the panel seals against the reveal it excludes draughts as a side-effect — and unlike everything else on this page it also addresses heat and noise coming through the glass itself. It is the most effective option and the most expensive, and the panel has to be removed to open the window.

Shrink film, snakes and other stop-gaps

Shrink-on film creates a still air layer in front of the glass and does work, but it is seasonal, it looks like what it is, and the window cannot be opened while it is up. Fabric draught snakes on the sill do essentially nothing on a sash window, because the air is coming in around the sashes rather than across the sill — they are a door product.

Which goes where

How long each type lasts

Being straight about this: there is no independent published service life for brush pile, and any firm quoting you “twenty years” or “lifetime” is stating a sales figure, not a tested one. What can be sourced:

• Q-Lon foam compression seal — the only one here with a published figure: a 10-year manufacturer guarantee, with weather-resistance testing claimed to 20 years.
• Routed-in brush pile and sealed beads — the long-life option in practice, because it is held mechanically rather than by adhesive. No published lifespan exists, so treat that as trade experience rather than a specification.
• EPDM on meeting surfaces — durable provided the adhesive went onto a clean, sound, dry surface. Old flaking paint is what usually defeats it.
• Self-adhesive brush strip — a season or two on a window in regular use, longer on one that is rarely opened.
• Foam tape — Historic England simply say foam strips have “a short life”. Weeks to a winter, in our experience.
• Shrink film — one season by design.

Whatever is fitted, the thing that actually kills a seal is paint. Historic England are explicit that painting the flexible part of a seal is not recommended, and that seals should be capable of being removed and put back at redecoration. If a decorator is coming, tell them.

Frequently asked questions

What is the best draught excluder for sash windows?

For the sliding parts, brush pile fitted into a routed groove or built into replacement beads — nothing else survives the movement. For the meeting rails, top rail and bottom rail, an EPDM compression profile or a wiper blade, together with a properly working meeting rail fastener. Most windows want both types, in their respective places.

Can I just use foam strip from the DIY shop?

On the top rail, bottom rail and meeting rail it will do something for a short while. Anywhere the sash slides, it will either jam the window or shred. If you are buying strip anyway, EPDM costs a little more and lasts many times longer.

Why does my draught excluder keep coming off?

Almost always one of three things: it is a compression seal in a sliding run and is being rubbed off; the adhesive went onto dusty or flaking paint; or the profile is too thick for the gap, so the sash is forcing it out of place every time the window moves.

Will a draught excluder stop my sash windows rattling?

A correctly sized seal in the sliding runs will, because it takes up the slack that lets the sash move in its frame. A strip on the meeting rail alone usually will not. If the rattle is bad, the gap may be wide enough that the sashes need packing or the beads renewing rather than just sealing.

Does draught proofing need doing again later?

A properly fitted, routed-in seal should not need periodic replacement. If a length does eventually flatten or get damaged, it can be renewed on its own without redoing the whole window.