Sash Window Draught Proofing in London

Draught proofing is the most worthwhile thing that can be done to an old sash window. It stops the draught, stops the rattle, cuts the noise from the street, and it does all of that without altering anything you can see. The window keeps its original glass and joinery and still slides by hand. Done properly it is invisible from the room.

This page sets out exactly what the job involves, in the order we do it, so you can tell the difference between a proper draught proofing job and a strip of foam pressed into the rebate. They are not the same thing and they do not cost the same thing.

Why sash windows are draughty in the first place

The gap is deliberate. A sliding sash has to have clearance all round or it could not slide, so the window was built with a working tolerance between the sash and the frame. When the joinery was new that tolerance was small and the draught with it.

Then a hundred and fifty years happen. The timber dries and shrinks across the grain. The staff beads get knocked off and refitted a fraction out. The parting bead wears thin where the sashes rub past it. Somebody replaces a broken bead with whatever moulding the merchant had. The sash itself shrinks. Every one of those adds a millimetre or two, and a two millimetre gap around the whole perimeter of two sashes is a considerable hole in the wall.

Where the draught actually comes in

It is worth knowing the individual paths, because a job that only closes some of them will only fix some of the draught.

• Between the sash and the staff bead — down both sides of the bottom sash, on the room side.
• Between the sash and the parting bead — the strip that keeps the two sashes in their separate runs. Usually the worst offender, and usually the most worn.
• At the meeting rails — where the top of the bottom sash crosses the bottom of the top sash. If the top sash has dropped even slightly, this becomes an open slot straight across the width of the window.
• Under the bottom rail — where the bottom sash sits down on the sill. Worn or shrunken and it never quite closes.
• At the top of the upper sash — the head of the frame, which nobody thinks about because the top sash never gets opened.
• Through the pulley openings — the slots the cords run through open directly into the weight cavity, which is a large void inside an external wall.
• Around the frame itself — between the box frame and the masonry, where the original mortar or lime pointing has gone. This is builder's work, not window work, but it is often what people are actually feeling.

What our draught proofing includes

The whole perimeter gets sealed, not the easy two thirds of it. In order:

1. The sashes come out. The paint seal is cut, the staff beads are eased off and kept, the bottom sash comes out, the parting bead comes out and the top sash follows. There is no way to draught proof a sash window properly without this step, and it is the reason the job costs what it does.
2. The sashes are overhauled. With them out on the bench we check the joints, ease anything binding, and deal with any soft timber at the bottom rail. There is no sense sealing a sash that is about to need taking out again.
3. Grooves are routed into the edges of the sashes — the stiles, the meeting rails and the bottom rail — to take the seal. The groove is cut to the carrier, not approximated with a chisel.
4. Brush-pile seal is fitted into the grooves. The pile is sized to the actual gap on your window, which is why it is measured rather than assumed.
5. A new parting bead with the seal integrated into it goes back in place of the old one, in the correct section for the frame.
6. New staff beads are made and fitted, also carrying a seal, and pinned so they can be removed again in future without being destroyed.
7. The meeting rails are sealed so the two sashes close tight against one another. Where the fastener is missing or worn we fit one, because a meeting rail fastener is what pulls the rails together.
8. The cords are dealt with. The sashes are out, so this is when tired or broken sash cords get renewed and the weights re-balanced. On most jobs this is included rather than extra.
9. Everything goes back and is adjusted so the sashes run smoothly by hand against the new seals, and the window is left ready for paint.

What we seal it with

The seal is a brush pile — a dense strip of fine polypropylene filaments held in a slim carrier that sits in the routed groove. It is the standard for sliding windows for a good reason: it seals against a surface that moves past it, which foam and rubber strip cannot do without either failing quickly or making the window impossible to open.

Pile is specified by the height of the brush, and it is chosen to suit the measured gap. Too short and it does not touch; too tall and the sash is stiff and the pile is crushed flat within a season. Some carriers include a thin central fin to improve the seal against wind driven rain. On the meeting rail, where the surfaces meet rather than slide, a wiper or blade-type seal is sometimes the better choice, and we will use one where it suits the window.

What draught proofing does — and what it does not

Being straight about this matters, because there is a lot of overclaiming in this trade.

It does: stop the perceptible draught around the sashes; stop the sashes rattling in wind; noticeably reduce the cold-draught feeling near the window, which is what most people actually mean when they say a room is cold; reduce noise coming through the gaps; and, by cutting uncontrolled air leakage, reduce the heat the room loses.

It does not: turn single glazing into double glazing. The glass is still a single pane and heat still passes through it, so the window will still feel cold to the hand and will still get condensation on a cold morning. If you want to address the glass rather than the gaps, that is secondary glazing or a slim double-glazed unit, which is a different job at a different price. Anyone telling you that brush pile alone will transform the glass is selling rather than advising.

How much difference does it actually make?

The most directly relevant research is Historic England's, because it is about old buildings rather than housing stock in general. Their published position is that air infiltration through a sash window in good condition can be reduced by as much as 86% by adding draught proofing. Two things travel with that number and are usually dropped by people quoting it: it is air infiltration that falls by up to 86%, not heat loss and not your bill; and it applies to a window in good condition — that is, one that has been repaired first, which is why we overhaul the sashes as part of the job rather than sealing a worn window.

Historic England also give the context that makes sense of it: older buildings lose around 15–20% of their heat through draughts, about a fifth of a home's heating goes out through the windows, and less than a quarter of the heat lost through a typical traditional window escapes by conduction through the glass — the rest is draughts. That last point is the whole argument for this work. The SPAB make the same case: owners tend to blame the glazing, when the major source of loss is air getting past the edges of the sashes, so eliminating draughts should be the immediate consideration.

Worth knowing too: Historic England put the gain from straightforward repairs alone — before any seal is fitted — at up to a third of air infiltration and heat loss. That is why we overhaul the sashes rather than sealing a worn window, and it is a large part of why this job is done with the sashes out. (Their broader figure for draught proofing windows and doors generally is a 33–50% reduction in air leakage; the 86% above is specific to sash windows in good condition.)

Their conclusion on where to spend money on a period window is blunt, and we agree with it: “Draught proofing will provide the biggest energy saving and conservation benefits for the lowest environmental and financial costs.”

Is it worth the money?

Here are the published figures rather than our opinion. The Energy Saving Trust estimates that draught proofing around windows, floors and doors could save around £55 a year in Great Britain, and puts the cost of professionally draught proofing the windows and doors of a whole house at around £250 (figures current as at May 2026).

Two honest caveats about those numbers. First, the £55 is the whole-home figure — windows are only one part of it, so nobody should tell you that draught proofing your sashes alone saves £55 a year. Second, the Energy Saving Trust's £250 is for ordinary strip and brush draught proofing, not for the specialist job described on this page, which involves taking the sashes out and renewing the beads and cords. Real sash prices are higher, and we set them out on the costs page.

The Trust also notes a second, larger saving that follows from the first: a draught-free room is comfortable at a lower temperature, and turning a thermostat down from 22°C to 21°C saves around £120 a year in Great Britain. That is worth more than the draught proofing itself — but only if you actually turn the thermostat down.

For comparison, the same source puts A-rated double glazing throughout an entirely single-glazed semi at around £4,800, saving about £85 a year. Neither measure pays for itself quickly on fuel savings alone. People do this work because the room stops being unpleasant to sit in, not because the sums are dramatic, and we would rather say so.

Sources: Energy Saving Trust, Draught proofing and Energy efficient windows and doors.

Ventilation — the part most companies leave out

An old house was built leaky and it relies on that. Once you seal the windows properly you have removed a real part of the air change in the room, and in a London flat with people, cooking, showers and drying washing, that moisture has to go somewhere. If it cannot leave, it condenses on the coldest surface it can find — which, in a room with single glazing, is the window and the reveal around it. Historic England put it plainly: the most significant risk in altering the thermal performance of an older building is creating condensation, which can give rise to mould, health problems for the occupants, and decay in the building fabric.

There is a sensible target to work to. Historic England reckon that about 0.4 air changes an hour is enough to keep the air fresh in a room used for living and sleeping, and recommend between 0.4 and 0.8 air changes an hour for older buildings in reasonable condition — adding that because air infiltration in many older buildings is well above that, draught proofing is normally beneficial. That is the honest shape of it: most London period houses are leaky enough that sealing the sashes is an improvement, but it is not a case of the tighter the better.

What must not be sealed

• Rooms with an open fire or any gas or oil burning appliance. Historic England advise specialist advice before sealing these, to avoid starving the appliance of air, and the Energy Saving Trust are explicit that fixed ventilation openings serving flueless or open-flue gas heaters must not be blocked. If there is a working fire or a gas heater in the room, say so before the work starts.
• Unheated voids — roof spaces and sub-floor voids are designed to be cross-ventilated and should not be draught proofed. Damp cellars may not benefit either.
• Internal doors, per the Energy Saving Trust — air has to move between rooms.
• Windows behind existing secondary glazing. This one surprises people: Historic England advise that where secondary glazing is installed, the original windows should not be draught proofed, to avoid condensation forming in the cavity between the two. If you already have secondary glazing, tell us — it changes the recommendation.

What to do instead

Keep trickle vents and air bricks clear rather than blocking them, run the kitchen and bathroom extract fans and let them over-run, do not seal an unused chimney completely, and open the windows properly for a few minutes a day rather than leaving one on the latch all day. The Energy Saving Trust also make the point that if a home is already fairly airtight, with no trickle vents, wall vents or mechanical ventilation, further draught proofing may not be suitable at all.

We will raise this on site if we think a particular room needs thinking about. We would rather have that conversation than take the money and leave you with a damp patch.

Does it need building regulations approval?

No. Draught proofing your existing sash windows does not trigger Part L. The energy efficiency standards in Approved Document L apply to a “controlled fitting” — an entire new window unit including the frame — and the document states directly that replacing glazing, or a window or door in its existing frame, is not providing a controlled fitting and does not need to meet the energy efficiency requirements. Approved Document F makes the complementary point that draught-proofing measures might not, on their own, constitute building work at all. Historic England's guidance says the same: there are no specific requirements for draught-proofing existing buildings.

Two caveats worth knowing. Building regulations do come into play if the windows are beyond repair and are being replaced rather than repaired — replacement windows in an existing home have to meet a maximum U-value of 1.4 W/(m²·K), or Window Energy Rating band B, though there is a relaxation where that cannot be met without harming the character of the building. And trickle vents are not required when you draught proof existing windows — that obligation attaches to replacing them. Anyone telling you the law now requires trickle vents as part of a draught proofing job is mistaken.

What Approved Document F does require, where energy efficiency work is done, is that the ventilation of the home ends up no less satisfactory than it was before — which is the regulatory version of the section above.

Sources for this section: Approved Document L (controlled fittings, para 10.6; replacement standards, Table 4.2), Approved Document F (paras 3.2, 3.6, 3.14–3.15 and Table 3.1), and Historic England, Draught-proofing Windows and Doors. This is general information, not a building-control decision on your property.

Draught proofing and re-cording together

These two jobs share nearly all of their labour. Both need the paint seal cut, the beads off, and both sashes out of the frame. Once that is done, renewing the cords is a small additional step and draught proofing is the routing and the seal. Having them done on separate visits means paying for the access twice and having the disruption twice, for no benefit.

If your cords have already gone, treat it as the moment to do both. If they have not gone yet but the window is a hundred years old and has never been touched, they are not far off.

How long does it last?

Brush pile in a domestic window is a long-life component — it is not something that needs revisiting every few years. What shortens its life is the sash being forced, badly adjusted or painted over: paint on the pile ruins it. In practice a properly fitted seal outlives several repaints, and if a length does eventually flatten it can be replaced on its own without redoing the window.

Listed buildings and conservation areas

Draught proofing is generally regarded as the right approach for historic windows, because it improves the performance of the window while keeping the original fabric — the glass, the joinery, the proportions and the glazing bars all stay. Historic England go further than that and actively encourage owners to conserve significant historic windows rather than replace them, noting that most traditional windows can be repaired and that even those in very poor condition can be returned to good working order.

But there is a consent point that a lot of companies gloss over, and you should know it. Historic England's guidance is that adding simple self-adhesive draught-proofing strips does not require listed building consent, but that more invasive methods — and they name grooving specifically — may affect the special interest of the building and may require consent. Routing a groove into the sashes to take a brush-pile carrier is exactly that method. It is the standard trade approach and it is what gives the durable result, but on a listed building it is not automatically permitted.

So: if your property is listed, speak to the local authority conservation officer before the work starts, not after. We will put the method in writing for that conversation. In a conservation area the building may also be covered by an Article 4 Direction removing permitted development rights, which is worth checking with the council at the same time.

Two related points that are more straightforward. Renewing broken sash cords like-for-like on a listed building is treated as repair and does not need listed building consent. Neither, generally, does secondary glazing. Replacement windows, by contrast, will need consent.

Sources: Historic England, Draught-Proofing and Modifying Historic Windows; SPAB, Upgrading timber windows. This is general guidance, not advice on your particular building — the council's conservation officer is the authority on that.

Frequently asked questions

Will I still be able to open the window?

Yes. That is the whole point of using brush pile rather than a fixed seal. A correctly sized pile adds a little friction and no more — the sash should still run under one hand and hold where you put it.

Can you draught proof a sash window without taking it out?

Not properly. You can stick self-adhesive strip into the rebate without removing anything, and it will help a little for a while, but you cannot seal the parting bead run, the meeting rail or the sash edges without the sashes coming out. Anyone offering a full draught proofing job that leaves the window in place is doing something less than that.

Will it stop condensation?

Not on its own, and it can make it worse if the room has no other ventilation. Condensation forms because warm damp air meets cold glass; draught proofing does not warm the glass. See the ventilation section above.

Will it make the room quieter?

Somewhat. Sealing the air gaps removes the direct path that street noise comes through, and most people notice the difference. It will not do what secondary glazing does, because the single pane of glass is still the weak point.

Is it worth doing on just one bad window?

It can be, if that window is the obvious problem. But the price per window drops a good deal when several are done in the same visit, so if the whole floor is draughty it is worth pricing them together.

Can I do it myself?

Parts of it, yes. There are DIY options that are genuinely worth the money and others that are a waste of an afternoon — we have set out honestly which is which on the DIY draught proofing page.