There is a tidy story about old windows that almost everyone believes. Single glazing is the weak link in an old house; rip it out, put sealed units in, and you have done the responsible thing. It is a good story, and the arithmetic underneath it is weaker than it sounds. The Energy Saving Trust price A-rated double glazing across a single-glazed semi at roughly £4,800, and put the saving at about £85 a year. That is the headline number from the organisation that exists to promote energy efficiency, and it is not a number that flatters replacement. Meanwhile the window you already own has one advantage nothing on a showroom floor can match: it exists. Nothing had to be extracted, fired, shipped or landfilled to put it there.
Where the Heat Actually Goes
Start with the physics, because the sales pitch quietly gets it backwards. Historic England’s research puts about one fifth of a home’s heating loss through its windows — a real number, and the reason windows get blamed. But the same research splits that fifth in a way that changes the whole decision: the majority of it leaves as air moving through gaps, not as heat conducting through the glass. Less than a quarter of a traditional window’s heat loss is conduction through the pane.
Read that again, because it is the crux. When you replace a single-glazed window with a sealed unit, the thing you are buying is better conduction through the glass — the smaller share. The bigger share, the moving air, is a gap problem, and gaps can be closed for a fraction of the price without removing anything. Historic England put draughts at 15 to 20 per cent of an older building’s total heat loss on their own, which is the same order of magnitude as everything the windows lose combined.
Their conclusion is unusually direct for a heritage body, and it is worth quoting in full because it settles the environmental question rather than dodging it: draught proofing “will provide the biggest energy saving and conservation benefits for the lowest environmental and financial costs”. Not the biggest saving full stop — the biggest saving per unit of money and materials spent. That is precisely the metric that matters if the motivation is carbon rather than comfort.
What You Get Before You Buy Anything
The most under-sold step costs nothing but labour. Historic England note that straightforward repairs alone — freeing sashes that have been painted shut, renewing worn cords, refixing loose beading so the sashes close square — cut air infiltration by up to a third. No new materials, no disposal, no delivery. A third of the leak, closed by making the existing machine work as designed.
Only after that does sealing earn its keep, and here the numbers get genuinely striking. Historic England cite research showing that draught proofing a sash window in good condition can reduce air infiltration through it by as much as 86 per cent. Two qualifiers travel with that figure and are routinely dropped by people quoting it: it is air infiltration, not heat loss and not a bill saving, and it applies to a window in good condition — which is the entire argument for overhauling first rather than sealing a worn window and hoping.
This is the sequence that specialist firms work to, and it is the opposite of the showroom sequence. Restoration outfits like Scott James Sash Windows Specialists assess whether the existing joinery is sound before anything is quoted, because a window that can be made to close squarely does not need replacing to perform, and a window that cannot be is a different conversation. Repair first, seal second, glass last is not sentimentality about old timber. It is the order that gets the most result for the least spend, which is also the order that emits the least.
Repairs alone close up to a third of the air leak — before a single new component is bought.
The One Product Distinction That Decides Whether It Works
If you take one technical thing from this piece, take this, because getting it wrong is why so many people conclude that draught proofing “doesn’t work”. Sliding surfaces need a wiper seal. Surfaces that press shut need a compression seal. They are not interchangeable.
A sliding sash window is a machine with clearance built into it — each sash has to glide in its channel, so nothing can grip it tightly. Stick a squashy self-adhesive foam strip into that channel and the sliding action shears it off within a season. The Energy Saving Trust say it plainly: for sliding sash windows, foam strips do not work well, and the answer is brush strips or a professional. Save the foam for hinged casements and door frames, where two surfaces genuinely close against each other and compress it.
There is a counterintuitive companion rule from Historic England that saves money and prevents damage: if secondary glazing is installed, the original window should not also be draught proofed. Sealing both sides traps moisture in the cavity between them. Specifying both on the same opening is a common and expensive mistake.
The Regulations Are on the Side of Keeping Them
People often assume that touching old windows drags you into building control. For repair and sealing, it does not. Part L is not triggered by draught proofing existing windows, and replacing glazing or a window within its existing frame is not a controlled fitting for energy efficiency purposes. Draught proofing may not, on its own, constitute building work at all. Trickle vents are not required when you draught proof either — the background ventilator requirements attach to replacement windows, at 8,000mm² for habitable rooms and kitchens and 4,000mm² for bathrooms.
Full replacement is where the regulatory weight lands. Then Part L applies properly: 1.4 W/(m²·K) or Window Energy Rating Band B, with the timber relaxation having expired on 14 June 2023. There is a character exemption for windows in protected buildings, met by a centre-pane value of 1.2 or better, or by keeping the single glazing and adding low-emissivity secondary glazing — an explicit acknowledgement in the regulations themselves that retaining the original glass is a legitimate route to compliance.
On listed buildings the balance tips further. Replacement needs consent and is frequently refused where the windows are original. Repair generally needs none, and like-for-like sash cord renewal needs none provided the new cords are of the same material. Historic England’s position on sealing is that self-adhesive strips need no consent, though grooving the timber to take a seal may — and grooving is the standard trade method, so it is a real question rather than a technicality. Islington’s 2025 guidance offers the workable test: no consent required provided the work is not visible and does not harmfully weaken the window. In a conservation area, check whether an Article 4 Direction has removed permitted development rights before assuming replacement is even available.
There is a VAT signal pointing the same way. Professionally installed draught stripping is treated as an energy-saving material and zero-rated until 31 March 2027, moving to 5 per cent after that. The associated repairs — cords, pulleys, rot, redecoration — stay at 20 per cent. The tax system is, in its own clumsy way, subsidising the option with the smaller footprint.
Draught proofing delivers the biggest benefit for the lowest environmental and financial cost — Historic England’s own conclusion.
If You Do Want Better Glass, There Is a Middle
Suppose the gaps are sealed, the sashes run properly, and the room still feels cold against the window on a January evening. That is the point at which glass genuinely becomes the remaining variable — and it is worth knowing that the choice is not binary between original single glazing and a skip full of Victorian joinery.
Secondary glazing is the least invasive route and the one the regulations treat most kindly. It is a separate internal frame set behind the existing window, so the original is untouched, nothing is thrown away, and it generally needs no listed building consent. Building regulations name it explicitly: for windows in protected buildings, the character exemption can be satisfied by retaining single glazing and adding low-emissivity secondary glazing. That is the regulations conceding that keeping the original glass is a legitimate compliance route rather than a heritage indulgence.
The other middle option keeps the frames and changes only the pane: slim double-glazed or vacuum-glazed units fitted into the existing sashes. The joinery, the glazing bars and the street elevation survive; the sash gets heavier, which is why it is a job for someone who understands counterbalance rather than a glazier working alone. Both routes share the quality that makes them defensible on environmental grounds — the great majority of the existing window, which is to say the part that took materials and labour to make, stays exactly where it is.
Whichever route appeals, the sequencing point stands. Do the gaps first. Glass upgrades are priced per pane and draught proofing is priced per window, so spending on glass while the window still leaks air is paying the expensive fix to solve the smaller half of the problem.
When Replacement Genuinely Is the Answer
None of this is an argument that old windows are sacred. Some are beyond economic repair: where decay has travelled into the joints and the box frame rather than sitting in the bottom rail, the repair cost climbs toward the specialist tier and the case for keeping the window weakens. On published UK ranges, overhaul alone runs about £280 to £480 per window and overhaul with draught proofing and re-cording about £250 to £600 in the mainstream London market, against £2,080 to £3,380 for full replacement — but those repair figures assume there is sound timber to work with.
There is also a safety point that argues for a professional look rather than a weekend attempt. On anything predating the early 1980s, assume lead paint: lead in paint was not prohibited in the UK until February 1992, and lead pigments were common in ordinary paints into the 1960s. The hazard is specifically dry sanding, and the controlled routes are chemical stripping, wet abrasive methods or on-tool extraction. Hot air below 500°C is acceptable — heat guns are not banned; blow lamps and gas torches are.
And one caution that catches people out: Historic England record cases where a window stuck firmly in place for decades had effectively taken on a structural role, and part of the surrounding wall collapsed when it was forced open. A window that will not move is not automatically the easy job it looks like.
The Honest Summary
The environmental case for keeping single-glazed windows does not rest on nostalgia. It rests on three things: most of what a window loses is moving air rather than conducted heat, so the cheap intervention addresses the bigger share; repairs alone close up to a third of that leak with no new material at all; and the body most associated with conserving old buildings has concluded in plain language that draught proofing delivers the biggest benefit for the lowest environmental and financial cost.
What that does not license is the reverse overclaim. Draught proofing does not make a single-glazed window the equal of a sealed unit, and anyone offering you a payback period for this work is extrapolating from figures that do not exist. The Energy Saving Trust’s own draught-proofing saving is around £55 a year in Great Britain and £80 in Northern Ireland — and note the scope, because that covers windows, floors and doors together, not windows alone. The honest position is narrower and more useful than either sales pitch: the window already in your wall is usually worth keeping, keeping it costs less carbon and less money than replacing it, and the order of operations is repair, then seal, then glass.
Frequently asked questions
Is it actually greener to repair old windows than replace them? On the evidence available, usually yes — and the strongest support comes from Historic England, who conclude that draught proofing provides the biggest energy saving and conservation benefit for the lowest environmental and financial cost. The mechanism is that most of a window’s loss is air through gaps rather than heat through glass, so the intervention that closes gaps addresses the larger share while consuming almost no new material. Beware of anyone attaching a precise carbon figure to your specific windows; that number is not credibly published.
How much will draught proofing actually save me? The Energy Saving Trust’s figure is about £55 a year in Great Britain, £80 in Northern Ireland — but read the scope carefully, because it covers windows, floors and doors combined and is routinely misquoted as a windows-only saving. The larger and less measurable return is comfort: a sealed room stops feeling cold at the same thermostat setting, which is why people who have it done describe the difference in terms of how the room feels rather than what the bill says.
Why do stick-on foam strips keep failing on my sash windows? Because they are the wrong category of product for a sliding surface. Foam is a compression seal that needs two faces pressing together; a sliding sash shears it off instead. The Energy Saving Trust’s advice is explicit that foam strips do not work well on sliding sash windows and that brush strips or a professional are the answer. Foam is fine on hinged casements and door frames, where the surfaces genuinely close against each other.
Do I need permission to draught proof windows in a listed building? Possibly, and the method decides it. Historic England’s position is that self-adhesive strips need no consent, but that grooving the timber to take a seal may — and grooving is the standard trade approach. Islington’s 2025 guidance gives the practical test: no consent provided the work is not visible and does not harmfully weaken the window. Like-for-like sash cord renewal needs no consent where the new cords are of the same material.
Does repairing windows trigger building regulations? No. Part L is not triggered by draught proofing existing windows, and replacing glazing within an existing frame is not a controlled fitting for energy efficiency purposes. Trickle vents are not required either — those requirements attach to replacement windows. Full replacement does bring Part L into play at 1.4 W/(m²·K) or WER Band B, with a character exemption for protected buildings that can be met by keeping the single glazing and adding low-emissivity secondary glazing.
Can I draught proof a house too much? In older housing it is rare. Historic England recommend background ventilation of 0.4 to 0.8 air changes an hour for older buildings in reasonable condition and note that most sit well above that, so sealing normally helps rather than harms. The firm exclusions are real though: never draught proof a room with an open-flued fire, stove or boiler without specialist advice, and leave roof voids, sub-floor voids and damp cellars alone. And do not draught proof an original window that already has secondary glazing — that traps moisture between the two.
