Heritage Double Glazing: Modern Units in Traditional Sashes

The front room is cold from October to March, and you want that to stop without the house looking like it has had new windows. Every firm you ring says it does heritage double glazing. Before you compare a single price, one measurement decides which of those quotes are even possible, and you can take it yourself in a minute.

Take the measurement before you take the quote

The glazing rebate is the step machined into the inside face of the sash that the glass sits in. It was cut for 3mm glass and a putty fillet, and on most Georgian and Victorian sashes it runs 9mm to 13mm deep. The sash itself is usually only 41mm to 45mm thick. Everything else follows from those two numbers.

1. Open the bottom sash and find a pane where the putty has shrunk or cracked away at a corner.
2. Push the corner of a steel rule into the gap between the edge of the glass and the shoulder of the rebate until it stops against the back.
3. Read off the depth: that is what you have to work with.
4. Measure the sash thickness at the bottom rail, front to back, while you are there.
5. Push the sash up to head height and let go. If it drops more than an inch, the balance is already out.

A standard modern sealed unit is 24mm or 28mm overall: 4mm glass, a 16mm or 20mm argon cavity, 4mm low-E glass. Carrying one needs a rebate around 22mm deep, so a sash nearer 56mm thick. You cannot machine that out of a Victorian sash. Try, and you cut through the ovolo moulding on the inside face and leave a bottom rail with nothing in it.

So heritage glazing runs the other way round: the unit is made thin enough for the sash you already own.

Two thin technologies, working on different physics

Slim sealed units are ordinary double glazing squeezed down. Two panes, usually 3mm or 4mm, separated by a narrow spacer, giving 11mm to 16mm overall. The catch is the cavity. Every fill gas has a width at which it works best, and below that width plain conduction takes over: roughly 16mm for air, 15mm for argon, 11mm for krypton and around 8mm for xenon. Squeeze argon into a 6mm gap and it performs barely better than air. That is why the better slim units are filled with krypton, or a krypton and xenon mix, and why they cost what they do.

Vacuum glazing deletes the problem instead of managing it. Two panes are sealed at the edge, the air is pumped out, and an array of near-invisible support pillars stops atmospheric pressure crushing the panes together. With no gas in the cavity there is almost nothing left to conduct or convect. AGC's FINEO is the unit most often specified in London. Its standard thin unit is 7.7mm overall, with the range running from 6mm to 12mm depending on the build-up. There is no visible evacuation port at the edge, and AGC guarantee it for twenty years, against an industry standard of ten. There is more on the fitting side in our piece on vacuum glazing in original sash rebates.

Centre-pane figures are the best case, taken mid-pane. The whole-window number the regulations use is always worse, because of the frame, the edge of the unit and the glazing bars: a slim-glazed timber sash realistically lands between 1.6 and 2.2 W/m²K. If those figures mean nothing to you yet, what U-values actually measure is worth ten minutes.

The arithmetic that decides whether your sashes still slide

This is where cheap heritage glazing comes apart.

Float glass weighs 2.5 kg per square metre for every millimetre of thickness. Take an ordinary Victorian lower sash with a glass area of 0.63 m², roughly 900mm by 700mm. In 3mm glass that is about 4.7 kg of glass. Put a 4-6-4 slim unit in and you are carrying 12.6 kg. The sash as a whole might go from 9 kg to 17 kg. Vacuum glazing is no escape: 7.7mm of unit is still 7.6mm of glass.

A box sash stays where you put it only because the two weights hanging in the pockets add up to the weight of the sash, each one carrying half. So every kilo added to the sash needs half a kilo added to each weight, and there is a hard limit on what will fit down a pocket that is typically 40mm to 50mm wide inside.

Three other things get checked at the same time. The cord goes up a size, usually to No. 8 waxed cotton rather than No. 6, because thinner cord chafes through faster under the extra load. The axle pulleys have to be sound, since a worn brass wheel on a corroded steel axle will squeal and then seize under a heavier sash. And the pockets need opening and looking at rather than guessing at. Anyone quoting to double glaze your sashes without mentioning re-weighting has not thought the job through.

Glazing detail decides whether the unit survives

Specify the finest unit made, fit it badly, and it mists up within a few winters.

1. Prime the rebate. Bare timber wicks water straight to the edge seal, and moisture sitting against that seal is the commonest reason slim units fail.
2. Set the unit on packers. Plastic glazing packers at the quarter points, so the glass never touches timber anywhere around the perimeter.
3. Back bed on silicone or glazing tape, not putty. Traditional linseed oil putty must not be used against a sealed unit; the oils attack the hot-melt butyl in the primary seal and the unit fails from the edge inwards. Historic Environment Scotland's follow-up research floated exactly this, putty incompatible with the edge seal, as a possible cause of the deterioration it measured. Our piece on linseed and acrylic glazing putties sets out where putty still belongs.
4. Pin with non-ferrous sprigs. Copper, sherardised or stainless. Ordinary steel brads rust, blow the paint film and open a water path straight to the seal.
5. Seal the perimeter and let it cure. Twenty-four hours before anything else happens to that sash.
6. Paint 2mm onto the glass. That thin overlap is what bonds the paint film to the glass and keeps water out of the rebate. Miss it and every pane draws water in along the timber edge.

Slim units are usually finished with a timber bead rather than a putty fillet, which alters the external profile. A good joiner runs a bead with a putty-like chamfer so the sightline from the street is unchanged. A poor one uses a square bead, and the window reads as modern from thirty feet away.

How long these units really last, on the evidence

Sealed units fail. The seal degrades, moisture enters the cavity, and the unit mists permanently. There is no repair: a failed unit is replaced, which means taking the sash out and cutting off the beads.

What nobody can honestly tell you is how long yours will last. The only serious in-situ study in the UK is Historic Environment Scotland's. Ten slim-profile systems were fitted into listed Georgian tenement flats in Edinburgh's Old Town and measured in the winter of 2009/10; eight of them were measured again two winters later. The follow-up was inconclusive by its authors' own account: most of the change fell inside the margin of error. One xenon-krypton unit showed a real loss, a 19% rise in U-value over two years. The air-filled unit and the vacuum unit showed no significant deterioration at all. The recommendation was to keep measuring over a longer period before anyone drew conclusions.

So the published evidence suggests gas-filled slim units drift towards the performance of an air-filled unit as the fill escapes, and that vacuum units, with welded edges rather than a conventional seal, behave differently. It supports no confident service-life figure. A salesman who offers you one, five years or thirty, is quoting marketing.

The one thing anyone commits to in writing is the guarantee. Ten years is the industry standard on a sealed unit; FINEO's twenty is the outlier. On a job where the glass is the expensive part, the length and terms of that guarantee deserve more attention than a decimal place on a U-value.

What gives it away from the pavement

Three things betray double glazing in an old sash.

The spacer bar sits about 10mm in from the edge of the glass and reads as a hard grey or black line around every pane. Warm-edge spacers in a dark finish are far less noticeable than bright aluminium. On a six-over-six Georgian window that is twelve such lines, which is why heavily barred windows are hardest to glaze convincingly.

The double reflection is unavoidable with any two panes. Stand at an angle in daylight and you see the building opposite twice. Vacuum glazing shows it too, though less, the panes sitting closer together.

The flatness of modern float glass reads differently from old cylinder glass, which has faint ripples that move as you walk past. If your windows still hold original panes, breaking them out is a permanent loss worth weighing first.

Draught first, glass second

A timber-framed single-glazed window sits around 4.8 W/m²K, and slim double glazing might halve that. Genuine, but it is not the whole story of a cold room.

Most of what you feel as cold from an old sash is moving air rather than conducted heat: the gap at the meeting rails, the gap behind the staff bead, the gap where the parting bead has shrunk. Historic England's research found heat loss through a window as a whole can be cut by over 60% using secondary glazing with a low-E coating, much of that from stopping air movement rather than the extra pane. Proper draught proofing alone changes how a room feels more than most people expect, at a fraction of the cost of re-glazing. Do it first, live with it a winter, then judge whether you still want the glass.

Noise is a separate question, and slim units are weak at it. Two panes of similar thickness with a narrow cavity share a resonance and sound passes through at that frequency almost unimpeded. If traffic is the reason you are doing this, secondary glazing with a 100mm-plus cavity beats any slim unit comfortably.

How much original fabric survives, and why the rules follow from that

At the lightest end, the glass comes out of your existing sashes and a slim or vacuum unit goes back in; your timber survives intact. In the middle, only the sliding parts are renewed: fresh sashes matched to the profile of the old ones but run in a deeper section, so the rebate will hold a thicker unit, then hung on the existing pulleys. The box, pulley stiles and cill stay put. That is the commonest London job. At the heavy end the box frame goes too, which is a replacement window rather than a repair.

Approved Document L draws its line in the same place. It defines a controlled fitting as the entire unit of a window including the frame, and says plainly that replacing glazing, or a window in its existing frame, is not providing a controlled fitting and need not meet the energy efficiency requirements. So re-glazing carries no U-value duty and no FENSA certificate, FENSA covering replacement windows rather than glass or repairs. Fit a complete new box sash and that reverses: since 15 June 2023 a replacement window in an existing dwelling must achieve a whole-window U-value no worse than 1.4 W/m²K or a Window Energy Rating of Band B, the old concession letting timber sit at 1.6 having expired.

Building Regulations are not planning, though, and planning is usually the binding constraint. If the house is listed, altering the glazing almost always needs listed building consent whatever Part L says, because consent turns on character rather than heat loss. In a conservation area with an Article 4 direction you need planning permission too. Approved Document L steps back here itself: work to listed buildings, conservation areas and scheduled monuments need not comply fully where that would unacceptably alter character or appearance, and building control is told to take the conservation officer's advice. Conservation officer first, specification second. Our guide to what conservation officers approve covers the sticking points.

Four questions that sort the quotes out

Put the same four to every firm, in writing.

1. What is the overall unit thickness, and what rebate depth did you measure on my windows?
2. What is in the cavity — argon, krypton, or nothing at all?
3. How are you re-weighting the sashes, and are new cords and pulleys in the price?
4. What is the guarantee on the seal, and who honours it if the unit mists in year eight?

The third question separates firms who have done this from firms who have sold it. Vague answers there mean sashes that will not stay up by spring.

If you would rather someone looked at the actual windows first, our double glazing work for period sashes starts with a survey of what your rebates and pockets will genuinely take, and the wider sash window repair service covers everything from a snapped cord to a full overhaul across every London borough.