Secondary Glazing vs Double Glazing for Sash Windows

You have single-glazed sashes, a bedroom that never quite warms up, and a bus route that wakes you at six. One firm has quoted to take the sashes out and reglaze them with sealed units. Another says leave the windows alone entirely and fit secondary glazing inside. The quotes are thousands apart and both promise the same things.

They are not the same job and they do not fix the same problem. Almost everything that follows comes down to the size of the air gap, and how honestly the person quoting describes it.

Two pieces of kit doing two different jobs

A double-glazed unit is a sealed sandwich. Two panes bonded either side of a spacer bar, the cavity filled with dry air or an inert gas, the perimeter hermetically sealed. It goes into the sash, replacing the old glass, or into a brand new sash made to take it. In a conventional unit of the sort that goes into a modern casement, the cavity is around 12mm to 16mm and the whole sandwich is 20mm to 28mm thick.

Nothing of that depth goes into a rebate cut for 3mm of Georgian glass. So slim-profile units are built to a different scale entirely. In the Historic Scotland study that measured ten of them retrofitted into real windows, the build-ups ran from 3-3-3 to 4-8-4. That is an overall depth of 9mm to 16mm, with cavities of only 3mm to 8mm. Eight millimetres was the widest cavity in the whole set.

So be clear which number a salesman is quoting you. An "11mm unit" is a 3mm pane, a 4mm cavity and a 4mm pane. The gap is 4mm, and it is the gap, not the sandwich, that does the insulating. The one exception is vacuum glazing, built 4-0.2-4, where the cavity is a fraction of a millimetre but evacuated, so there is nothing left in it to carry heat by convection at all.

Secondary glazing is a separate window fitted on the room side of the original. The old sash stays where it is, untouched. The new pane sits anywhere from 50mm to 200mm inboard, usually in a slim aluminium frame fixed to the reveal or to a timber sub-frame. Crucially there is no seal between the two layers. The cavity is left open to the room, deliberately, so moisture can escape rather than sit on the cold outer glass.

That distinction drives everything else. A sealed unit works by suppressing convection in a thin, dry, still cavity — it is a thermal device that happens to do a little for noise. Secondary glazing works by putting a large, structurally unconnected air space between two sheets of glass — it is an acoustic device that happens to do a lot for heat.

The heat figures, measured rather than modelled

The useful numbers on traditional windows come from Glasgow Caledonian University, which measured real sash windows in real buildings for both English Heritage and Historic Scotland, rather than modelling a pane in isolation.

Lower is better. The first rows are in-situ measurements of single-glazed sashes and secondary systems. The last two are the calculated whole-window figures from the Historic Scotland slim-profile study, in which the single-glazed control worked out at 5.2 for the Georgian window and 5.1 for the Victorian one — so read those two rows against 5.2, not against 4.8.

Two things jump out of the bottom half of that table. The first is how wide the slim-profile range is, and how much of it sits at the wrong end. The 1.9 in the Georgian window and the 1.4 in the Victorian one were achieved by a single product, the vacuum unit, which is a different technology at a different price. The ordinary gas-filled units, the ones most firms actually offer, landed between 2.5 and 3.0 in the Georgian sash and between 2.0 and 2.8 in the Victorian one. The only air-filled unit tested was worst of all at 3.4 and 3.0.

The second is that secondary glazing held its own against all of them. In the same calculations a single-glazed sash with secondary glazing came out at 2.0 in the Georgian window and 2.1 in the Victorian — better than every gas-filled slim-profile unit in the Georgian sash, and beaten only by the vacuum glazing and, by a tenth of a point, one xenon-krypton unit in the Victorian.

The reason is not really the glass. Measured at the centre of the pane, those slim units ran from 1.0 to 2.8, most of them close to 2.0, against 5.4 for a single sheet. The glass does its job. The losses happen at the edges: a slim unit needs a slim edge seal, which conducts, and a six-over-six sash carries an enormous length of glazing bar and edge-of-glass relative to its glass area. That thermal bridging is exactly why the same units perform better in a Victorian one-over-one than in a Georgian six-over-six.

Cavity width matters too, and not in the direction you would guess. The optimum gap depends on what is in it: about 16mm for air, 15mm for argon, 11mm for krypton and 8mm for xenon. A 3mm cavity is too narrow for any of them to work properly. The one 8mm unit in the study, the widest cavity of the lot, was filled with argon, a gas that wants nearly twice that gap, so it never got the benefit of its own dimension. Historic Scotland's conclusion was blunt: 8mm with krypton or a xenon-krypton mix is about the optimum, but the units on sale are not designed that way, because slimness is bought at the expense of thermal performance. There is more on how these units are specified and glazed in our guide to slim-profile units for period sashes, and if the numbers themselves are unfamiliar, what a U-value means in a period house is worth ten minutes.

In an occupied house the draughts matter more than the glass, which is why a well-executed draught-proofing job on an untouched sash frequently makes a cold-feeling room comfortable for a tenth of the price of reglazing.

Noise is where secondary glazing wins outright

This is the part where the two options are not close.

Sound reduction in glass is quoted as a weighted index, Rw, in decibels. A single 4mm pane sits at roughly 29 dB. Ordinary sealed double glazing sits at roughly 29 to 33 dB. That is a barely perceptible improvement, and homeowners who paid five figures for quiet are regularly baffled by it.

The reason is coupling. Two panes of similar thickness, held rigidly a short distance apart in a sealed unit, behave like a single vibrating diaphragm at certain frequencies. Around the mass-spring-mass resonance the assembly transmits sound better than one thick pane would, and a narrow cavity puts that resonance uncomfortably close to the frequencies of traffic rumble.

Secondary glazing sidesteps the problem by decoupling. The two panes sit in separate frames, mechanically independent, with 100mm or more of unsealed air between them. Systems built with a 100mm to 200mm cavity and a 6.4mm acoustic laminate on the secondary pane test in the mid-40s dB and above. As a rough perceptual guide, a 10 dB reduction is heard as roughly halving the loudness.

Three things degrade real-world acoustic performance and no brochure mentions them:

• The cavity depth you can actually achieve. A shallow Victorian reveal with a plastered return may only give you 60mm. That is a serious downgrade from 150mm.
• Flanking sound. Noise arriving through the chimney, the letterbox, the floor voids or an uninsulated single-brick spandrel panel below the window will not be touched by any glazing.
• The original sash itself. Secondary glazing works with the primary window, not instead of it. A rattling sash with perished putty leaks sound straight into the cavity. Overhaul first.

If you want the practical version, covering achievable cavity depths, glass specification and the ventilation you still need, it is set out in our article on cutting traffic noise through sash windows.

What each route costs in London

Prices swing hard on access, condition and borough. Take these as the shape of the market rather than a quote.

Two cost points people miss. First, retrofitting a sealed unit into an original sash almost never happens on its own. The rebates have to be deepened, sometimes the glazing bars have to be remade entirely, and the sash weights have to be increased to rebalance for the extra glass. That is why we price double glazing for sash windows as part of a restoration rather than as a glass swap, and our room-by-room breakdown of restoration costs covers what the surrounding work adds.

Second, VAT. Draught stripping for windows and doors currently sits at the zero rate for domestic installations, running to 31 March 2027, after which it reverts to 5%. Secondary glazing and double glazing are explicitly excluded from that relief and are standard-rated at 20%. Across a whole house that is real money, and it is one more argument for doing the sealing work properly and separately.

Consent often settles the argument before cost does

Approved Document L treats a replacement window in an existing dwelling as a controlled fitting, and sets the standard at a whole-window U-value of 1.4 W/m²K or a window energy rating of Band B. Repairing an existing window is not a controlled fitting, and neither is replacing the glazing in a frame you are keeping, so overhauling sashes, replacing cords, splicing in new timber and draught-proofing sit outside it entirely. Where meeting the full standard would unacceptably alter the character or appearance of a historic building, the guidance allows a lesser standard, and it names low-emissivity secondary glazing over retained single glazing as an acceptable route.

Secondary glazing is generally the easier proposition in a listed building because it is reversible and does not alter the historic fabric, provided it is fixed sensitively and does not damage shutters, panelling or the original frames. Conservation officers vary, but the direction of travel is consistent, and we go into the detail in secondary glazing for listed buildings. If your house is in a conservation area with an Article 4 direction, the picture is different again and replacement windows may need planning permission where they would not elsewhere.

Never take a glazing firm's word on consent. Ring the conservation officer at your borough, describe the proposal, and get the answer in writing.

Working out which one your house needs

Work down this list and stop at the first line that describes you.

1. Is the room noisy rather than cold? Secondary glazing, the deepest cavity the reveal allows, acoustic laminate on the inner pane. Nothing else comes close.
2. Is the house listed? Start with repair, draught-proofing and secondary glazing. Sealed units in original sashes need consent and are frequently refused.
3. Are the sashes rattling, sticking or cord-broken? Fix the mechanism first. Half the "cold window" complaints that reach our sash window repair service across London are draught complaints wearing a disguise, and a sash that will not close onto its beads cannot be made warm by any glass.
4. Are the sashes genuinely beyond repair — soft bottom rails, failed joints, split stiles? Then new double-glazed sashes in the retained box frame become sensible, and you are buying joinery as much as glass.
5. Do you want the original glass kept? That rules out reglazing. Old cylinder and drawn glass cannot be built into a sealed unit without losing what makes it worth keeping.
6. Is the window on a bathroom or a north wall with black spotting round the edges? Deal with the moisture as well. Cold glass is only half the story of condensation on the inside of the pane.

The questions that separate a good quote from a bad one

Get two surveys and insist both surveyors open every sash rather than working from photographs. Then put these questions, and ask for the answers written on the quote rather than said at the door.

To the secondary glazing firm:

• What cavity depth can you achieve at each opening, measured rather than assumed?
• How are you fixing to the reveal, and what happens to the shutters and the architrave?
• What is the glass — 4mm float, low-E, or 6.4mm acoustic laminate?
• How do I get at the outer sash to clean it and repaint it?

To the double glazing firm:

• What is the whole-window U-value, not the centre-pane figure?
• What is the cavity width, what gas is in it, and what is the overall unit thickness?
• How much timber comes out of the rebate, and do the glazing bars survive it?
• What are you doing about the sash weights once the glass gets heavier?
• What is the guarantee on the edge seal, and who honours it in twelve years' time?

A firm that answers all of that in writing has done the work before. And if the honest answer at the end of it is that the windows need overhauling before any glazing decision makes sense, that is the ordinary outcome rather than a disappointing one. It is usually the cheapest useful thing anyone can do to a cold, noisy, rattling sash.