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A broken pane, a failed double glazed unit, or an upgrade from single to double glazing — we replace glass in all types of timber sash windows across London. Here is everything you need to know about the glass options available and which is right for your window.
Replacing glass in a timber sash window is not the same as replacing glass in a modern casement. The box sash frame has a fixed rebate depth — the groove that holds the glass — typically 10–14mm on Victorian and Edwardian windows. This limits how thick a unit can be and therefore which glazing options are available.
There is also a weight consideration unique to sash windows. The counterbalance weights inside the box frame are sized to offset the weight of the original glass. Replacing 4mm single glazing with a heavier unit — such as laminate or double glazed glass — increases the weight of the sash considerably. In some cases, replacement weights are needed to restore correct balance, which adds to the cost and complexity of the job.
This guide covers the glass types we fit and the practical differences between them, so you can make an informed decision before requesting a quote.
Standard 4mm float glass is the original specification for the vast majority of Victorian and Edwardian sash windows. It is still the correct choice in several situations:
4mm Float Glass
Standard clear glass. Like-for-like replacement for original panes. Lightest option — no weight adjustment needed.
4mm Toughened Glass
Required by Building Regulations in critical locations — low-level panes, doors, and areas near stairs. Breaks into small blunt fragments rather than sharp shards.
4mm Obscured Glass
Satin or acid-etched finish for privacy. Common in bathrooms, side returns, and ground floor windows overlooking public areas.
Laminate glass consists of two or more sheets of glass bonded together with a plastic interlayer (PVB). If broken, the interlayer holds the fragments in place — making it significantly safer than standard glass and much harder to force through.
6.4mm Safety Laminate
Two sheets of 3mm glass with a PVB interlayer. Meets BS EN 12600 safety standards. Popular for ground floor and vulnerable windows.
Acoustic Laminate Glass
A thicker acoustic PVB interlayer significantly reduces sound transmission. Available in 6.8mm and 8.8mm combinations. Excellent for London streets without requiring double glazing.
Weight note: Replacing 4mm float glass with 6.4mm laminate adds approximately 3kg per square metre to the sash. On a typical Victorian upper sash this can mean an additional 2–3kg, which may require weight adjustment to maintain smooth balanced operation.
A double glazed unit (DGU) consists of two panes of glass separated by a spacer bar, with the cavity filled with argon or dry air. The key challenge with sash windows is fitting a double glazed unit into a rebate designed for a single pane of glass.
Standard double glazed units are too thick for most original sash frames. Slimline double glazed units have been engineered specifically for heritage timber windows and are what we fit at Golden Ratio.
| Unit specification | Total thickness | Suitable for sash? | Best for |
|---|---|---|---|
| 4–6–4 slimline | 14mm | ✓ Yes | Heritage sash windows, listed buildings with consent, shallow rebates |
| 4–8–4 low-e argon | 16mm | ✓ Yes | Best all-round thermal performance in a sash-compatible thickness |
| 4–12–4 standard | 20mm | ✗ Rarely | Modern windows with deep rebates — usually too thick for period sashes |
| 4–16–4 standard | 24mm | ✗ No | New-build windows only — will not fit original sash frames |
4–6–4 Slimline Unit (14mm)
The thinnest double glazed unit available. Fits original Victorian rebates without modification. Good thermal improvement over single glazing. The first choice for heritage properties with shallow-rebated frames.
4–8–4 Low-E Argon (16mm)
The most commonly fitted unit in period sash windows. The low-emissivity coating reflects heat back into the room. Argon-filled cavity adds further insulation. Fits most sash frames with rebates of 16mm or more.
Acoustic DGU Options
Acoustic double glazed units use different glass thicknesses on each pane (e.g. 6mm + 4mm) to break the resonance frequency that allows sound to pass through. Significantly better noise reduction than standard DGUs of the same overall thickness.
Weight note: A 4–8–4 unit weighs approximately twice as much as a single 4mm pane. On most period sash windows, the counterbalance weights will need to be supplemented or replaced to maintain smooth operation after double glazing is fitted. We assess this during the free site survey.
A double glazed unit is made up of three components: two panes of glass and the spacer bar between them. The total thickness is the sum of all three. For example:
For sash windows, the rebate depth limits the maximum unit thickness that can be fitted. Most original Victorian and Edwardian sash frames have rebates of 10–16mm, meaning a 14–16mm slimline unit is the maximum that can be accommodated. We measure the rebate during survey to confirm which specification is achievable in your window.
Glass replacement on a listed building requires listed building consent if the nature of the work changes the character of the window. Replacing a broken single pane with like-for-like glass generally does not require consent. Installing double glazing in a listed building usually does — though in some cases consent is granted where slimline units are used that are not visible from outside.
In conservation areas, double glazing is not automatically restricted, but permitted development rights may be limited. We always advise confirming with your local planning authority before proceeding.
Where double glazing is not permitted, acoustic laminate glass is often the best alternative — it dramatically reduces noise without changing the appearance of the window from the outside, and typically does not require consent.
Pricing depends on the glass type, pane size, number of panes, and whether weight adjustment is needed. The following gives a general indication.
Single Pane Replacement
From £80
4mm float or toughened glass. Like-for-like replacement.
Slimline Double Glazing
From £350
Per sash. Includes rebate preparation and weight assessment.
Acoustic Laminate
From £150
Per pane. Better noise reduction than standard DGU in same thickness.
All prices are indicative. For a full breakdown see our sash window price guide. We provide a free written quotation after a no-obligation site survey.
Yes — using slimline double glazed units of 14–16mm total thickness. These are engineered specifically for shallow-rebated heritage frames and retain the single-pane appearance of the original window. Not all sash frames can accommodate even slimline units without rebate modification, which we assess during the free site survey. Read more on our double glazed sash windows page.
The thinnest widely available double glazed unit is the 4–6–4 slimline specification, which measures 14mm in total. This is the minimum thickness at which a meaningful insulating cavity can be achieved. Units thinner than 14mm are available but offer very limited thermal benefit and are rarely used in practice.
The most common specification fitted in new-build properties is 4–16–4, which is 24mm in total. For comparison, most original sash window rebates are only 10–14mm deep, which means a standard double glazed unit will not fit. Slimline units at 14–16mm are specifically manufactured for heritage applications such as timber sash windows.
For maximum sound reduction in a sash window, acoustic laminate glass or an acoustic double glazed unit with an asymmetric specification (e.g. 6mm + 4mm panes) outperforms standard double glazing of the same thickness. The acoustic interlayer or asymmetric pane combination disrupts the resonance frequencies that allow sound to pass through. See our soundproofing sash windows page for a full comparison.
No — a misted or cloudy double glazed unit means the hermetic seal has failed and moisture has entered the cavity. The unit cannot be repaired and must be replaced. The good news is that the frame itself does not need to be replaced — we remove the failed unit and fit a new one to the existing sash. This is a straightforward job and significantly cheaper than replacing the whole window.
Quite possibly, yes. The counterbalance weights inside a box sash frame are sized to match the weight of the original glass. A slimline double glazed unit is heavier than single glass — a 4–8–4 unit weighs roughly double a 4mm pane of the same size. If the weights are not increased to match, the sash will be harder to lift and may not stay open reliably. We assess this during the site survey and advise on whether weight supplementation is needed.
Yes — we work on listed buildings regularly. Like-for-like single pane replacement does not require listed building consent. Upgrading to double glazing on a listed building does require consent in most cases, though consent is sometimes granted for slimline units. We can advise on the correct approach for your property. See our period property window repairs page.
We carry out glass replacement across all of Greater London. Free site survey and written quotation — no call-out fee, no obligation.
Our full double glazing retrofit service for timber sash windows across London.
Acoustic glass options and draught proofing for noise reduction in London properties.
Full pricing guide for all sash window repairs, glazing, and restoration services.
Here at Golden Ratio Windows we have built up an excellent reputation in our local area by offering the most comprehensive and expert service in London. From the planning stage to the finished article; we are with you every step of the way, making sure that whatever the task, you are completely satisfied with the end result.
Get a Free QuoteWith so many years working in the trade, we are happy to answer any queries and provide quotes for any work that you are looking to have done. Rest assured that you will be receiving a professional, yet personal service. If you are in need of any work in London or the surrounding areas, get in touch.
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