Double Glazing
for Sash Windows
Upgrading your timber sash windows to double glazing is one of the most effective improvements you can make to a London period home. You get better thermal performance, less noise, and reduced draughts — without losing the character of your original windows. At AMB Joinery, we specialise in double glazing for sash windows across London, using slim sealed units that preserve the traditional profile and sightlines your property deserves.
Whether your sash windows are in good structural condition or in need of a full replacement, we can advise on the most appropriate glazing upgrade for your situation.
Why Double Glaze Your Sash Windows?
Traditional single-pane sash windows account for a significant proportion of heat loss in London period homes. A single 4mm pane of glass has a U-value of around 5.8 W/m²K — roughly seven times less efficient than a modern double-glazed unit. Upgrading to double glazing typically reduces this to between 1.0 and 1.8 W/m²K depending on the specification.
Beyond heat retention, the benefits are practical:
- Noise reduction — particularly relevant in London, where road traffic, railway lines and flight paths make acoustic performance a real priority for many homeowners in Hackney, Lambeth, Hounslow and beyond
- Reduced condensation on internal glass surfaces — though external condensation on a well-performing unit is entirely normal and not a cause for concern
- Improved security — modern double-glazed units are harder to break quietly than single panes
- Lower energy bills — meaningful savings for houses with many sash windows
How We Upgrade Sash Windows to Double Glazing
Slim Sealed Units in Existing Frames
In many cases, we can upgrade your glazing without replacing the entire window. If your sash frames and cased boxwork are in sound condition, we remove the existing single-pane glass and fit slim sealed units in its place.
Standard timber sash windows have a glazing rebate designed for a single 4mm pane. Modern slim sealed units — typically 12mm total thickness (4mm glass / 4mm cavity / 4mm glass) — fit within the original rebate with only minor adjustment to the rebate depth or the addition of slim glazing beads. For the slimmest possible profile, vacuum glazing units are available at just 6mm total thickness, requiring no rebate modification at all. The result is a double-glazed window that looks virtually identical to the original from the street.
In many cases, we can upgrade your glazing without replacing the entire window. If your sash frames and cased boxwork are in sound condition, we remove the existing single-pane glass and fit slim sealed units in its place.
Standard timber sash windows have a glazing rebate designed for a single 4mm pane. Modern slim sealed units — typically 12mm total thickness (4mm glass / 4mm cavity / 4mm glass) — fit within the original rebate with only minor adjustment to the rebate depth or the addition of slim glazing beads. For the slimmest possible profile, vacuum glazing units are available at just 6mm total thickness, requiring no rebate modification at all. The result is a double-glazed window that looks virtually identical to the original from the street.
Replacing Sash Panels Only
Where the frames and box casing are sound but the existing sashes themselves are warped, damaged or beyond economic repair, we can manufacture new sash panels to the original dimensions and glaze them with double-glazed units from the outset. The box frame, pulleys, weights and architraves all remain in place.
This approach is cost-effective, faster than a full replacement, and keeps the amount of disruption to a minimum.
Where the frames and box casing are sound but the existing sashes themselves are warped, damaged or beyond economic repair, we can manufacture new sash panels to the original dimensions and glaze them with double-glazed units from the outset. The box frame, pulleys, weights and architraves all remain in place.
This approach is cost-effective, faster than a full replacement, and keeps the amount of disruption to a minimum.
Full New Sash Windows with Double Glazing
When the entire window — casing, frame and sashes — is beyond repair, we manufacture a complete replacement unit and fit it with double glazing from the start. All our new sash windows are made to measure in our London workshop. We match the existing moulding profiles, horn details and glazing bar configurations precisely.
When the entire window — casing, frame and sashes — is beyond repair, we manufacture a complete replacement unit and fit it with double glazing from the start. All our new sash windows are made to measure in our London workshop. We match the existing moulding profiles, horn details and glazing bar configurations precisely.
Glazing Options
Standard Double Glazing
Our standard sealed unit is an argon-filled 4-4-4 or 4-6-4 configuration with a low-emissivity (low-e) coating on the inner pane. This gives a centre-pane U-value of around 1.4–1.6 W/m²K — a substantial improvement on single glazing. It is the most cost-effective option for the majority of London sash windows.
Acoustic Double Glazing
For homes on busy roads, near railway lines or under flight paths, we recommend acoustic laminated glazing. This uses a laminated inner pane — a layer of PVB film bonded between two sheets of glass — which dampens sound transmission significantly. An acoustic unit can reduce ambient noise by 35–40 dB in many situations. If your home is in Hammersmith, Ealing, Islington, or anywhere near a major route, acoustic glazing is often worth the additional investment.
Fineo Vacuum Glazing
For properties in conservation areas, or where listed building consent imposes strict requirements on profile depth, Fineo vacuum glazing is the premium solution. A Fineo unit is approximately 6.7mm total thickness — thinner than most single-pane sash windows, which are typically glazed with 4mm glass. It achieves a centre-pane U-value of around 0.7 W/m²K, outperforming standard double glazing at a fraction of the thickness. This allows double glazing in conservation areas where a standard sealed unit would visibly alter the profile of the window.
Will Double Glazing Affect My Planning Permission?
In most cases, upgrading the glazing within existing frames is treated as like-for-like maintenance and does not require planning permission. If the external appearance of the window does not change — same profile, same dimensions, same colour — it will generally fall within permitted development rights.
If your property is a listed building, any alteration to the windows requires Listed Building Consent from your local authority, regardless of how minor. If your property is in a conservation area subject to an Article 4 Direction, you may need to apply for planning permission even for replacement windows. We can advise you on the requirements for your specific property before any work begins.
FENSA Certification
All window replacement work we carry out — including work where existing frames are retained but sashes are replaced — is registered with FENSA. You will receive a FENSA certificate on completion, which you will need if you sell your home. Find out more about FENSA compliance and what the certificate covers.
Frequently Asked Questions
Can sash windows really be double glazed without losing the traditional look?
Yes. Slim sealed units are designed specifically for traditional timber sash windows and can be fitted within the original rebate depth with minimal visible change to the profile. On a well-executed installation, the difference from the street is essentially undetectable.
Yes. Slim sealed units are designed specifically for traditional timber sash windows and can be fitted within the original rebate depth with minimal visible change to the profile. On a well-executed installation, the difference from the street is essentially undetectable.
What is the best double glazing for a noisy London street?
Acoustic laminated glazing is the most effective option for noise reduction. The laminated pane breaks up soundwaves in a way that a standard float glass unit cannot. For properties near Heathrow flight paths or major roads, the improvement in comfort is often very noticeable.
Acoustic laminated glazing is the most effective option for noise reduction. The laminated pane breaks up soundwaves in a way that a standard float glass unit cannot. For properties near Heathrow flight paths or major roads, the improvement in comfort is often very noticeable.
Do I need planning permission to double glaze my sash windows?
Usually not, if you are upgrading the glazing within existing frames. If you are replacing the entire window in a listed building or conservation area with an Article 4 Direction, you will need consent. We will advise you on your specific situation as part of the free survey.
Usually not, if you are upgrading the glazing within existing frames. If you are replacing the entire window in a listed building or conservation area with an Article 4 Direction, you will need consent. We will advise you on your specific situation as part of the free survey.
How much does it cost to double glaze sash windows in London?
Costs depend on the number of windows, current condition, glazing specification and access. We provide detailed written quotes following a free site survey. As a rough guide, upgrading glazing within existing frames is considerably less expensive than full window replacement.
Costs depend on the number of windows, current condition, glazing specification and access. We provide detailed written quotes following a free site survey. As a rough guide, upgrading glazing within existing frames is considerably less expensive than full window replacement.
Will double glazing stop condensation on my sash windows?
Double glazing dramatically reduces condensation on the internal glass surface, because the inner pane stays much warmer than a single-pane window. If you are seeing condensation between the panes of an existing sealed unit, that unit has failed and needs replacing — contact us to arrange a survey.
Double glazing dramatically reduces condensation on the internal glass surface, because the inner pane stays much warmer than a single-pane window. If you are seeing condensation between the panes of an existing sealed unit, that unit has failed and needs replacing — contact us to arrange a survey.
Get a Free Quote
Ready to upgrade your sash windows to double glazing? Contact us to arrange a free, no-obligation survey. We work across London and typically respond to enquiries within 24 hours.
