Double glazing sash windows london UK

Double Glazing Sash Windows

Can Wood Sash Windows Be Double Glazed?

For Over 20 Years We Have Been Double Glazing Sash Windows.

double glazing sash Windows Will improve energy efficiency and save on fuel bills

Double glazing traditional sash windows by retrofitting new timber sashes into your existing box window frames.

  • Our energy-efficient (Low-E), double glazed sliding sashes are custom made timber replicas of your existing sash windows. 
  • New sashes are retrofitted into your current sash window frames, in one day. 
  • Only the top and bottom glazed sash frames are replaced, leaving the original box window frame in place.  

Replacement sashes can make old windows look like new again, at a fraction of the cost of replacing period windows. 

The sliding sashes in a traditional sash window make up the biggest part of the visible exterior area.  Replacing them with our paint finished energy efficient replicas will transform your period home.  

The new glass will let more light into your room, whilst the Low-E coating reflects heat to keep your home warmer in winter and cooler in summer.  

The discreetly hidden draught seals are low friction to improve sliding functionality, but tight enough to stop rattles, ensuring a cosy and energy-efficient period home.


  • An energy efficient home is a comfortable home
  • Double glazing reduces your homes carbon footprint.
  • Double Glazed Units can reduce heating bills by around 20%.
  • Reduced UV prevents furnishings from fading
  • Reduced condensation on glass
  • Air borne noise, dust and other pollutants that are suspended within draughts are also greatly reduced.
  • Low friction draught proofing strips aid smooth window operation and stop annoying rattles


  • Survey of the existing frames.
  • Our standard Draught Seal & Restore service.
  • Bespoke replica timber sashes.
  • Balance the weight system.
  • Large selection of double glazed units.
  • A choice of paint or stained finishes.
  • A wide range of ironmongery in a choice of finishes.

Unaltered Appearance

We can replicate any heritage glazing pattern or intricate window shape, to ensure the original charm of your heritage home remains unaltered.

Long Life

Time-proven traditional construction, combined with a 50-year guarantee from Accoya wood, ensures that our double glazed sash windows will last a lifetime. 

Less Mess

Replacement sashes are paint finished and double glazed at our workshop, allowing us to completely finish double glazing your sash window in just one day.  

Contact Your Local Branch

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Low-e Double Glazing

Replica Double Glazed Sash Frames

Double Glazed Wooden Sash Window.

We carefully reproduce a copy of your original sliding sashes, paying close attention to the original horn shape, timber mouldings & sectional details.  Preserving these fine details, that vary by region and era, is important to maintain the architectural significance of your heritage property. 

If the original sash frames are missing then we can work from old photos or copy the architectural details from a neighbouring property.

By increasing the glazing rebate depth of the sash we accommodate a sealed double glazed unit with a 6mm – 16mm spacer bar.    With this increase in rebate depth, we must also offset the mortise and tenon sash joints to maintain the structural integrity of the sashes.  

The majority of the new sashes we install have the glazing bars bonded to the glass face of a single glazed unit.  Duplex bars are fitted inside the double glazed unit for support and improved aesthetics.

Alternatively, individual Slimlite double glazed units or Pilkington Spacia vacuum units can be chosen.  Production and installation of many individual glazed units is a more expensive option.


If your traditional sash windows have been converted to casement or louvre windows then we can reinstate them to their original double hung operation with our bespoke sashes.  See our Reinstate Double Hung Service.

Double Glazing Old Sash Windows

It is possible for us to retrofit slim double glazed units into some existing single-glazed sash windows.  The construction and condition of the existing sash frames are deciding factors to their suitability for conversion.  However, due to recent advances in Low-E glass technology we are now performing less of these conversions to period homes.  Instead, it is often advisable to fit single glazed Low-E laminated glass that offers comparable energy efficiency instead. 

This sash window double glazing conversion process may become popular again with the recent release of Pilkington Spacia in the UK.  Specialist double glazed units with a vacuum that are only the width of single glazing but offer the warmth of double glazing.  Currently, they are an expensive choice but historically new glazing technology drops in price as it becomes more widely adopted.

Retrofitting double glazing into existing sash windows remains a more viable option for the heavier duty sash window frames commonly found in public buildings and manor houses.  Often these windows are constructed from oak or mahogany with a natural finish that needs to be preserved.  Slimlite units are a good choice.

Counterbalancing Heavier Sashes

Sash Window Weights used to counterbalance double glazed sashes.

Additional weight is required to counterbalance the heavier double glazed timber sashes.  The original cast iron weights are replaced with lead weights; these make up a considerable part of the total service cost.

An important consideration is the additional weight that will be carried by the pulley.  Whilst the majority of British sash windows have sturdy axle pulleys, Australian sash windows often used cheaper pressed pulleys that may need to be replaced.  Our standard replacement sash cords are more than sufficient to carry the additional weight.

Energy Efficient Period Windows

Regardless of the temperature outside a window with good insulation properties will help to improve the comfort of your home.

Single glazing contributes to energy loss by allowing heat to conduct directly through the glass.  Double glazing overcomes energy conduction by introducing an air space between the two panes of glass.  Low-e technology further improves the insulation performance by actively reflecting heat back to its source.

Heat loss through single glazed windows is in excess of 25%.​

Draught proofed & double glazed sash windows offer significant savings on heating and cooling costs in a typical home. It is estimated that double glazing a period window can reduce the amount of heat transfer through a home’s windows by up to 40%. This ability to retain heat makes double glazing more energy efficient compared to single-pane windows.  Reduced heat conduction through your window glass means less energy is required to heat and cool your home, cutting energy bills.

Sash Window Weather Seals

Sash window weather stripping | Draft Seal.

Simply installing double glazing into sash windows will not make them energy efficient without also ensuring effective weather seals are fitted to stop draughts and air leakage. 

Draught-stripping period sash windows can reduce the draughts by almost 90%, and is an essential step to improving energy efficiency in period windows.  We have been using the same top quality British made draught seals for over 20 years.  Brush draught seals are the best option for long life in a sliding sash window.

Selecting Double Glazed Units

The thickness of your existing sash window frames dictates the room we have to work with in order to include the best energy-efficient double glazing.  

Spacer Bars

The spacer bar inside the glass unit governs the cavity between the two panes.  The wider the cavity the better the energy efficiency will be, but there is a law of diminishing returns.  The first 5mm of the cavity is responsible for the most significant insulation improvement but a 10mm void is not twice as good as 5mm.

12mm is the average thickness of the spacer bar that we use in new sashes, but with thin sashes or if you choose to incorporate a thick laminate or acoustic glass then we may use a thinner spacer bar.  A 16mm spacer bar is reserved for much thicker sashes, typically found in heritage public buildings.

Spacer bars can be either rigid aluminium duplex bars or ‘warm edge’ flexible foam spacer tape.  A warm-edge spacer bar performs slightly better at reducing heat conduction around the unit perimeter.

Double Glazed Timber Sash Windows With Castellated.Style Glazing Bars.
Double glazed unit with warm edge spacer bar. Castellated pattern.

Not all double glazing is created equal.  When double glazed units (dgu) first came to the market they simply comprised 2 panes of glass separated by an air space.  Whilst we can still supply these budget d.g.u. the saving over low-e units is not a sensible cost cutting measure.

Low-e double glazing outperforms those early double glazed units by a large margin.  Firstly the air space is now filled with an inert gas that conducts heat less readily.  Secondly, Low-E coatings baked into the glass surface reflect any heat energy back to its source, resulting in significant improvements to the insulating properties.  

Pilkington K glass is the best-known energy efficient, low-e glazing, but all glass manufacturers offer comparable products.

How Does Low-Emissivity Glass Work?

low e glass for period casement and sash windows

Low-e glass has a special coating that reflects heat back into the room.  In winter heat is reflected back indoors, in the summer heat from the
sun is reflected away keeping the room cooler. 

Because a Low-E coating reflects some wavelengths of the light spectrum it will slightly alter the colour tone of the light passing through the glass.  There are various Low-E coatings offered by different glass manufacturers that each have their own colour signature and performance characteristics.  Ask your local branch representative about the options on offer in your area.  


Argon vs. Other Inert Gas-Filled Units

A gas-filled unit features an inert gas in the cavity between the glass panes.  Using an inert gas that is denser than air stops circulation within the unit, which may lower your overall efficiency.  Argon is the most commonly selected and the best value.

  • Xenon – Xenon gas offers very high efficiency but does not warrant the cost.
  • Krypton – Krypton gas glazing offers up to 27% increased energy efficiency in comparison to air.
  • Argon – Argon typically reduces the insulating value by about 16% in comparison to air and is the most cost-effective and most widely used option.
Gas fillings should not lose more than 5% of the total gas over a period of 25 years.

slimlite Vs. Single Unit

Single Unit

Double Glazing Traditional Sash Windows. Berkshire UK.Using a single, sealed glazed unit per sash is cheaper for us to produce and supply. 

Timber beads are bonded to the exterior pane of glass mimicking the interior glazing bars that are mortice and tenon jointed to the outer sash frame.  Sealed inside the glass unit are matching spacer bars to fill the void, creating a solid glazing bar effect.  Internal spacers also brace the panes and reduce any flexing of the glass. 

Our bonded beads are custom made to the same thickness as your original sashes, ensuring the overall charm of the period property is preserved.

  • Bonded using high-performance glazing tapes
  • Shaped to represent the traditional putty lines of single glazing.
  • Intricate glazing patterns can economically be reproduced.
  • Timber beads can be painted in any colour. 

We have been offering this system for over 20 years, it is a very durable solution.  

Glazing this way allows us to recreate more intricate glazing patterns that are prohibitively expensive when using many separate, shaped glazing units.

Slimlite/Slimline Glazed units & Pilkington Spacia Units

Manchester Sash Window Repair SpecialistsFitting many individual, Slimlite or Spacia, double glazed units is desirable because it can add a little more character to the glazing of multi-paned frames.  Each unit will sit slightly out of level with its neighbour, as traditional single glazing does, eliminating much of the ‘flatness’ that a single pane unit provides.  Whilst some energy efficiency is sacrificed by selecting the slimlite 5mm spacer bar option, some people prefer the look of a thinner unit with slightly more timber visible.

Both producing and fitting individual glazed panels not only costs more but they are also more prone to misting up in the future.  The simple fact of having more cut edges of glass creates more potential weak spots where a double glazed unit could fail and subsequently mist up.

Slim units can be beneficial for double glazing existing sash frames.

For sashes with no glazing bars, a single unit is used anyway.

Our Double Glazing Options Include

  • A choice of inert gas fills.
  • Acoustic DGU configuration.
  • A choice of low-e, energy efficient glass.
  • Etched or patterned glass.
  • Safety Glass.
  • Slimlite units or duplex spacer bars.
  • 5 – 16 mm air space depending on the sash thickness.
logo.logo.Double Glazed Sash Windows 2
Historic Glass

Before choosing this service some consideration should be given to the benefits of your existing glazing.  Heritage window glass was hand made resulting in an uneven surface with flaws and distortions.  Historic glass adds to the character of a heritage window in a way that modern float glass can not match.

If your property has unique historic value or your existing sashes are still in good structural order then first consider if our Draught Seal & Overhaul Service might be a better, cheaper option. 

Read more in our article on window glass

Can old lead light windows be double glazed?

Leaded Light Encapsulation is possible where the existing leaded panel is sandwiched inside of a double glazed unit. 

First, it must be cleaned and repaired.  Secondly, the perimeter must be trimmed back approx 25mm on all edges, this loss of the pattern is unavoidable. Finally, it is encased inside a sealed glazing unit.  Gas fills or coated glass are not recommended.  The minimum thickness of the finished unit is 20mm (4 x 6 Y 6 x 4.) making this unsuitable for many windows.  The excessive weight requires these encapsulated sashes to be fixed shut in place.

Wood is a natural insulator

The natural insulating properties of timber make this an extremely energy efficient building material.  Timber window frames do not conduct heat like metal frames. 

Wood is a sustainable building material when derived from a renewable source.  Any home owner choosing green living should consider preserving or reinstating timber windows.  In order to grow one kg of wood the tree requires about 1.55 kg of carbon dioxide, extracted from the air.  A rule of thumb is that 1 cubic metre of wood stores 1 tonne of CO2.

Timber Selection

We offer bespoke joinery constructed from a range of premium timbers.  Accoya wood is the recommended option for double glazed sash windows, as no other timber has ever been able to provide a 50-year warranty against wood rot.

Read our Guide to Timber Type for more information.

What Is Accoya® Wood

Light weight and durable; Accoya is the perfect choice for double hung sash windows.

80 years of research resulted in Accoya, a high performance wood that exceed those of the best tropical hardwoods. Using nontoxic processes fast growing pine is structurally altered to produce a stable, very durable light weight timber.  The manufactures guarantee painted windows for 50 years against wood rot and even in its natural unpainted state for 30 years. If the paintwork is maintained in good order, Accoya wooden windows will theoretically last ‘forever’.

Whilst Accoya is an expensive wood, we do not mark up our prices on this product like some sash window companies.  We charge you only the difference that it costs us to buy each plank of wood.  That way we can ensure that this excellent product remains an affordable option for every customer.

Read the Accoya website FAQ

accoya wood logo.

Window Security & Hardware

Sash window security locks.Various catches effectively bring the window sashes into a locked position, however, the screws holding these in place are only small and these catches should not be considered an effective security measure. We have a range of sash window security options available. 

See our range of sash window security and furniture, in a variety of finishes.

Heritage Preserved