Double Glazed Sash Windows

Double Glazing Sash Windows

Can wood sash windows be double glazed?

For over 20 years we have been double glazing sash windows.

Double glazed timber sash windows for period homes.

Double Glazing with low-e timber sashes | Winchester UK
Georgian type double glazed sash windows

Double glazed timber sash windows will improve the energy efficiency of period homes.  We manufacture & install replica wooden sliding sashes into your existing timber sash window frames.  Only the 2 glazed sash frames that slide up and down are replaced, leaving the original window frame in place.

Our replacement sashes are paint finished and double glazed with energy efficient low-E glass at our workshop, allowing us to completely finish a double glazed sash window in just one day.  Accoya wood, with it’s 50 year guarantee, ensures that our double glazed sash windows will last a lifetime. 

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

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

Double Glazed Castellated Sash | London UK
Double glazed timber sash windows with a castellated pattern.

Replica Double Glazed Sash Frames

We carefully reproduce a copy of the original sliding sashes, paying close attention to the original horn shape, timber mouldings & sectional details.  Preserving these details, that vary by region, is important to preserve 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 can accommodate a sealed double glazed unit.  With increased rebate depth we must also offset the mortise and tenon sash joints to maintain the structural strength of the sash.  

Glazing bars are bonded to the glass face, with duplex bars inside the double glazed unit for support. Alternatively  individual Slimlite sealed double glazing units can be fitted, although this is a more expensive option.


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 pulleys in British sash windows are sturdy axle pulleys, occasionally some may require replacing.  Many Australian sash windows used cheaper hardware that frequently need to be replaced, at additional cost.  Our standard replacement sash cords are more than sufficient to carry the additional weight.

Sash Replacement Service includes:

  • Survey of the existing frames.
  • Our standard Draught Seal & Restore service.
  • Bespoke replica timber sashes.
  • Balancing the weighting mechanism, to ensure ease of operation.
  • Large selection of double glazed units.
  • A choice of paint or stained finishes.
  • A wide range of ironmongery in a choice of finishes.
Double glazing sash windows in Surrey UK.
Double glazing sash windows

Benefits of our double glazed sash windows:

  • An energy efficient home is a comfortable home
  • Double glazing reduces your homes carbon footprint.
  • Double Glazed Units should 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

Double Glazing Old Sash Windows

It is possible for us to retrofit double glazing 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.

Otherwise 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.

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 has further improved 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.

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 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 paintwork is maintained in good order Accoya 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.

Double Glazed Timber Sash Windows With Castellated.Style Glazing Bars.
Double Glazing for sash windows with a castellated pattern.

Selecting Double Glazed Units

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.  Low-e double glazing far out performs those early double glazed units.  Firstly the air space is now filled with an inert glass that conducts heat less readily.  Secondly low-e coatings baked into the glass surface reflect heat energy back to its source, resulting in significant improvements to the insulating properties.  

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 1

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 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.  


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. 

Window Security & Hardware

Sash window security locks.
Double glazed sash windows with security stops

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. 

We also supply a wide range of sash window furniture, available in variety of finishes.

Sash Window Specialist - Double Glazing.
Double glazed sash windows with low-e glazing