simply sash: double glazing

Double Glazing Your Sashes

We are often asked if it is possible to double glaze existing sash windows and generally the answer is yes, but please read on.

Things you need to note:

Insulated Glazing Units (IGU's) this is the correct name for the glass unit in a doubled glazed sash. They normally consist of two peices of 4 mm thick clear or obscured glass bound together with glazing tape with a 'spacer' to hold the glass apart.

The IGU is then filled with an inert gas, which effectively does all the work of stopping the heat transferring through the unit. Most IGU's will work okay with Argon which is a cheap and plentiful gas, but if your sashes are narrow and the gap between the glass is very small, less than 8 mm,  then it is better to use Krypton.  Krypton is harder to extract from the atmosphere and therefore considerably more expensive - but not to be skipped if the unit requires it to function at the u-value you require.

Fitting IGUs in traditional wooden sash windows brings a maintenance program as it is not possible to drain the sash in the way that uPVC windows are drained and, therefore, the tape that seals the IGU can sometime fail and the gas escapes leading to misty IGUs.  This will happen in all windows (uPVC and wooden), just potentially a bit more often in wooden frames. IGU's can be replaced when they fail.

Fitting IGU's in double glazed sashes on a Victorian single pane sash is relatively straight forward, installing them in multiple paned sashes such as the classic Georgian 6/6 is more difficult.  Essentially the glazing bars (the wood that divides and supports the glass panes) in a Georgian or Regency 6/6 were made as slim as 16 mm - it was a matter of pride for the carpenters to achieve this finesse - and the meeting rails as narrow as 21 mm.  When it became possible to produce large glass sheets the Victorians opted to remove glazing bars to let in more light, but recognised that the sash was going to be heavier as a result of the thicker glass and compensated by adding horns (the swirly bits to the top of the sash) to help strengthen the joints at the meeting rail. 

To produce a double glazed copy of a Georgian 6/6 sash that is unlikely to have horns, requires that we install glass IGUs that weight over 150% more than what the sash was originally designed for!  This commonly results in building sashes with larger timber sections and therefore changing the appearance of the window - this is why modern uPVC sash windows don’t look very much like the original product.  One frequently used 'work around' is to make a sash frame without the glazing bars, install a single IGU and then surface mount (stick with sticky tape) the glazing bars to give the appearance of the Georgian sash.  This practice, although effective in achieving the improved thermal performance, is generally frowned upon by Conservation Departments as it is not in anyway reflective of the original work.. 

With all these issues is double glazing the right solution?

When thinking about double glazing, we need to keep in mind the historic nature of our properties.  The original glass is likely to have been hand blown in a relatively local workshop using traditional knowledge and skills.  The timber for the sash is probably imported pine from the Baltics cut from virgin forest up to 200 years old and the heartwood separated from the sapwood to give you the best possible durability - which is why so many old windows still exist.  Once that glass and the wood is gone, it is gone forever

Perhaps the most compelling argument for double glazing sash windows is climate change and the need to look at the energy efficiency of the housing stock, especially pre - 1919 dwellings.  However recent research has shown that the carbon cost of installing potentially short life modern timbers (typical lifespan of a modern pine up to 25 - 35 years if regularly painted) and IGUs that make up the replacement sashes, may not actually represent a massive carbon reduction compared to wearing more clothes indoors, fitting blinds or shutters and curtains to windows and generally insulating our houses, its floors, walls, roofs etc,. where we can.  Secondary glazing, although universally unpopular, is without doubt one of the most effective thermal improvement options that we could employ.

In many instances, double glazing is simply not appropriate. Indeed in most listed buildings you will not be given permission to double glaze the sash windows.

The key message is, if you are going to double glaze your sash windows to retain the elegant style and appearance that make them so unique and special, then make sure you do so using the right materials, the right IGU's and a professional installation service to maximise their service life.

Double Glazing Service

We can either replace your sashes in the existing box or supply complete new windows. The former starts from around £1075 plus VAT to include a pair of copied sashes with IGUs to match what you have, with weights and installation. Any repairs to the original sash box are additional. Alternatively we can supply and fit complete new double glazed and draft proofed timber sash windows, made to measure and installed from around £1750.99 per window using vacuum treated softwood with hardwood sills and beadings or from £2500.00 using Accoya which has a greater durability and extended painting intervals. These are starting prices for windows up to 1.65 m tall.

Installation starts at around £200 per window but is priced separately and depends what is involved, access etc. Once the remainder of the frames have been brought up to gloss by your decorator, your sash window will look like new. All figures plus VAT.

See terms and conditionsterms and conditions for service details and pricing.

We will do our best to give you all the information you need to make an informed decision.
If you have any questions, please contact us

See our glossary  page for an explanation of sash window parts and terms.

Simply Sash Windows

Restoration - servicing - draught proofing

Page Theme