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 pieces 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 with Argon, but if your sashes are narrow then it is better to use Krypton. 

Fitting IGUs in traditional wooden sash windows brings a maintenance program as all IGU's can, over several years, fail as the gas escapes leading to misty IGUs.  This will happen in all windows (uPVC and wooden). IGU's can easily be replaced when they fail.

Fitting IGU's into a Victorian style 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 and the meeting rails as narrow as 21 mm. 

To produce a double glazed copy of a Georgian 6/6 sash requires that we install glass IGUs that weigh over 150% more than the original design tolerances.   This commonly results in building sashes with marginally larger timber sections (25 mm glazing bars) therefore slightly changing the appearance of the window. One frequently used 'work around' is to make a sash frame without the glazing bars, install a single IGU and then surface mount the glazing bars onto the glass to give the appearance of the Georgian sash. 

Owners of pre 1919 housing often ask if double glazing is the right thing for their homes. Our response is that whilst it is true that to upgrade your windows requires the sacrifice of old hand produced glass and the loss of durable heartwood timber, often derived from 200 year old virgin forests, we need to set this against the cost of living and climate change making energy efficiency more essential as time moves on. Upgrading to double glazing can be done sensitively and in ways that retain the visual appearance and function of the original windows.

That said in some instances, double glazing is simply not appropriate and in most listed buildings you will not be given permission to double glaze the sash windows.

Double Glazing Service

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

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