Sash Window Double Glazing
Traditionally, the structure of the sash window has not allowed the fitting of double glazing while retaining the delicate appearance of the sash detail. The double glazed units were simply too thick.
But in the last ten years or so different types of double glazing have come to the market.
These include slender/slim units that vary from 12mm down to just 7mm in thickness. The narrow gap between the panes in these units can have a Vacuum (in the narrowest of units) and a Krypton gas or similar in the slightly wider units.
Double glazed units can now be made fine enough to fit in to the sashes that require a traditional style glazing bar. The U-value of these units can vary from 1.1 to 2.5. This allows the more traditionally detailed sashes to successfully accommodate these new type of double glazed units.
New sash windows can be made with this in mind. Essentially the weight of the average sash increases by between 30% and 50% by introducing double glazing and so the weights must increase by the same ratio.
Where a property is under a more strict conservation listing the existing glass may well have to be kept in place for future generations.