Double glazing and sash windows
Before considering the development of Double Glazed Sash Windows it would be helpful
to understand what is meant by the term 'Sash' window.
A 'Sash window' is made up of two or more moveable panels or 'sashes' that form
a frame to hold panels of glass separated from each other by glazing bars. The sash
window may open vertically or horizontally in which case it is referred to as a
Sash windows first came into use in the late 17th century when the heavy glass was
contained by thick oak frames and glazing bars. From the 18th century onwards, cylinder
glass was produced which was thinner and available in larger pieces, and this allowed
the production of the elegant, more lightweight sashes of the Georgian period. Often
part of a formal, classically inspired elevation, sash windows provided a completely
different appearance to the more mundane leaded casements of the previous centuries
and quickly became highly fashionable. After about 1850 the introduction of plate
glass allowed sash windows to be much larger with only one or maybe two panes of
glass to each sash, so 'horns' had to be added to strengthen the junction of the
meeting rail with the frame.
The sash offered many advantages, being better suited to the wet British climate.
The sash could be closed down to a narrow gap, allowing for good ventilation whilst
reducing the chance of rain entering. Sashes are less susceptible to distortion
and rot than a hinged casement adding greatly to their life span. From a design
point of view the sash was constructed from delicate sections of wood, with comparatively
large areas of glass, as such they added a certain grace, even when open they do
not detract from the façade, as an open casement does. It is not surprising that
because of these benefits particularly their attractiveness that they were favoured
by architects and builders through the ages.
The sash window has continued to remain popular if for no other reason than it is
seen as the most attractive window in its setting. Modern developments have sought
to preserve the look of the window whilst adding benefits to it such as incorporating
not only 'Double Glazing' but features which improve the ability to clean them more
So how does the sash work. In the traditional sash window imagine two frames sitting
in front of each other, each referred to as a sash. It is possible to slide one
up or down in front of the other, the two being separated by what is referred to
as a parting strip. This ability to raise the sash is brought about by a series
of pulleys and weights which are embedded within the frame of the window. These
slide up and down in a similar way to the movement of the sashes. With today's improvements
the pulleys and weights have been replaced by a series of tension loaded springs
set into the frame. When they are both closed there is a turning lock situated on
the top of the bottom frame which enables them to be locked together in order to
seal them, provide a tight fit and avoid any draughts.
A question often asked is how does a modern sash window compares with a traditional
sash one. There is no doubt that some modern 'sash' windows fail to replicate the
detailing of the historic originals, although from a cursory glance they may appear
similar. It is possible however to obtain modern windows that are very difficult
to tell apart from the traditional window. Modern sash windows can be made in wood
frames or in upvc which can replicate the original materials.
The cords on the original sash window although capable of lasting many years did
need replacing when they broke requiring he removal of the sash to replace the cord.
One of the criticisms of a traditional sash window is the difficulty of cleaning
the outer surface. This is particularly true of windows above the ground floor.
Those responsible for cleaning windows often express difficulty in reaching all
parts of the outer surfaces from within the building. Modern Double glazed windows
have recognised that difficulty and are now provided in a form known as Tilt and
Turn windows which can be opened in two ways – turn the handle once so that the
window opens at the top to allow ventilation; turn the handle again so that the
window opens inwards on side hinges to allow easy access to the outside surface
of the glass.
In addition to the above benefits the modern Sash Double glazed window is much more
secure than a traditional one. Locking devices on the window frame are less likely
to give under pressure of forced entry than a traditional window.
Where a listed building or conservation area is in operation, the regulations imposed
by the planning Authority are likely to be far more stringent. If a building has
sash windows fitted the following advice is relevant. Usually, like-for-like repairs
to a listed building do not require Listed Building Consent but you may be asked
to submit details of your proposed repairs to your sash windows so that the appropriate
Planning authority can be sure of what you intend to do. For more extensive work,
including the total replacement of any window on a listed building, Listed Building
Consent will almost certainly be required and detailed drawings may be asked for
to ensure that the detailing of the new windows is precisely right. The company
supplying the window can sometimes provide these drawings.
For all unlisted historic buildings, particularly in conservation areas, Councils
often encourage owners to repair their windows using matching materials and reinstating
the original details. Further, more stringent controls may apply in some conservation
Where sash windows have been fitted previously it is as well to bear in mind that
they have perhaps been fitted to maintain the character of the building. It would
be wise before proceeding to make changes to ensure that the authority in which
the building is situated do not have objection to the changes you are considering