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 sliding sash.

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 easily.

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 areas.

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 making.