The Heritage Directory

                                                                                                                                 The Heritage Directory has provided the following articles as an informative guide to aspects of the historic environment. The articles are grouped into the following areas : General & Legal, Architectural History, Interiors, Exteriors, Gardens, Preventative and Remedial and Professionals and Contractors.  


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Sash Windows

Sash windows are one of the most common forms of window found on historic buildings, particularly those of the 18th and 19th century. They are made from vertically sliding sashes, usually in an opening with a vertical emphasis.

  French sash 1630s

French sash windows in a mid 1630’s engraving. The word sash is from the French, chassis, meaning frame.

History

Sash windows were introduced from France in the mid 17th century following the Restoration, but the problem with the French sash was that the size of the low sash was limited by its weight. In the late 1660’s, the sash pulley system was invented so the lower sash could now always be equal in size to the upper sash. The ease with which the new version of these windows opened and stayed in place, together with their classical symmetry ensured their popularity in England. The first recorded sash pulley system was made by Thomas Kinward, Master Joiner in the Office of Works when Sir Christopher Wren was Surveyor of the King's Works,  but the designer may have been Wren’s colleague, Robert Hooke, the scientist and inventor, who had experimented in pulley systems. 

Pane, Window and Glazing Bar Size

An important element of the French sash window was the use of glass set in the timber glazing bars. Before their introduction, the use of leaded lights had been universal. Whilst the pattern initially varied, the vertical emphasis of the pane size used at Hampton Court in the 1690’s was widely adopted, which resulted in the standard pattern for smaller houses of 12 pane sash windows, or six over six, three panes wide to 4 panes high. Pane size was based on the Golden Section at a ratio of 5 parts wide to 8 parts high, which was devised in ancient Greece as a harmonious proportion based on the human body. Occasionally the structural opening was also based on the Golden Section but proportions based on the cube were also important, sashes most typically being one and three quarters parts high to one part wide. Glazing bars gradually reduced in width to less than 15 mm in the 19th century. By the mid 19th century the use of plate glass meant that sash windows were often just divided into four panes, and eventually two. 

Construction

The weight of the sash is counterbalanced by a lead or cast iron weight within the outer frame or box. This weight is connected to the window by a waxed sash cord with a pulley at the top of the frame. Spring balances are sometimes used on modern sashes, but are not generally acceptable in listed buildings.  

Position of the Window and Reveals 

The position of the sash within the open is important. On most houses early sashes had their box frames exposed and finished with an architrave surround, which was set flush with the outer face of the wall. This style was common until the mid 18th century and was revived in the early 20th century. The London 1709 Building Act required the sash box to be set back 4 fours inches and the 1774 Act required the sash box to be hidden as well, behind the reveal. This was ostentiously for fire protection, but driven by the move from the English baroque love of timberwork to the austerity of Palladian design.

 sash window history

The four above windows show the most common window forms in the evolution of the sash.  Top left is the flush exposed box sash window in use until the early 18th century. Top right shows an early 18th century sash with the 4 inch set back required by the 1709 London Building Act but with the sash box still fully exposed. It also has the segmental arch which was fashionable at that time. Bottom left shows the sash after the 1774 London Building Act with the sash box hidden behind the reveal. This was to prevent the sash falling out in a fire. Bottom right shows a Victorian sash using plate glass which became popular form the 1840’s, with less panes and with sash horns.

Paint 

Externally sashes windows have generally been painted white, and rarely were different colours were used until the late 19th century. White was an off white, rather than the pure brilliant white of today. 

Horns   

In the 19th century, projections appeared on the ends of the sides of the windows, know as horns, began to appear, to strengthen the joints. Visually it is important to avoid the use of horns where they have not been used historically on a building, as this is a common mistake. They can look very odd when used on a Georgian house.  

Position of the Window and Reveals   

The position of the sash within the open is important. On most houses early sashes had their box frames exposed and finished with an architrave surround, which was set flush with the outer face of the wall. This style was common until the mid 18th century and was revived in the early 20th century. The London 1709 Building Act required the sash box to be set back 4 fours inches and the 1774 Act required the sash box to be hidden as well, behind the reveal. This was ostentiously for fire protection, but driven by the move from the English baroque love of timberwork to the austerity of Palladian design.  

Replacement Sashes and Double Glazing   

Sash windows can be double glazed, though this is usually unacceptable on listed buildings. Where replacing sash windows on listed buildings and also in Conservation Areas, always check with your local Conservation Officer first as to what is acceptable. Replacement windows also need to be in accordance with Part L of the Buildings Regulations. This allows flexibility in historic locations but it is important to obtain the Buildings Regulations approval first, following discussions with your Building Control and Conservation Officers. FENSA registered joiners do not require Building Regulation applications as they work through a certification scheme, but it is important to contact your local Conservation Officer to ensure that that the proposed work is acceptable. In the first instance retention and refurbishment of the existing sashes is often required as they form an important part of the history of the building. The timber of historic sash windows also can be very durable as it was sourced from slow growing timber with a fine grain.  

Common Problems 

Existing windows can be upgraded by refurbishment and draughtproofing. Problems include decay due to rot, sashes stuck due to overpainting or distortion and broken sash cords. The cill is the area most prone to decay. Ensuring the paint is important in preventing decay. Properly seasoned and sourced new timber can be spliced in where existing is decayed and severely rotten cills is the most common replacement. Sash cords should never be painted.  

Security    

To improve security sash locks can be fitted to meeting rails. Sash bolts can also be fitted to provide ventilation with restricted opening.  

Glass   

Original glass should be retained as the variation in the glass adds to the character of the window and there are a number of manufacturers of old style glass.  

Energy efficiency   

Draughtproofing windows is a relatively cheap way of making your windows more thermally efficient. Secondary glazing is also preferred to double glazing by conservation bodies and is good for noise and thermal insulation.  Thermal curtains are also available, and recent studies have shown close fitted blinds can improve efficency. Where they exist, internal timber shutters can help reduce heat loss.