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Conservation of Windows
This article explores the development of windows in historic buildings, and some of the conservation implications of their repair and replacement.
Windows have enormous inflence on the character and appearance of historic buildings. The ratio of window openings to wall areas, their proportions, the patterns made by glazing bars and the reflections of historic glass are often significant. Since window designs have changed with the times, they can also provide important clues to dates. The decision as to whether to repair or replace windows can have implications both for the appearance of buildings and for their historic value.
History
Many ancient buildings were originally equipped with small 'wind-eyes', openings with wooden shutters rather than glass, but these were soon replaced by wooden windows with moulded glazing bars. Cast glass were already being used by the Romans in Scotland around 80AD, but during the Middle Ages many windows were glazed with thin sheets of marble or horn, or oiled linen or paper stretched on timber frames.
These were not durable and in due course were replaced by glazing using small rectangular squares of glass, still known by their French-derived name as 'quarries', held in H-shaped lead 'cames', wired on to horizontal 'saddle bars' and fitted snugly into grooves in the stone reveals. The varying textures and colours, and the slope of the individual quarries, which cause them to glitter as they catch the light, lend special interest and beauty to medieval windows. Medieval stained glass is one of the glories of European architecture, the earliest examples being from the Saxon monastery of Monkwearmouth in Northumberland, founded around 674. Later medieval opening lights were usually wrought iron casements with large ornamental catches and stays, containing leaded panes. Despite their beauty, the glazed area was restricted by the small panes, which could work loose over time, rattling and letting in water, while the metal frames corroded letting in drafts.
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Sash windows
Timber horizontal sliding windows, known as 'Yorkshire' sashes, were a logical development of the medieval casements, especially for the ranges of windows required to light weavers' cottages. Properly maintained, timber windows could last indefinitely, were easy to manufacture, repair and replace, and warmer to the touch than the old metal sashes.
Windows with a vertical sliding sash in the lower half were only invented in the mid 17th century, the word 'sash' deriving from French: châssis, referring to the sliding frame. Coinciding with the rebuilding that followed the Great Fire of London and the works of Sir Christopher Wren, the sash window suited Classical proportions; the counterbalanced double-hung sash developed before 1701 rapidly became Britain's main window type throughout the eighteenth century, though in Europe the hinged side-hung casement remained dominant.
The sliding sash and case window was also found to be versatile, capable of being adapted to a wide range of architectural styles, and by the mid 18th century the characteristic 6 over 6-pane glazing pattern was usual. Initially the glazing bars were broad Grecian sections usually run in softwood and in Scotland the original name of the moulding used: 'astragal', still means a glazing bar. As the century progressed however, a range of more refined mouldings were developed including Roman 'lambs tongue' mouldings and Gothic styles, as the bars became more and more slender and delicate, and the panes of glass larger and larger, until by the late 18th century the bars could be made of hardwood or have metal inserts or 'feathers' to strengthen them, or even be entirely made of metal.
Window tax (1696-1861), The London Building Acts (1709 and 1774), all influenced the appearance of windows and led to replacements, but the Georgian sliding sash remained common until 1950. Many are still in excellent working order even after 300 years' use, though early softwood sashes were not treated with timber preservative (first available 1810), so that regular painting is essential to prevent rot. As the designs developed, windows were repeatedly replaced or adapted to reflect the latest fashion
Window or 'flat' glass
The thick cast glass used in ancient buildings, and 'muff' or cylinder glass, blown as a tube then split and flattened out, was replaced in most Georgian buildings with 'crown' glass, manufactured by blowing and spinning a thin disk of molten glass. Crown glass can be found in windows dating from the end of the middle ages, but glass duty (1746-1845), levied on the weight of glass used, helped to popularise the thinner, lighter and clearer panes, with their 'music lines', 'bow' and 'fire sparkle', which still give many 18th century buildings a uniquely attractive appearance.
As well as the need for highly skilled craftsmen to make it, crown is very fragile and like all glass tends to become brittle with age. It was also limited in size. The invention of sheet glass (1838) and steam polished plate glass (1851) made glass available in ever larger sheets, reducing the cost of frames and eliminating the visual obstruction of glazing bars. Many sashes were replaced or had their glazing bars removed. Despite this, crown glass survived in Britain until at least the end of the 19th century, simply because of its appearance.
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20th Century replacements
The early 20th century saw a resurgence of metal casement windows, with steel windows with slender glazing bars being viewed as more suited to the modern age of industrial prefabrication and large horizontal panes reflecting modernist aesthetics, although they were also produced with leaded panes for use in traditional buildings. Initially, steel windows were not galvanised and early examples are particularly subject to rusting, jamming, warping and failure, although much can be done to repair and draught strip them. The horizontal format resulted in many traditional buildings being spoiled by alteration of the window proportions, although in modernist buildings the ribbon window and the picture window are entirely appropriate as windows of their time.
When timber windows reappeared in the domestic architecture of garden cities and 'homes fit for heroes' more influenced by the arts and crafts style, after the two World Wars, the use of cheap, fast grown low quality softwood without preservatives led to rot at the joints and failures which gave wooden windows a bad name. Although the problems were soon overcome by vacuum preservative treatment, sadly, many people even today still view softwood windows with suspicion, and this prejudice has been strengthened by plastic window manufacturers' marketing.
The use of tropical hardwood replacement windows is often proposed as a more durable alternative to softwood but can be environmentally irresponsible, unless the wood is sourced from proven sustainable forest logging. The narrow, internal glazing bars, or 'stick on' bars used on many hardwood windows also fail to replicate the varied reflections of historic glazing.
Some modern buildings have been fitted with anodised aluminium windows but the production of aluminium is a high energy process that contributes to global warming and pollution. In fact, softwod is are still the most versatile, sustainable window material available, and wooden windows have proven service lives in excess of 300 years.
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Plastic windows
By contrast, UPVC replacement windows are the ultimate industrial product, requiring massive energy input in manufacture and producing large amounts of harmful pollution in manufacture, as by products of the cadmium and lead stabilisers, and disposal, when dioxins are released. The European Parliament has actually passed legislation to phase them out. In the meantime their success is mainly due to intensive marketing which claims they are maintenance free. The reason they have spoiled the appearance of so many conservation areas in Britian is because the material is inherently weaker than timber, so that the sections need to be thick and clumsy, imparting a 'toytown' appearance to traditional buildings.
Far from being maintenance free, the Britsh Plastics Federation freely admits that they require maintenance. They can be degraded by pollution and UV light, leading to discolouration, powdering, cracking and eventual failure unless they are painted or replaced, and they are not easily repaired: even replacing a broken pane can require return to the factory. The Building Research Establishment suggests a service life for plastic replacement windows of a mere 35 years. In practice, many need replacing in half this time.
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Consents
On listed buildings and properties in conservation areas, consents are likely to be required for replacing windows.
No consents are required for repairing traditional timber windows, except in some churches and Scheduled Ancient Monuments. In fact, it is seldom necessary to completely replace historic timber windows and often the perception that replacements are required is based on insufficient knowledge of the options.
Repairing traditional timber windows
The Society for the Protection of Ancient Buildings and other conservation bodies publish detailed guidance on the repair of historic timber windows, such as replacing rotten timber by piecing-in matching wood. Broken glass can be replaced with panes which closely resemble the quality of the original Crown or plate glass, available from specialist suppliers, and draught strips and discreet secondary double glazing systems can be added to match or exceed the performance of modern high performance windows.
Stained glass and metal framed windows can also be repaired and upgraded by specialists, and closely matching epoxy coated steel sections can usually be found to replace heavily rusted early 20th century windows that are beyond repair. The additional cost of repair over cheap replacement windows will usually be more than offset by the longer life of the repaired windows and the added value to the property.
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Occasionally however, historic timber windows that have been neglected or damaged beyond repair may require replacing and then the approach should always be to replace 'like with like'. Timber from managed forests is a low embodied energy, sustainable material and its use for replacing timber windows will be environmentally responsible and will also encourage traditional craftsmanship, keeping costs down. Original mouldings should be recorded and matched, and ironmongery and sometimes even glass can be saved and reused. Government policy encourages the use of sustainable, low embodied energy materials in building, and local councils should also encourage the use of traditional materials and local trades in line with English Heritage and Historic Scotland guidance.
Windows deserve to be treated with special care as an important part of the character and appearance of historic buildings. We have carried out a number of surveys of historic windows and specified repairs and replacements which have helped to prolong the life of the windows without unnecessary destruction and loss of character. Sometimes this has led to the discovery of important historic features that might otherwise have been overlooked and lost, for example where historic glass or window mouldings survive. Window repairs should always be based on a 'minimum intervention' approach so that buildings retain their historic interest and appearance, and their value is preserved and enhanced.
References
Looking After Your Sash and Case Windows: A Short Guide for Homeowners, Historic Scotland 2003 (www.historic-scotland.gov.uk/sashcase.pdf)
Maintaining Sash and Case Windows, Historic Scotland, 2007 (www.historic-scotland.gov.uk/inform sash and case.pdf)
The Conservation of Timber Sash and Case Windows: Guide for Practitioners 3. Historic Scotland 2002.
Thermal Performance of traditional Windows. Technical Paper 1. Historic Scotland 2008.
ROBIN KENT
NOTE | This article is copyright. No responsibility is accepted for errors or omissions. It provides pointers to general principles and should not be viewed as a comprehensive guide. Each historic building will need separate consideration.