During the recent heatwave, as temperatures rose due to climate change, how many people turned on fans, turned up the air conditioning or had an ice cream to cool down?

For the last 50 years or so double-glazed hinged casement windows, mostly made from UPVC, have been installed to keep houses warm and reduce consumption of energy by keeping heat in.

But as our climate changes, pandemics affect our lives, and we recognise the need for good ventilation and better air quality, can we learn from more traditional and less energy-demanding means of keeping cool in our homes?

The Great Fire of London saw some 13,200 homes, 87 churches and 52 livery company halls (or guildhalls as you might know them in York) destroyed.

The Government passed an Act for rebuilding the City of London in 1667 which attracted craftsmen from across Britain and Europe – especially the Netherlands and France. This was not a simple act of rebuilding but acted as a period of intense innovation spurred on by innovators, scientists and revolutionary thinkers.

One of the clauses of the Act of Parliament was that windows were to have sills and they must be four inches (10 cm) deep to mitigate against any future fires spreading so easily.

Joiners, carpenters and glass producers experimented with developing new types of windows to adapt to these new requirements and to modernize and to improve the environmental conditions within buildings. Fixed and hinged opening windows were the norm before the Great Fire but a new innovation appeared - the sliding sash window.

Sliding sash windows aesthetically looked good within the new deeper apertures; they could be opened easily and were highly effective in ventilating rooms. However, their design requires a high degree of precision in their manufacture and they are best made in workshops rather than on site. But they do lend themselves to prefabrication and mass production.

Through advocates like Sir Christopher Wren, who used them extensively in his rebuilding of Royal Palaces, London soon became the manufacturing base for a growing national and international market.

The nobility soon placed orders for these modern sash windows for their great country houses. By the 1680s some speculative housing schemes were being provided with ready-made sash windows. In 1701 a merchant in Boston, Massachusetts, imported sash-windows fully assembled and glazed from London as local craftsmen were not able to construct them.

Improvements were made to the early designs with ‘buttons to keep the sashes from rattling... putting pieces on the outside to keep out the weather’ and terms appeared in the English language with sash windows needing to ‘eased’ and ‘hung’.

The sash windows of Hampton Court - of which there were many from the early 1700s -became the model for many private builders whose projects were to spread the sash-window throughout Britain.

John Cossins produced a map of York in 1748 with all the notable houses of the day depicted at the side.

Every single window is seemingly a sash window – with Mr Thompson of Micklegate having 27 sash windows facing onto the street alone.

Wooden sash windows were fashionable but they were also immensely effective in ventilating spaces by removing air-borne pollutants and heat.

Opening either sliding pane allows an immediate intake of fresh air far more quickly than opening hinged casement windows.

The real genius of sash windows is in opening both panes at the same time so that cool clean fresh air can enter at the bottom and warm polluted air can exit at the top. Recent research by the University of Cambridge engineers has shown that the natural ventilation flow rate of air through a sash window – a 17th century innovation - can be up to four times more effective.

Unlike hinged casement windows which can actually trap stale air, moisture and spores in the top half of a room, sash windows typically exceed four complete room air changes, every hour.

Smart building systems are now using programmable sash windows to regulate natural ventilation to address pressing modern challenges - our changing climate and removal of airborne contaminants.

Using 360-year-old technology could be an answer to keeping cool and well-ventilated in the years ahead – so ease your wooden sash windows, don’t replace them – and celebrate with an ice cream!

Andrew Morrison is the chief executive of York Civic Trust

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