Sash Windows

Sash windows can be slid open at both the top and the bottom. This simple arrangement permits passive ventilation to occur (a form of natural air-conditioning).  It is a method of providing clean fresh air throughout the house that our ancestors in the Victorian era would have been all too aware of, but which has been forgotten down the ages.

The science

As any high school physics student will know, warm air is light and rises while cool air which is heavier sinks.

The top opening of a sash window allows the warm internal stale air to rise and escape.  This permits fresh cold air to be drawn in from the outside through the lower opening and, in turn, causes more of the internal warm stale air to be pushed upwards. The cycle perpetuates, providing an eco-friendly, cost free form of air-conditioning.

Sash window passive ventilation

Sash windows can be opened in a way that provides natural air-conditioning (passive ventilation)


A friend of mine who has recently had sash windows installed in his house told me that by using his windows in this way, the fresh air that was drawn into his rooms has drastically improved the air quality and helps him to breathe and sleep easier.

Sash or casement windows?

Modern windows mostly have single opening casements which are side or top hinged.  Although they can be opened relatively wide to seemingly give the impression of good ventilation, they cannot be used in the same way as traditional sash windows to purge stale warm air from a room.

Sash windows can be relatively expensive to fit and maintain. However, unlike modern air-conditioning units, they have no running costs, take up no extra space and are carbon neutral.

As I sit writing this article with my modern casement windows wide open but with no noticeable fresh air entering my room, I am beginning to appreciate just how well old buildings were designed – both aesthetically and from a practical point of view.

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