Sash windows are a very old design and have been known to go back as far as the early 1660s. The earliest in England being in the 1670s, is Ham House which was and still is located beside the River Thames in the area of Ham near London. This building is now owned by the National Trust. These particular windows all have one thing in common and that is the sash window weights which are used to raise and lower the window are hidden from view.

These weights, said to have been invented by Robert Hooke an inventor and scientist who lived in England, are a necessary part of the design of these types of windows. Before the actual window is put into its fitting, there has to be an allowance made at either side of the frame for the window weights to fit and to be able to slide up and down.

The sash window weights are made from lead, cast iron or heavy steel and once installed act as a counter balance when the window sash is raised or lowered.  They usually hang from a cord or chain and this is positioned on a pulley at the top of the frame on the same side as the window, so that as it is lifted the weight drops down and holds the window up. The reverse of this happens when the window is physically lowered by hand with the weight inside the framework being raised. The sash weight at this stage is not heavy enough to re lift the window by itself.

The weights are hidden from view once the window is installed and they slide up and down in the allowance area at the side. One set is used when only the bottom sash is able to open or close, this allows for half of the bottom window area to be open as the top one is fixed in place.

When there is a hung or double sash window this refers to both sashes being able to move. The upper window is able to be pulled down or lowered so that the top half on the upper section is open, which allows ventilation through this area of the window and the bottom is raised up and lowered as usual. This requires an extra set of sash window weights to be attached inside the framework of the upper section, exactly the same as the lower section but on the opposite side where the upper window is fitted.

With sash windows these may have a few problems which are caused by these weights, such as wear which may cause a rattling effect in windy weather and sticking after the application of a coat of paint. There may also be swelling of the wood during wet weather which does not allow the weights to either lift or lower the window sash.

The double sash windows are however great for ventilation and cooling of a room on a hot day as the upper and lower sections can be positioned halfway to allow hot air