Traditional sash windows work using a counter balance system that holds the sashes in desired open position. There are generally two types of balance mechanisms for timber sash windows, a traditional weights and pulley mechanism and a spiral balance system, sometimes referred to as a sprung sash. We generally recommmend the traditional weights and pulley system over the sprung system due to it performing much better in terms on ease of operation, smoother sliding sashes and longevity. The design of a weighted box sash window hasn’t changed much over the years although with the advent of double glazing being introduced, the box frame that houses the weights has increased in size ever so slightly to be able to accomodate larger weights to be able to balance the heavier double glazed units. Old sash windows were usually hung on waxed cotton cords but we use nylon cords these days as they don’t degrade over time like cotton does. A spiral balanced sash windows works by hanging the sashes on a pre tensioned spring that supports the sashes under tension. One of the reasons we would usually recommend a weighted window over a sprung window is because the springs come with a 5 year guarantee and can be prone to failing because of the high tension the springs are under, especially when used for large sashes with acoustic glass which weighs considerably more than standard double glazing units.