A window sash is a pane of glass contained in a rigid frame, usually wood, vinyl or aluminum. Single and double hung windows are made up of two window sashes installed into the window frame.
A window sash can either contain single, double or triple pane glazing options within their frame. More glazing installed will translate into a higher insulation factor for the sash. The sash can also contain mullions, which divide the sash into equal “lites”. The mullions can either be superimposed on the glass, or installed in between panes.
Wood window sashes are the most common type, although they are known to exhibit warping and cracking over time. To counter this condition, manufacturers began to use aluminum and now vinyl sashes. These materials are lightweight yet offer increased strength and stability over wood. Yet despite their drawbacks, wood continues to be the predominant choice for constructing window sashes.