Reduce your carbon foot print with sash window draught proofing this winter and next summer. Sash windows have been around since the thirteenth century. Often they are found in older homes that were built before the development of modern day insulation materials. The word sash refers to the wood casing that surrounds the glass. A single sash window is actually made up of two windows that slide horizontally or vertically to be in an open position. These windows are often characterized with each sash have two rows of three panes of glass, that is, six small panes of glass form the single sash.