Casement windows are considered to be the most energy efficient type of window because of their amazing insulating properties, multi-point locking system and low-E coated glass. Their strong seal allows for maximum airtightness with the sash pressed tightly against the frame. To further increase their energy efficiency, double-pane and triple-pane casement windows are filled with argon gas, which is less conductive than normal air and thus reduces cold or hot air escaping through the windows.