The oldest strategy for improving a window’s energy performance—used by Thomas Jefferson more than 200 years ago—is to add storm windows on the outside of the existing windows. A fairly recent development is the availability of low-emissivity (low-e) storm windows, which have special transparent coatings that reflect heat back into the house.
Exterior storm windows can be fixed, requiring seasonal installation and removal; triple-track, which have moveable upper and lower sashes, along with a moveable screen; and double-track, in which only the lower sash and screen are operable—significantly reducing the crack length and air leakage, compared with triple-track. Most exterior storm windows are aluminum; higher-quality ones are available with a durable anodized finish. Fixed storm windows are often made of wood.