Storm windows are windows that are mounted outside or inside of the main glass windows of a house. They can be mounted externally or internally; can be made of glass, rigid plastic panels, or flexible plastic sheets; and may be permanently or temporarily mounted. They function similarly to insulated glazing. Aside from insulation, storm windows provide an additional measure of protection for homes against damages to costly glass panes during inclement weather.

These windows are a typical energy upgrade solution for cold climates. Storm windows do not make sense in buildings with energy-efficient windows, or in hot or moderate climates but are mostly intended to improve the insulation value (U-value) of existing windows, especially single-glazed units.