Windows are the most inefficient part of the building envelope and a primary cause of thermal discomfort.
A single pane window loses 20 times as much heat as the same area in an adjacent well insulated wall, and a double-glazed window loses ten times as much. In reality, windows are thermal sink holes.
While the amount of heat loss or heat gain through windows depends on whether the windows are single pane glass, clear double pane, Low-E coated, or gas filled, the fact is if you have windows you have heat gain and or heat loss every day of every year.
Control of solar gain in the summer and thermal heat loss in the winter is not only necessary in current highly glazed, poorly insulated buildings, but is critical in the design of new energy efficient commercial buildings.