Blocking Air Passages – The Three General Types of Windows
While the entirety of a window’s structure and component materials contribute to its overall U-factor and solar heat gain coefficient, the specific design of the window itself pretty much determines the air leakage rate of the window. Windows fall into three general types, with variations within the general classification forming different subtypes based on how the window opens and can be manipulated:
Fixed windows have glass panes that do not open. These types of windows are the most airtight (i.e., have the lowest air leakage rates); consequently, they also provide little or no ventilation. A picture window is a large fixed window with no grilles or bars, providing an unimpeded view of the area beyond; they are so called because the lack of window pane dividers makes the view look as if it were part of a picture frame.
If air circulation is important in the area where the window is being installed, then a fixed window is most likely not the best choice of window type for that area.
Sliding windows have a window pane or panes on rails that slide to open the window. Single-hung windows open vertically, and have one pane, typically the lower one, that can be slid up to let in the air; in double-hung windows, both panes are mounted on rails and can be slid open. Similarly, single-sliding windows have one pane that slides open horizontally, while with double-sliding windows either pane can be slid open.
Of the three general window types, sliding-type windows provide the most air circulation when opened. They also have the highest air leakage rates of the three window types. They can also be difficult to open or close, especially for older homeowners.
As the name implies, hinged windows have a hinge that allows the window to open by swinging outwards; the different types of hinged windows are classified according to the location of the hinge. Awning windows are hinged at the top, hopper windows have the hinge at the bottom, and casement windows are hinged on one side. Hinged windows have more air leakage than fixed windows, but not as much as sliding windows.
Stay tuned for Part 3 to learn about how the window’s frames and how they effect energy efficiency.