Choosing New Windows
New windows are rated by their capacity to retain heat in winter and block solar heat in summer. The heat-retention rating is called U-factor, which is usually decimal that is less than one (between 0.50 and 0.20). The lower the U-factor (smaller number), the better the window insulates during the heating season. The sun-blocking rating is called Solar Heat Gain Coefficient (SHGC), which is also a decimal less than one. The lower the SHGC, the better the new window blocks the sun in summer. These ratings are listed on the National Fenestration Rating Council (NFRC) label found on better windows.
If you expect to save energy by replacing your existing windows with new windows, the new windows must have significantly lower U-factor and SHGC than your existing ones. In general, replacing single-pane windows with single-pane windows or double-panes with other double-panes won’t save any energy.
Replacement windows commonly cost between $30 to $70 per square foot of window area, installed. Vinyl and aluminum-clad wood windows dominate the window market. Vinyl window frames have good thermal resistance, low cost, and no maintenance. However vinyl windows are generally less durable than aluminum-clad wood windows. Aluminum-clad wood windows have excellent life span and low maintenance but are significantly more expensive than vinyl.
Single-pane glass starts at a U-factor of 1.1. A U-factor of 0.40 is considered the maximum U-factor (heat loss) for an energy-efficient window for cold climates. Achieving a U-factor of between 0.40 and 0.20 requires advanced window features, including low-e glass, double-pane glass, Argon-gas filling between the panes, and an insulating spacer between the panes.
A special new low-e coating blocks most solar heat, while admitting most visible light. This innovation is being widely employed by window buyers in the South where air conditioning is a major expense. The heat-blocking low-e glass is sold under the brand names: Sungate 2 and Low-e2.
For more information on choosing new windows see The Homeowners Handbook to Energy Efficiency.