U-Factor is probably the most important efficiency measurement for windows. Just as R-value is a measure of thermal resistance in walls, U-factor gives indication of heat retention. Unlike R-value, which increases with thermal resistance, the lower U-factor the better for cold weather climates. Typically, a U-factor of 0.30 or lower is a good choice of window. For warm weather climates, a low U-factor is not needed, but would not hurt. If your home needs to use the air conditioner in the summer a lower U-factor window would reduce cool air losses.
In addition to U-value, windows are available in single, double, and even triple pane. In most climates, but especially in cold climate, the more panes the better. In most climates the difference in savings from changing to double pane windows from single pane windows saves you roughly twice as much on the bill. The Department of Energy has published single pane/double pane savings by region of the USA.
|USA Region||Single Pane||Double Pane|
|West North Central||$145||$353|
|East North Central||$160||$372|
|East South Central||$88||$195|
|West South Central||$159||$273|
Still another factor in choosing an Energy Star window is the amount of solar gain provided by the window. In the winter in cold weather states when the sun is low in the southern sky, you want as much sunlight to penetrate the home as possible. This helps heat the house and reduces the amount of work your furnace needs to do to keep the house warm. Solar Heat Gain Coefficient (SHGC) is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The higher the number the better for cold climates. However, warmest climates may want a little more sun deflection.
Here's an example specification of a sliding glass window from Lowes.
Tags: energy star windows