Every window or glass door has numeric energy efficiency ratings assigned to it by oversight agencies. The best-known of these is the ENERGY STAR program run by the US EPA and the Department of Energy and verified by the National Fenestration Rating Council (NFRC). Windows are tested and rated according to two measures: The U-Factor and the Solar Heat Gain Coefficient (SHGC). The U-Factor tells you how well the window keeps heat indoors. The ratings range from 0.20 to 1.20. The lower the number the better, especially in Northern Climates.The SHGC describes how well the window keeps heat out. It ranges from 0 to 1, with lower numbers indicating higher efficiency. It is more important in Southern climates. So regionality plus the U-Factor and SHGC determine if a window earns an ENERGY STAR sticker. The NFRC rating label is perhaps a more instructive guide for choosing a window because it reveals the U-Factor and NFRC metrics, along with a couple of others, which allows you to make one-to-one comparisons of the energy efficiency of windows you are considering.

When considering materials for the window frames, jambs, and trim, there are four main categories of materials from which to choose. In some cases, you can choose to have two materials—wood for the interior and aluminum clad exterior, for example. Each material has unique characteristics, from design elements to performance features. To select the right material for you, consider factors like the climate you live in, the style of your home, and aesthetic preferences. When it comes to wood species, the most common is pine. It may be used on window interiors and exteriors, although it is sometimes covered, or clad, on the exterior with extruded aluminum or fiberglass cladding that makes the outside virtually maintenance-free. Wood is also an efficient insulator. Douglas Fir may be substituted for normal pine to provide a strong vertical wood grain.

You’ll find a dozen or more general types of windows offered by manufacturers, but most windows installed in homes fall into six basic categories:
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Double Hung Windows
Consisting of two sash that slide up and down in tracks, double hung windows are very familiar and very popular. They offer excellent ventilation that can be adjusted according to which sash is opened.  

Single-Hung Windows
These have a fixed top sash and an operable bottom sash.

Casement Windows
Sometimes called crank-out windows, casements are single-frame windows hinged on one side, so they open outward, usually by means of a hand crank mounted in the frame sill. They offer full ventilation when open and seal very tightly closed.

Awning Windows
Hinged on top, awning windows swing outward, usually by means of a crank. Mostly horizontal in configuration, they are often installed high on the wall.

Picture Windows
A non-operable window, generally large in size and installed in a prominent spot. Usually square or rectangular, but available shapes include round-top, octagon, and other specialty shapes.

A quality window unit has two or three panes of glass in the frame. These insulating glass units greatly decrease the amount of heat loss through the window, as well as limiting condensation or frosting. In most cases, triple panes (tri-pane) windows have somewhat better insulation performance than dual-pane units. Quality window glass is also treated with a low-emissivity (Low E) coating of microscopically thin metal such as silver. The Low E coating blocks UV radiation and radiant heat transfer.Tempered glass is required in areas where someone could fall into the glass, such as floor-level windows and windows in stairways.