Windows, Skylights & Doors

More about Windows, Skylights & Doors

There are three basic components of a window that determine its energy efficiency: the frame, the glazing or glass, and the operation.

The frame of the window conducts heat, and therefore contributes to the overall energy efficiency of the window. Traditional wood frames offer good insulating power, but can expand and contract with air temperature. They are also generally the heaviest type of frame and can be the thickest. Aluminum or other types of metal frames are very lightweight, strong, and require very little maintenance. Unfortunately, they conduct heat very effectively and thus require an insulating strip between the frame and sash. Another option is composite framing, which are more stable than regular wood and offer better structural and thermal properties, as well as improved moisture and decay resistance. Vinyl frames (made of PVC with special UV stabilizers to protect the material from the sun) are very popular due to their good insulating quality, moisture resistance, and their pre-finished character—they do not require paint. However, there is risk of expansion, warping, cracking, and fading with exposure to extremely cold or hot temperatures. To combat these issues, some high-performance vinyl frames feature cavities filled with insulation and are used in conjunction with high-performance glazings. Fiberglass frames are also available and share many properties with vinyl frames, including the option for insulation.

The glass and glazing of your windows will dramatically affect their energy efficiency. Many technologies can be used together, depending on your climate, the orientation of your home, and the location of the particular window within your home. The “glazing” of your windows refers to the number of panes—double-glazed windows feature two panes of glass with a space between them. Insulated window glazings are commonly referred to as “storm windows”.

The first technology is “gas fill”. Gas fills utilize an inert gas, such as argon or krypton (both nontoxic, clear, and odorless), to fill the space between the layers of glazing. These gases act as additional insulation by resisting heat flow more effectively than air. Another tactic that manages the amount of heat and light energy transmitted into the home (as measured by the solar heat gain coefficient and visible transmittance) is heat-absorbing glazing or glass. The glazing contains tints (usually gray or bronze, and sometimes blue or green) that absorb varying wavelengths of solar radiation. A similar concept is reflective coating to reduce the transmission of light (more than heat). Reflective coatings come in metallic colors and are used in hot climates to reduce visible transmittance, glare, and solar heat gain coefficient. It is important to remember that the energy savings gained by blocking light can sometimes be lost by the need for additional electrical lights.

A very popular technology for improving window energy efficiency is low-emissitivity (Low-E) coating for glass and glazings. Windows that feature this technology are generally 10-15% more expensive but reduce energy loss by as much as 30-50%. The coating itself is a virtually invisible layer of oxide metal applied directly onto the surface of one or more panes of glass in the window unit. The basic function of Low-E coating is to reduce heat transfer while admitting solar gain. In climates where air-conditioning is more frequent than heating, low solar gain Low-E glazings applied to the outside pane of glass are best, allowing the least amount of solar heat gain. The reverse is true in heating-dominated regions, where the coating is intended to keep heat inside the home. Do-it-yourself Low-E coatings are also available as an alternative to window replacement.

Finally, a special type of Low-E coating allows specific (visible) wavelengths to pass through the glass, while blocking others (infrared, or heat). These coatings create a window with a low U-factor and solar heat gain coefficient but a high visible transmittance.

The operation of the window is largely determined by its location in your home. However, some types are more energy efficient than others. Awning (hinged at the top and opening outward), casement (hinged at the side), and hopper (hinged at the bottom and opening inward) windows generally leak less air than single- and double-hung or sliding windows because the sash closes by pressing against the frame.