Windows come in all styles, types, shapes and sizes, but unless you’re building a new house, all of the above are largely predetermined. There are of course some exceptions. Perhaps a previous homeowner replaced the original windows with units that are historically inappropriate or inferior. (Today, historic window styles are readily available from manufacturers like Andersen.) Or maybe you’re adding a family room at the back of the house, where it would be okay to deviate from the double hung windows in the front; in this situation, you might decide to use casements. Sometimes a homeowner will want to increase or decrease the size of the window being replaced, but if you’re like most homeowners, the real decisions will have more to do with energy-saving features and ease of maintenance.
With regard to energy saving, the first thing to focus on is glazing. Efficient windows typically have two layers of glass and are called dual-pane or double-pane. The small gap between the glass layers creates a barrier to heat flow, which may be enhanced with an additional layer of glass (two separate insulating chambers), in which case it’s called triple-glazed. The gap or gaps between layers of glazing are often filled with a gas that further reduces heat flow by conduction. Argon and Krypton, or a combination thereof, are commonly used gas fills.
REFLECTIVE FILMS, TINTS, AND COATINGS
Reflective films, tints, and low-emittance (low-E) coatings are some of the other ways window manufacturers are improving window performance. Reflective films block much of the radiant energy striking a window—keeping occupants cooler—but they also block most of the visible light. In addition to giving windows a mirror-like appearance, they often cause occupants to use more electric lighting to compensate for the loss of daylighting. Bronze- and gray-tinted glass reflect radiant energy and reduce cooling loads without reducing as much the visible light entering the home. A visual transmittance (VT) of 60% (versus 90% for clear glass) is common.
Replacement WindowsLow-E coatings are more versatile than either reflective films or tints and are virtually invisible. Microscopic metal or metallic oxide particles suppress radiant heat flow out of the window and can be formulated to allow varying degrees of solar radiation in. In climates where heating is the dominant concern, low-E coatings may be used to prevent radiant heat transfer out of the house while allowing high solar heat gain. In climates where both heating and cooling are required, low-E coatings can reduce radiant heat loss while allowing moderate heat gain. In climates where the dominant concern is cooling, low-E coatings are primarily used to reduce solar heat gain. It’s even possible to fine-tune solar heat gain by choosing a low-E coating with a high solar heat gain coefficient (SHGC) for south-facing windows and a lower coefficient for other orientations.
The material with which the window frame is built will also significantly affect its efficiency. Insulation-filled vinyl frames and fiberglass perform better than wood, wood-clad, and vinyl that is not insulated. Aluminum and steel perform worse than any of the above.