The options in modern windows relate to how each component is designed and manufactured. The most familiar part of the window is the sash—the frame that holds the glazing. Sashes often have two and, occasionally, three or four layers of glazing, referred to as double-, triple-, or quadruple-paned windows.
Frames hold the sash in the wall. They can be made of different materials, which vary in their thermal conductivity. Metal window frames, typically aluminum, are great conductors, and generally a poor choice for energy-efficient windows. Metal frames can be improved by having a thermal break within the frame to slow conduction. Wood frames have better thermal performance. They can be challenging to maintain, however, since condensation and rain can cause rot and peeling paint. Vinyl windows are rot-resistant, but need to be insulated to be energy efficient. Fiberglass and composite windows offer low maintenance combined with high strength and good insulating properties, but with additional cost.
Operating type describes how the sash is opened to allow ventilation. The two most commonly available varieties are double-hung and casement windows. Double-hung windows have two sashes, one at the top and one at the bottom, that slide up and down, with the screen on the outside. Casement windows are hinged on one side and generally crank outward, with a screen on the inside. From an energy efficiency standpoint, casement windows are superior. When open, they allow much more airflow, since the entire area of the sash is open compared to only about half in a sliding window like a double-hung. Casement windows seal better than sliding windows—when they are closed (and locked in place) the entire sash is pulled against the weather stripping. In sliding windows, the sides remain relatively loose. Keep in mind that not all windows need to open, however, and that a fixed window can be less expensive and more energy efficient than an operable window.
Other styles of windows are oriented differently but behave in a similar fashion to casement or double-hung. For instance, a slider has two sashes that slide horizontally and performs like a double-hung, and an awning window is hinged at the top but otherwise performs like a casement window. A tilt-turn, which can hinge on either the side or the base, is becoming an increasingly popular style.
Holding up all of those panes of glass in such a small area is not easy, and the first generation of windows mostly accomplished this by using aluminum spacers to hold the glazing in place. But aluminum is a great conductor and this thermal bridge wicked away large amounts of heat, negating much of the benefit of multiple panes. Another problem was condensation resulting from the differences in temperature across the window unit. Spacers made of composite materials or stainless steel, along with thermal breaks in the spacers, have greatly reduced these problems. The latest windows have also made improvements in the seals that lock in the argon or krypton gas between the panes, meaning much greater sash longevity, which previously had been prone to eventual leakage.
Passive solar design captures solar gain, admitted through south-facing windows, to help with winter heating. Every window, however, replaces insulated wall space with an area that is much less insulated. In a well-built home with walls reaching R-21, adding even a triple-glazed window will make the wall space that contains the window only about a quarter as effective at stopping heat transfer.
In the past, passive solar design meant lots of south-facing glazing coupled with lots of thermal mass to absorb the solar energy. Insulating shades were often part of the design to reduce heat loss at night. Newer designs tend to be less aggressive in their glazing applications. The reason for this is simple—windows (in addition to being costly) can lose many times as much heat per given amount of wall space as an insulated wall.
For new construction, a solar site assessment that considers winter irradiance is crucial in deciding the amount of windows. If, say, only an hour or two of winter sun is available, that means heat is escaping the other 22 or 23 hours of the day. A net heating gain from lots of south-facing windows will be close to impossible. There’s going to be some windows on your south-facing wall no matter what, and it’s a shame not to take advantage of the gain that does come through them.