Part II: Understanding Windows And Energy Efficiency – Framing the Discussion

Much of a window’s energy efficiency is determined by the material that a window’s frame is made of. Except for the pane itself, all of a window’s other parts are generally made from the same material, from jambs to sill to rails to stiles. Because of this, the material’s particular energy efficient aspects dictate much of the same for a window.

Aluminum, being a metal is largely, not very energy efficient. It gains and loses heat readily (low R-value), and thus contributes a lot to heat loss (during the cold) and heat gain (during the summer). Aluminum is, however, generally the least expensive window material option, and in some cases is more practical than others, especially in places where the weather poses severe impact stress on windows.

As the other less expensive option, much is touted about vinyl’s cost-effectiveness in addition to its good R-value. Generally, good, “tight” construction also means that vinyl generally has a low amount of air leakage.

Wood is by far the most expensive of the different window materials. It is, however, a better insulator than even vinyl. The spacing between the panes in a wooden frame tend to be set farther apart than vinyl or aluminum, which provides a better buffer against solar heat gain.

One of the most energy efficient materials is Fibrex®. It blocks thermal transfer almost 700 times better than aluminum helping to reduce heating and cooling bills. Made with wood fiber and thermoplastic polymer it is twice as strong as vinyl, allowing windows do be made with less bulky frames and is virtually and maintenance free. Fibrex® is available in a wide range colors and contours, creating unique styles including real wood interior options.

Aside from considerations of material, the style or design that a window is made in also affects its energy efficiency.

Double-hung windows are, by far, the most common window style in the United States. While generally efficient, there is a lot of potential air leakage in the design, especially if the window has aged and is no longer properly operable.

Casement windows are generally more energy-efficient than double-hung, and are actually even more efficient when the wind is blowing hard, sealing the window tighter. Their construction requires regular maintenance to maintain the level of energy efficiency, however.

Picture windows are generally energy-efficient, especially since they don’t open at all. The relative energy efficiency of picture windows, however, is affected in large part by the structure of the windowpanes, though this applies to other window styles as well.

Part III, coming soon, will deal with the other main source of a window’s energy efficiency, the structure of the pane, or panes.