Measuring Heat Loss in Energy-Efficient Windows by Alex Wilson Last month I discussed the most important factor controlling the energy efficiency of windows: the R-value of the glass. The glass, however, is by no means the whole story. This month I'll discuss two other important factors that determine a window's energy performance: sash/frame material and air leakage. Conductivity Through Window Sash and Frame Window sash and frames, like any building-skin component, contribute to heat loss. Of the materials used in manufacturing windows—wood, vinyl, aluminum and steel—wood usually is the most energy efficient, followed by vinyl, then aluminum and steel. Aluminum frames typically have thermal breaks (layers of lowconductivity material separating the inside and outside of the frame) to reduce conduction, but not always. If you're building
