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