The effect of the window frame and spacer on the total window heat loss has generally been overlooked when estimating building heat loss. The complex nature of the heat flow and the wide variety of window frames make it difficult to characterize frame and spacer effects. This paper presents the results of two-dimensional finitedifference computer modeling of a variety of window frames. The results show that double-glazed windows with insulating spacers have a 6% lower U-value than those with aluminum spacers. Furthermore, insulating spacers can reduce total window U-value by as much as 12% in high-performance windows, as compared with aluminum spacers.

The frame material has the greatest effect on total window U-value. The U-value for aluminum frames is 39% higher than wood frames for standard double-glazed windows and up to 52% higher for high-performance glazings.