THE EFFICIENCY TEST
Despite the aesthetic challenges, replacement is an obvious and attractive choice, especially given the growing demand for better energy efficiency. Classic single-pane, double-hung windows, after all, were not designed with building science in mind. Today’s double- and triple-glazed windows, on the other hand, cut down on heat loss in winter by providing insulating airspace. And low-E coatings reduce solar heat gain in the summer.
Tax credits of up to 10% of purchase price for Energy Star-rated windows make replacement even more attractive. Klement, the Ann Arbor remodeler, often works on pre-World War II homes and has won awards for designs that adhere closely to traditional architectural styles. Still, he rarely prescribes anything but replacement. “
A huge focus of our practice these days is energy efficiency and sustainability,” Klement says. “These [historic] windows have done their job and performed well in the environmental and energy-efficiency framework that existed when [these structures] were built, but where we are right now in terms of technology is so far ahead of that. Unless you’re doing a house that’s on the historic registry or in a historic district, I think it doesn’t make any sense [to repair windows rather than replace them].”
REPAIR AND RESTORATION
But a number of contractors argue that repair and restoration should be given more consideration. Energy efficiency, these contractors argue, should not be given such great weight because windows only contribute about 20% to 30% of heat transfer. Compared with modifications that improve insulation and reduce airflow, no window improvement will have as great an impact on the overall energy efficiency of a house, particularly an older house that loses heat in a number of other ways.
“For example,” Alward says, “a single pane of glass is rated as R-1; a typical stud wall” — a cavity wall with insulation, framing, and an exterior and interior — “is probably an R-13. If you go to dual glazing, you’ve increased it to about R-2.4; you’ve hardly done anything. It’s an overrated energy-conservation technique.”
In fact, research has shown that weatherizing, reducing airflow, and installing low-E storm windows can be just as effective as replacing a single pane with a double-pane low-E window. Research by the Oak Ridge National Laboratory found a negligible difference between energy savings produced by these two options. A study by the city of Boulder, Colo., found the same result, concluding that “replacing a historic window does not necessarily result in greater energy savings than upgrading the window.”