Historic metal windows are generally not energy efficient;
this has often led to their wholesale replacement. Metal windowscan,
however, be made more energy efficient in several ways, varying
in complexity and cost. Caulking around the masonry openings
and adding weatherstripping, for example, can be doityourself
projects and are important first steps in reducing air infiltration
around the windows. They usually have a rapid payback period.
Other treatments include applying fixed layers of glazing over
the historic windows, adding operable storm windows, or installing
thermal glass in place of the existing glass. In combination
with caulking and weatherstripping, these treatments can produce
energy ratings rivaling those achieved by new units.
(3) One measure of energy efficiency is the Uvalue (the number
of BTUs per hour transferred through a square foot of material).
The lower the Uvalue, the better the performance. According
to ASHRAE HANDBOOK1977 Fundamentals, the Uvalue of historic
rolled steel sash with single glazing is 1.3. Adding storm
windows to the existing units or reglazing with 5/8"
insulating glass produces a Uvalue of .69. These methods of
weatherizing historic steel windows compare favorably with
rolled steel replacement alternatives: with factory installed
1" insulating glass (.67 Uvalue); with added thermalbreak
construction and factory finish coatings (.62 Uvalue).