Techniques > Systems > Windows > The Repair and Thermal Upgrading of Historic Steel Windows >

 
 

Weatherization

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)

(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).

 
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