Testing the Energy Performance of Historic Windows in a Cold Climate (1997-16)

A study was undertaken to determine the feasibility of renovating and upgrading an original condition window to the extent that its thermal performance would be equivalent to a window using replacement sash or window inserts. The study was funded by the State of Vermont Division for Historic Preservation based on a grant received from the National Center for Preservation Technology and Training of the U.S. National Park Service.

Thermal losses associated with a window are the result of infiltrative and non-infiltrative losses. Infiltrative thermal losses are a result of air infiltrating through and around a window whereas non-infiltrative thermal losses are due to conduction, convection and radiation through the materials of the window. Infiltrative thermal loss rates were based on fan pressurization data for total window and extraneous air leakage rates from 151 field-tested windows containing of 64 original condition windows and 87 windows of varying upgrade types. Sash leakage characteristics for baseline typical, tight, and loose windows were assumed from the averaged original window data. The percentage of exterior air contained in the extraneous air volume was estimated during the test procedure based on temperature differences in the test zone during fan pressurization and added to the sash leakage for a total window leakage rate representative of the heating season. The Lawrence Berkeley Laboratory correlation model was used to convert leakage data to natural infiltration rates during the Vermont heating season. Non-infiltrative thermal losses were modeled using WINDOW 4.1, a fenestration computer simulation program.

Annual energy costs based on the combined infiltrative and non-infiltrative thermal loss rates for each upgrade category were estimated. A sensitivity analysis of the cost estimation method resulted in a variability of +/- 25%. Each upgrade type was compared to the three assumed baseline windows to estimate annual energy savings in 1996 dollars. Also investigated were differing configurations of replacement storm windows and the effect double-glazing had on energy costs versus those associated with single-glazing.

Estimated annual savings per window due to renovations or upgrades ranged from zero to a high of $3.60 as compared to a typical baseline window. Annual savings compared to a tight window ranged from $0.05 to $2.10 per window while savings compared to a loose window ranged from $12.40 to $16.60 per window. Pay-back period for any upgrade as compared to any of the typical windows was measured in decades.

A systematic upgrade of an original sash window can potentially approach the thermal performance of an upgrade utilizing replacement sash although decisions should not be based solely on energy considerations due to the similarity in savings between upgrades. It was found that approximately 85% of energy costs associated with thermal losses through and around a window were due to non-infiltrative losses. While tightening a window to prevent air infiltration around the sash and jamb and through the rough opening would reduce annual energy costs associated with a window, a more efficient use of time and resources would be to reduce non-infiltrative losses by using double- or triple-glazing and/or low-emission glass.

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