This report records the results of a field study conducted in a single historic home in Seattle, Washington, to document the performance of Indow Windows interior storm window inserts. The energy use and thermal performance of the house were monitored before and after the installation of the window inserts and changes in the two recorded metrics were examined. Using the defined analysis approach, it was determined that the interior storm windows produced a 22% reduction in heating, ventilation, and air-conditioning energy use and reduced building envelope leakage by 8.6%. While there were no measurable changes in the thermal comfort of the house, the occupant noted the house to be warmer in the winter and cooler in the summer and that the temperatures are more even (throughout the house). Despite the significant energy savings that resulted from the installation of interior storm windows, the payback period for this investment was extensive (approximately 80 years) due in part to the upfront cost of the Indow Window inserts as well as relatively low baseline heating bills of the case study home. Prior to the installation of interior storm window inserts, the case study home had implemented a series of energy-saving retrofits, which included replacing the homes heating and cooling system with a high-efficiency heat pump, duct sealing, air sealing, and additional insulation. If the Indow Windows interior storm window inserts had been installed prior to these retrofits (e.g., house heated with an older, inefficient oil-fired furnace) and the same economic analysis was conducted, the interior storm window inserts would have resulted in a simple payback period of 9.0 years. This field study has added to the body of knowledge about interior storm windows by presenting the measureable energy savings and evaluating the claims of increased thermal comfort.
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