Measured winter performance of storm windows

Direct comparison measurements were made between various prime/storm window combinations and a well-weatherstripped, single-hung replacement window with a low-e selective glazing. Measurements were made using an accurate outdoor calorimetric facility with the windows facing north. The double-hung prime window was made intentionally leaky. Nevertheless, heat flows due to air infiltration were found to be small, and performance of the prime/storm combinations was
approximately what would be expected from calculations that neglect air infiltration. Prime/low-e storm window combinations performed very similarly to the replacement window.
Interestingly, solar heat gain was not negligible, even in northfacing orientation.