Recoup your investment
In a modern home, it's estimated that 25% of all energy loss is through the windows. Which means improving them will show immediate returns. Although exact cost-recovery calculations are difficult since existing windows vary significantly in terms of R-value and air infiltration rates, actual experiences of real customers place the recovery period for interior storm windows offered by D.W. Philpot Company at between 2-1/2 to 4 years.
Save on energy costs year after year
According to the manufacturer, a homeowner in the Boston area would save approximately $440 year on heating and cooling costs using interior storm windows. (Based on calculations done using RESFEN - a window performance tool from Berkeley National Labs in California.)
All calculations are for a 2 story existing frame house with 300 square feet of basic windows, using an electric heat pump with a power cost of 8.5cents per kWh.