How does a storm window function?
A good storm window fits over the entire prime window frame and insulates by adding air space. A good storm window fits snugly. The addition of a storm window results in an overall lower U-value for the window, meaning that heat conduction (and thus heat loss or gain) has been reduced. Storm windows not only insulate the glass in the prime window, they also help reduce the amount of air leakage around the perimeter of the prime window. Adding a storm window to a single-glazed window cuts heat loss or gain roughly in half. Adding one to a double-glazed prime window reduces heat loss or gain by about a third.
