A.The storm window will always add to the insulating value, but it may be difficult to justify economically. Heat loss is inversely proportional to R-value. Each additional R-value is worth less than the preceding one. Installing a storm window over a single glazed window will raise the R-value from about 1 to 2, cutting heat loss by 50%. If the prime window already has R-4 insulated glass, however, raising the R-value to 5 will only save 20% of the heat lost through it. If the prime window has a center-of-glass R-value of 8, which seems to be the best available today, the gain would only be 12%. If the storm window is installed properly, it should reduce infiltration, as well. Here again, if you install the new window over a leaky, old sash you’ll get more of an improvement than if you install it over a new, well-sealed window.