Benefits of Storm Windows
Based on research presented in Home Energy magazine the average energy efficiency improvement of adding an energy-efficient single glazed exterior storm window to an existing single pane wood framed window is over 400%. Using an energy efficient fiberglass framed storm window increased the efficiency by 600% or more. The installation of a storm window with a wood frame is 300%. The most effective storm windows are considered energy efficient and have the smallest frames. Energy efficiency of windows is measured in U-value of the glass with the lower the score the better. Energy efficient U-values are in the 0.2 end of the spectrum while inefficient glass is in the 1.0 range. In R-value terms the 0.2 U-value is an R-value of 5.0 and the inefficient rating of U-value 1.0 also has an R-value of 1.0.
Another consideration in choosing the efficiency of the glass is the temperature that is created between the storm and the interior window. If the temperature outside is 10o F and the inside is 70o F, when using storms with a U-value of 1.0 the temperature between the storm and interior window will be 40o F. If you are using the energy efficient storm windows with the 0.2 U-value the temperature between the storm and window will be 60o F. The higher the temperature is between the storm and the window the lower the radiant heat loss is for the complete window unit. Also the risk of condensation on the interior window frame and glass of the window is greatly reduced.