You can raise the R value of your double pane storm windows by adding a fine layer of mylar film over them. An R value is the value assigned to the conductivity level of the material being used between the outdoor and indoor environment, getting windows new with higher R levels or U levels will cost more money, but add efficiency against heat and energy loss. No matter what the age of your windows, you are at the very least loosing 50 percent of your energy, and you can decrease this loss by adding mylar sheeting.
R Ratings on Windows
Most standard one pane windows offer an R1 rating, as double pane storm windows rate at around R2, you can buy them with a higher rating, however this is the standard. In other countries this rating is based on an SI rating instead but does take into account the same science and comparable units of measure. You can increase the R rating of your windows by individually adding maylar sheets to them, which is a transparent coating that you can attach yourself.
Clear Coating That Doesn't Blur the View
Adding maylar film to your double pane storm windows will not effect the view, or the images seen through the glass. it is a transparent, crystal clear material if applied evenly and correctly. Be sure to follow all applicable instructions when applying the maylar to your windows, to avoid visibility problems going forward. The maylar itself can also be purchased in different grades to raise the rating on the windows even further for a very good price.
The Science behind R and U Values
All homes using either single or double pane storm windows have the same basic equation attached to the formula when trying to calculate your ratings. Single pane is at a standard of R1, unless you buy the window with a higher rating to begin with. A double pane window will be the same value as the single pane x2, and can also be purchased at a higher rating right from the start. Regardless of the windows starting R rate, you can increase it with mylar sheeting. The greatest loss of heat and energy in a home, comes from the transfer of energy in and out of your home through your windows themselves.
The Mathematical Equation Behind your R Rating
The R rating for your double pane storm windows represents its thermal resistance, and is calculated from using an equation that comes from the temperature difference across an insulator, and the Heat flux through it. The larger the R rating number, the better the insultion. You can search and find many widow r rating related websites that will give you a basic check sheet for different types of windows.
If you don't know the brand of double pane storm windows you have installed, you can look for a factory label stuck to the underside of the window jam itself. The only other way to determine this is by knowing the age of the glass in the window, as window technology has evolved over the years and can would effect your R rating. The rating is determined by the windows area, and the material density, area by pi.