What Is It?
In the final analysis, the Energy Rating (ER) number is the bottom line for energy-conscious consumers as it is the indicator of the window’s thermal performance. The ER number is not a temperature rating. It is a scale rating the comparative performance of windows based on three factors: 1) solar heat gains; 2) heat loss through frames, spacer and glass; and 3) air leakage heat losses. It is a rating solely of a window’s energy performance, regardless of how or with what materials it was built.
The Higher The ER The Better
The ER can be used as an indicator of the effect the windows will have on an annual heating bill. A high ER (over 40) means that the window will add more heat than it loses during the heating season. A lower ER means that the window loses more heat than they gain during the heat season. The lower the number, the more heat is lost and the higher the heating costs.
Things to Remember
Most fixed windows will have better ER numbers than operable windows. There are two reasons for this. First because the standard test size for a fixed window is larger than most operable windows and thus have more glass area relative to frame area and the frames tend to be narrower because they do not need separate moveable sash. This translates into more solar gains and less frame losses and a higher ER number. Second, fixed windows have less air leakage than operable ones, simply because you can’t open them so the heat loss is less resulting in a higher ER number.
Compare Apples to Apples
The ER system is based on a formula which calculates a single ER number for a specified window size in each of seven window categories. Because all evaluations are done the same way, it makes comparisons between different manufacturers much easier. The ratings given will be for all windows in the standard size and not a particular window. It is essential to compare ER numbers within each window category as the test size of windows is different from one category to another. This results in different ER numbers for different categories, because the frame to glass ratio changes. For this reason, you cannot compare ER numbers across categories.