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 canΓÇÖt open them so the heat loss is less resulting in a higher ER number.
The Model National Energy Code for Houses suggests minimum energy performance values for windows and doors for various locations and fuels.
Compare Apples to Apples
The ER system is based on a formula which calculates an ER number for a specified window size for each operator type (i.e. casement, slider, etc.). Each specified window size has a different frame to glass ratio which affects the ER rating. The calculated ER number applies only to that standard size, however, because all evaluations are done the same way, direct comparisons within the same operator type can be made between different product designs and manufacturers. Since there are different specified sizes for each operator type, energy performance comparisons between different operator types is not direct but may be inferred. Other factors such as the inherent design of each operator type and air tightness
