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As windows get more energy efficient, window selection gets more complex. Gone are the days when R-value and air leakage were the only important window specifications. Open almost any building magazine, and you will probably see ads promoting the reduction of fabric fading with Heat Mirror windows, the increased solar heating of LOF Energy Advantage glass, and reduced condensation with Hurd Insol- 8 windows. These are a few examples of the latest performance features that window manufacturers are promoting. More are sure to follow. The following suggestions should help a builder or designer intelligently navigate this technological maze. R-value — A Moving Target When low-e windows were first introduced ten years ago, they carried a hefty cost premium over conventional windows with