Most, if not all current building codes require a certain U value to be met in window performance, in order to get a construction permit. the only way that U value is attained is with a Low E coating. If the desire is to keep heat out, the coating is on surface 3 of a IG unit. If heat retention is desired, the coating is on surface 2. Basically the low E coating reflects heat back to the direction it came from. ..The energy codes are getting more and more stringent in the next couple years, The only way they will be able to meet the requirements will be to utilize triple pane glass in new construction.