Green building design can be a complex process, and windows are an important part of that process. A wide range of performance parameters can make selecting the right options difficult, and some parameters provide conflicting performance trade-offs. Good information about how to design green windows is not readily available, as most people look through windows, but not many people look at them. Designed correctly, windows can greatly enhance the occupants' enjoyment of the space and enhance the energy performance of the entire building. Designed incorrectly, windows can be a source of irreparable failing of the building to meet the occupants' requirements for comfort, energy efficiency, and long-term durability of the building envelope.
This article discusses the physics behind window performance, describes some of the design parameters, and explains how to determine whether you can get the performance you want on site. This is not meant to be an exhaustive reference on the subject, as that would require several books. Rather, the intent is to provide an overview of the physical behavior of window components, so that appropriate design choices are based on sound knowledge and not guesswork. This article also provides some fluency with window-performance terminology for use in discussing design options with specialists.