When Building Green, it is important to think about the whole picture. The home is incomplete without windows and doors. The National Fenestration Rating Council (NFRC) is dedicated to helping those build green and by rating windows, doors, and skylights based on an energy performance rating. They are able to give ratings and certifications to windows and doors that are ranked with the quality needed for energy efficiency. The best way to check these is to see if they have an Energy Star seal of approval. Many windows and doors are capable of letting in excess heat or cold, or letting out excess heat or cold. The way that these transfers of energy happen is through general air leakage through and around the doors and windows, direct conduction through the glass, glazing, frame, and door, or by radiation of heat into or out of a house from room temperature objects (example: the sun beating down on a sofa).
The National Fenestration Rating Council rates doors and windows based on how much of the energy is allowed in or out of the door or window. The U-Factor is the rate of which a door, window, or skylight conducts non-solar heat flow. The lower the U-Factor, the more energy efficient a door or window is. Another way that they rate doors and windows is by Solar heat gain coefficients (SHGC). This is the fraction of solar radiation admitted through a window or door, this can be transmitted directly or absorbed, and releases the energy into the home. A high SHGC is better for the winter since it collects heat, a low SHGC is better for the summer since it repels heat. The SHGC has a passive solar window design that helps to stabilize these, but it all depends on where they are placed within the home. The NFRC also rates doors and windows by how much air movement is around a door or window. A product with a low air leakage rating is tighter than one that has a high air leakage rating.
Net Zero looks at these ratings and effectively uses them within their Green Building. We use insulated windows with Low-E glass, and fiberglass doors, to better insulate the home. These are able to fully protect the home from any sort of energy loss, or any extra energy that could come into the home, all caused by windows and doors that are not energy efficient.