High-Efficiency Windows include well-built, air-sealed windows with good thermal performance. Typically, double-glazed or triple-glazed windows with insulating gas between panes (argon or krypton) and a low-E coating are considered high-efficiency. Low-Emissivity (low-E) coatings prevent radiant heat from crossing the window plane, i.e. they keep heat inside in the winter and outside in the summer.
The frame material is as, if not more, important than the glass. Frame materials like Fibreglass and Wood have low rates of thermal energy transmission. Aluminium and PVCu (Vinyl) consistently have the highest negative environmental and health impacts according to life cycle assessment techniques (Source: USGBC).
Another significant energy pathway is the porosity of the construction including sealing. Installation is an important consideration for High-Efficiency Windows.