Windows can be the weakest points in a building's thermal barrier. Through windows, a substantial amount of indoor heat can be lost during cold weather, whilst also permitting unwanted indoor heat gain in warm weather. In either event, this may add significantly to indoor climatic conditioning bills and to a building's energy consumption (it is estimated that at least US$12,000 million worth of energy is lost through windows in the USA each year). In recent years, new window technologies have increased window thermal efficiency, which have led to energy savings in buildings.
Single-pane windows are most commonly used in buildings, yet they are comparatively thermally very inefficient. In the last decade, high-performance windows have been developed that allow natural lighting and views while insulating up to twelve times more effectively than single-pane windows. Innovations such as low-emissivity (low-e) coatings, inert gas fills between panes, suspended plastic films, improved air tightness, insulated edge spacers, and insulated window frames have improved building energy efficiency. Low-e windows glass surfaces are coated, allowing most visible light to pass, but controlling the amount of radiation passing through either way. Some manufacturers are upgrading the technology resulting in hot climate windows that block more solar radiation to reduce indoor cooling costs, and cold climate windows that admit more solar radiation to reduce heating bills. Superwindows are an improvement on low-e windows, and insulate four times better than triple-pane. They function as low-e windows, except that the low-e coating in superwindows is applied to thin plastic films suspended between the two panes of glass. As with low-e windows, superwindows can be adapted for cold and warm climates. Low-conductivity inert gases such as argon and krypton have been used in recent years to fill (and replace the air) between window panes, since this can result in reductions of heat loss or gain by 15-30%. Krypton is a better insulator than argon. Such gas fills are a standard feature of many low-e windows and of all superwindows. Among window frame material available at present, wood insulates best against heat transfer, whilst aluminium frames insulate the worst. Highly insulative frames made of pultruded fibre-glass and plastic are currently under development. High-performance windows also feature warm-edge technology that lowers heat loss through the metal spacers separating the panes of glass in sealed insulating windows. One manufacturer of warm-edge technology has developed a rigid foam thermal break that achieves 95% less conductivity than the dual-seal aluminium edge which is currently the industry standard. The most insulating superwindows incorporate most of the above window technology. These high-performance windows are more expensive and the pay-back period may be 15 to 20 years.