Though there have been many improvements in energy efficiency in home building products, none have had the impact of energy efficient windows. In almost all cases, the windows of older homes were the largest source of heat loss. Older style frame windows that only had single pane glass, easily let heat transfer into the home in the summer and out in the winter. But modern windows perform much better than older ones at limiting the amount of heat that moves in and out of a home. Modern windows use a variety of different design and material improvements to provide a level of energy efficiency that was unattainable in the past.
Modern windows typically use two panes of glass in order to provide a buffer between indoor and outdoor air. In fact, many windows use three or more panes of glass to add even more efficiency. The layers of glass provide insulation by trapping pockets of air which work to provide insulation and prevent the transfer of heat. Most multiple pane windows do not hold air, but rather some type of inert gas (often argon or krypton) which prevents the window from sweating internally and which provides better insulation than air.
Energy efficient windows often offer coatings that help to make the window more efficient. One type of coating is referred to as low-e. Glass that has low-e coating reduces the amount of solar energy that passes through the window which creates heat in the home. Low-e coatings do this by blocking some of the ultraviolet and infrared light from the sun, while allowing visible light to pass through the window. Essentially, low-e glass blocks heat while providing light. In addition to energy savings, blocking ultraviolet light will also reduce the fading of paint and fabrics inside the home from sunlight. Glass can also be tinted to further reduce the transfer of solar energy through the window.
Though wood is a good insulator, older wood windows were notorious for shrinking, expanding and warping over time. With these movements came air gaps that made the older windows even less energy efficient. Modern windows use frames that are typically made of aluminum or vinyl which resist warping and stay in place. Some modern windows are still made of wood, but are better sealed against the elements than older styles of windows. All modern windows use advanced sealants between the glass and frame to prevent air leaks that can cloud the glass and result in lost energy efficiency.