Windows are thermal holes. An average home may lose 30% of its heat or air-conditioning energy through its windows. Energy efficient windows save money each and every month. There are even some cases where new windows in strategic locations can be net energy gainers. The payback period for selecting energy efficient units ranges from two to ten years. In new construction, the higher initial cost can be offset because you will need a smaller less expansive heating and cooling system. More durable windows will also cost less in the long haul because of lower maintenance and replacement costs. Plus you will be more comfortable the whole time you live with them.
Windows lose and gain heat by conduction, convection, radiation and air leakage. This heat transfer is expressed with U-values, or U-factors. U-values are the mathematical inverse of R-values. So an R-value of 2 equals a U value of 1/2 or 0.5. Unlike R-values, lower U-value indicates higher insulating value.
Touch a hot skillet and you feel heat conduction from the stove top or flames through the pan. Heat flows through a window much the same way. With a less conductive material you restrict heat flow. Multiple -glazed windows trap low conductive gas such as Argon in between the glass panes. thermally resistant spacers and window frames reduce conduction also.
In a cold climate, heated indoor air rubs against the interior surface of window glass. The air cools, becomes more dense and drops to the floor. As the stream of air drops, warm air rushes in to take its place at the glass surface. This cycle is a connective loop and self perpetuation. You recognize this movement as a cold draft and turn up the heat. Each 1°F increase in thermostat setting increases energy use by 2%. Multiple panes of glass seperated by low-conductive gas fillings (Argon), combined with thermally resistant frames, raise interior glass temperature and slow convection to improve comfort.
Radiant transfer is the warm feeling on your face when you stand near a campfire. Conversely, your face feels cool when it radiates its heat to a cold sheet of window glass. But radiant heat loss is more than a perception. Clear glass absorbs heat and radiates it outdoors. Radiant heat loss through windows can be greatly reduces by placing low-E coating on glass that reflects specific wavelengths of energy. In the same way low-E coatings keep the summer heat out and block UV rays.
air leakage through windows is responsible for much of this loss. Well designed windows have durable weather stripping and high quality closing devices tha effectively block air leakage. Hinged windows such as casements and awnings clamp more tightly against weather stripping than do double hung windows. How well the individual pieces of the window unit such as : glass to frame, frame to frame and sash to frame; also affect air leakage. the technical specifications for windows list values for air leakage as cubic feet per minute per square foot of window. Look for windows with certified air leakage rates of less than 0.30 cfm/ft2. Lower values are best.