Windows used to be little more than a hole in the wall keeping out weather elements. Today’s attitude is that windows are a wall you can see through while remaining comfortable. In the average home windows cover from 10 to 15 percent of the total wall area, yet they are responsible for 20 to 30 percent of the total home-energy loss.
Windows lose heat, or in the summer when the air conditioning is operating, gain heat through the glass, the frame, the edge spacers separating the panes of glass and through cracks around the sill. These losses cost the homeowner big bucks.
Good windows keep energy costs down”. They reduce drafts, increase the home’s comfort level, block exterior noise, provide fade protection to carpeting and furnishings, and increase the value of the home.
Windows have three integral parts: glazing or glass, edge spacers and frame. Each contributes to, or detracts from, a home’s comfort level. Each conserves or wastes energy.
Single-paned windows offer little or no insulation. Two panes of glass are more energy efficient than one, and three panes are even better. However, triple-paned glass is heavy and expensive. Current technologies make the glass itself an energy saver.
Low-E glass makes double-paned windows equivalent to triple-paned ones by using a colorless, invisible coating that transmits high levels of light but reflects heat. Heat Mirror ™ , another energy-efficient glazing option, suspends a thin, clear, polyester film with low-e coating between two panes of glass. Some windows have krypton or argon gas between the panes. Because these gasses do not conduct heat as fast as air does, they can deliver efficiencies higher than any other type of window.
Edge spacers that hold the glazing layers apart used to be made of aluminum which conducts heat rapidly. Today they are made of materials such as structural foam that conduct heat more slowly.
Frames make up 10 to 30 percent of the total window area. The framing material used can make a difference in the efficiency of the window and, consequently, the energy cost to heat and or cool the home. Metal frames conduct heat faster that other window frames. Metal frames with a thermal break, a plastic insert that separates the inside metal framing from the outside, conserve energy. Wood frames conduct heat much more slowly than metal, but require more maintenance. Today some wood windows are available with metal cladding which can reduce the need to paint. Vinyl frames have an efficiency comparable to that of wood and they require less maintenance.
How do you know when you need new windows? A rule of thumb is, “if you feel air when sitting or standing next to the window, you know that moisture and air is coming through”.
Determining the best window to buy depends on a number of factors, including budget. Orientation of the window is one factor to be considered. Windows used to be rated with R-values, the measure of resistance to heat transfer at the center of the glass. The higher the R-value, the better the window’s insulation ability. A new system, the U-value, usually displayed on the window, measures the efficiency of the entire window unit. Typically, U-values range from 1.15 to 0.15. The lower the U-value the less heat transfer between the inside and outside air. The Shading Coefficient (SC) and Air Filtration (AF) are other measures of window energy efficiency.
No matter how energy-efficient a window is, proper installation is necessary to assure that the window performs to its potential. Windows need to be weatherized and insulated when installed. Qualified installers provide this service.