Energy efficient windows can save you a bundle of money by preventing heat loss or gain in winter and summer, respectively. These types of windows seal up your house and make it as air-tight as possible, which in turn keeps your heating and cooling appliances running at maximum efficiency. But what exactly does it mean for windows to be energy efficient?
Anatomy of an Energy Efficient Window
An energy efficient window consists of five major parts: Frame Material, Panes, Coating, Gas Fills, and Spacers.
- Frame Material – Energy Star rated windows can implement one of several different framing materials. Fiberglass is durable, low maintenance, and has good insulating properties. These frames can be hollow or filled with insulating foam. Vinyl frames also require little maintenance and have a good thermal insulation. These may also be filled with insulation. Wider sills may be reinforced by wood or metal. Aluminum frames are often recycled; containing at least 15% recycled material. They are also durable and low maintenance. They are constructed with thermal breaks to reduce the amount of heat lost through the metal itself. Wooden frames provide good insulation and are favored in historical neighborhoods for their look and strength. Though not the most durable on their own, they are often reinforced by aluminum or vinyl to cut back on necessary maintenance. Any of these materials may be combined in a single framework, giving a specific look with added durability or strength.
- Panes – Windows are primarily constructed with two panes of glass with a gap in between them. This insulates the window far better than a single pane of glass.
- Low-E Glass – The glass in each pane is coated with an infrared-reflecting coating to help repel heat during the summer and trap it during the winter. These coatings also reflect ultraviolet light which can damage furniture.
- Gas Fills – In the space between the two panes of glass, some windows are filled with argon, krypton, or other gases to help improve the level of insulation. They are odorless, colorless, and non-toxic, so there is nothing to worry about with these fillings.
- Spacers – Spacers keep the individual panes separated the proper distance for energy efficiency. These spacers also insulate the pane’s edges, further preventing heat loss.
All windows are given performance ratings based on their construction and insulating properties. These ratings are established by the National Fenestration Rating Council, or NFRC.
- U-Factor – This is the amount of heat transfer the window exhibits, which gives a clear indication of how well the window insulates. The value generally ranges anywhere from 0.25 to 1.25 with 0.25 being the better rating. The rating is derived by using the equation Btu/hour * ft^2.
- Solar Heat Gain Coefficient (SHGC) – This is the measurement of how well the window blocks incoming sunlight and usually ranges from 0.25 to 0.80, with 0.25 being the better rating.
- Visible Transmittance (VT) – This is the measure of how much light your window lets in and ranges from 0 to 1, with most ratings between 0.20 and 0.80. The higher this number, the more light is let in.
- Air Leakage (AL) – This is the measure of how much air passes through the joints of a window. The lower your rating, the less air escapes. Most building codes require a maximum rating of 0.3 cubic feet lost per minute over every square foot of window.
- Condensation Resistance – This is the measure of how much condensation builds up on the window and is scaled from 0 to 100. The higher the number, the less condensation will build up on the window.
It is important to note that Energy Star certification is only based on the U-Factor and SHGC ratings. All of these factors come together to give you an exact picture of how efficient your windows are at keeping your temperatures balanced where you want them. Using energy efficient windows will save energy by keeping your appliances like refrigerators, air conditioners, and heaters from running longer than they really need to.
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