You know that 50 miles per gallon (MPG) for a vehicle is excellent, 30 MPG is about average, and 15 MPG is just plain bad.
Ratings for window energy efficiency are similar. Windows are rated numerically in several categories to show if they provide great, average, or poor energy efficiency.
Let’s take a look at these categories right now…
U-Factor measures the rate of heat loss between 0 and 1. The lower the number, the better the window insulates.
For our climate, ENERGY STAR recommends a U-Factor of 0.30 or less. Windows with a U-Factor more than 0.30 won’t retain your home’s heat as well as you need them to, which can lead to higher energy usage.
Our windows have a 0.27 U-Factor*… the optimal rating for the Central VA climate based on ENERGY STAR standards.
SHGC rates how well a window blocks out heat on a scale of 0 to 1. The lower the number, the better the window prevents unwanted outdoor heat from entering your home.
SHGC is most critical in warm climates (Arizona, Texas, etc.). While it’s not quite as significant as U-Factor in the Central VA climate, it’s still an important rating. ENERGY STAR recommends an SHGC rating of 0.32 or lower for our region.
Our windows have a 0.30 SHGC*, which exceeds ENERGY STAR standards for the Central VA climate.
Air Leakage rates the cubic feet of air passes that through a square foot of window area. Air Leakage rating is expressed between 0 and 1, with lower numbers being better.
Poorly constructed windows will allow conditioned air to escape in the summer and cold outdoor air to leak into your home in the winter. The best windows have an Air Leakage rating of 0.30 or less.
Our windows have a 0.30 Air Leakage rating, which ENERGY STAR considers excellent.
This is the rating of how much visible light enters a window’s glass, expressed as a number between 0 and 1. The higher the number, the more visible light the window transmits.
Energy efficiency coatings can block out some light. Some companies that use inferior materials will try to “juice” the efficiency by adding more coatings. But when too many coatings are applied, the windows will start to have an unnatural grayish tint to them. That’s why it’s best to choose windows with high-quality coatings that allow in an ample amount of natural light.
Our windows have a 0.50 Visible Transmittance rating, which means they’re crystal clear.
*Rating is based on double hung windows. Certain types of windows (picture windows, for example) have even greater energy-efficiency properties.
Energy efficient windows consist of a multitude of parts working together to create an effective whole.
Energy efficient windows contain two to three panes of glass. Multi-paned windows provide more insulation for a few reasons. First, multiple panes create more thermal barriers. Second, the space between the panes can be filled with high-density gas that further insulates.
Low-emissivity (Low-E) coating is applied to the glass of a window to reflect infrared and UV light without reducing the amount of visible light allowed through. This lets sunlight into your home without fading your indoor belongings. Low-E coating also reflects heat from both inside and outside your home, which makes your home cooler in the summer and warmer during the winter.
Gas in between the glass panes provides windows with a big efficiency boost. The gas itself is usually krypton or argon. These gases are denser than air (for better insulation) and completely colorless, scentless, and non-toxic.
A quality frame goes a long way to determining a window’s efficiency. The most energy efficient frames have fusion-welded corners and high-density insulation within the frame. These two features are standard in vinyl windows, which make vinyl frames unbeatable for energy efficiency.
Spacers are what hold together the multiple panes of glass in a window. Spacers create insulating air space in which high-density gas (like argon and krypton) can be added. Be sure to get windows with non-metallic and metal/non-metal hybrid spacers, as these provide the best insulation and conduction.
The most energy efficient window in the world will fail if the contractor doesn’t install it right. Improper installation leads to air and water leaks, which can cause poor efficiency, higher energy bills, and potential water damage. It’s vital to select a contractor with a stellar installation track record. Doing so will ensure your windows are tight, straight, and invulnerable to leaks.
All reputable window brands have their windows rated by the National Fenestration Rating Council (NFRC), which is a third-party non-profit organization. This lets you, the consumer, know exactly how efficient a window is. Now that you’ve read this page, you can read this label!
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