Since windows are a vulnerable spot in the envelope of your home, it should be no surprise that windows can affect how much you spend on your monthly energy bills. There are many features of a window that can determine just how much energy waste or savings they can provide. The most well-known approval rating for window manufacturers comes from ENERGY STAR, which is an international standard for energy-efficiency consumer products.
The energy rating for all ENERGY STAR windows, skylights and doors need to be independently tested and verified according to specific procedures laid out by the National Fenestration Rating Council (NFRC).
What is the NFRC?
NFRC is a non-profit organization that provides rating and labeling to assist consumers in comparing the performance of windows, skylights, and doors. They do not select good or bad windows, set performance standards or provide performance requirements.
ENERGY STAR takes NFRC-certified products and ranks them according to their energy performance and then labels them so consumers can make a wise energy decision.
What are the performance factors used?
The NFRC label is placed on all ENERGY STAR rated windows, skylights, and doors and gives information regarding their performance in five key categories:
- U-Factor tells you the rate at which heat travels through the window, meaning how well does it insulate or resist heat transfer. This varies typically from 0.25 to 1.25 and is measured in Btu/h·ft²·°F. A low U-factor means a better insulating window. A high U-factors means heat can pass through much easier and will cost more to own.
- Solar Heat Gain Coefficient (SHGC) tells you how much solar energy is allowed to pass through the window. This is especially important in sunnier and warmer areas. The sun can add a lot of heat to your home while you are trying to cool it down, but a window with a low SHGC helps block that solar heat gain. SHGC is measured from 0 to 1 and typically ranges from 0.25 to 0.8. A high SHGC means that more solar energy will enter and can have adverse energy effects when you are trying to cool your home. In colder climates, this may not be as big of an issue since more of the time is spent in a heating cycle and solar energy may be a welcome addition to your house when you are trying to heat it up.
- Air Leakage (AL) tells you the rate at which air flows through the joints in your window. It sounds crazy because ideally, you want it to be 0, but there is a small amount of air that passes through all openings, especially if your house is positively or negatively pressurized. The lower the air leakage value, the less air leakage is shown in that window. Many industry standards specify a maximum of 0.3 cfm/ft3. A higher AL means more air can pass through the joints which is bad news for your energy bills.
- Visible Transmittance (VT) tells you the amount of light the window allows through. This is typically measured from 0 to 1, common values ranging from 0.2 to 0.8. The higher the VT, the more light you see and the lower the VT, the less light you see. This isn’t as directly related to energy unless you are considering allowing daylight to be used instead of electrical lighting in your home. But this is a good number to know since an energy efficiency window also needs to allow light in the way that you like.
- Condensation Resistance (CR) tells you how well the window resists the build-up of water. CR is assessed on a scale from 0 to 100, the higher the score, the less build-up the window will allow. The lower the CR, the more water build-up is allowed. This is important because water build-up can be damaging to your window and can help transfer heat where you don’t want it to transfer. Condensation can lead to the rapid decline of your window seals, joints, drywall, and insulation, so look for high CR when selecting your windows.
Windows do a lot for the aesthetic of your home and can also be a major factor in how much your home costs to operate. Be sure to check out the U-factor, SHGC, AL, VT and CR of the windows you are considering to purchase. And most of all, check to see if the windows you are looking at are ENERGY STAR rated.
If you need more help with a window replacement project, be sure to call the Pros at 888-776-1998 or get a free estimate today.