Details in the Label: What Do They Say About Your Windows?
It’s important to make wise decisions on your window replacement. To get the best value, you need to consider how the window you spend money on now will help you save money in the future. That’s where the National Fenestration Rating Council (NFRC) sticker labels come in.
The label allows you to compare the performance of different windows when it comes to energy efficiency. An energy-efficient window replacement, after all, plays a big role in keeping your utility bills under control. But what do these measurements tell you, exactly? How do they help you choose the right window? Here is a rundown on each of the ratings you see and what those numbers tell you about your window.
U-factor indicates the flow of heat through windows, measured in Btu/per hour/ft2/°F, which essentially tells you how well the window insulates. The rate of heat transfer generally ranges from 0.25 to 1.25; the lower the value, the less your heat loss. During winter, for example, there is a large temperature difference between the warm air inside and the cool air outside. A cold window’s glass has a tendency to cool the warm interior air adjacent to it. Windows with a lower U-factor allow the inside pane of glass to remain warmer, slowing heat loss.
The Solar Heat Gain Coefficient (SHGC) measures the solar radiant energy that enters through the glass, or the heat gain the window allows into the interior of the house. With values typically ranging from 0.25 to 0.80, it tells you how well the window blocks heat caused by sunlight. The rating refers to directly transmitted solar radiation as well as the fraction of the absorbed radiation that enters as additional heat. The higher the SHGC, the greater the heat gain.
Additional Performance Ratings
Visible Transmittance (VT) values generally range from 0.20 to 0.80, and measures the amount of light that a window lets through. A higher value means more light in your home. Older windows used to have visible transmittance and solar heat gain ratings that were basically the same, but with new technologies, windows can now let in lots of light while still fending off solar heat. Low-E and solar control coatings, for example, are placed on the window’s surfaces to reduce heat gain.
The Air Leakage number is a measurement of the rate at which the window allows air to pass through its frame materials. It is expressed in cubic feet per minute of air passing through a one square foot area of your window. The lower the air leakage value is, the less the air leakage – an important factor to those with homes that are exposed to harsh environmental conditions.
Condensation Resistance is measured on a scale from 0 to 100. The higher the condensation resistance factor, the better it resists water build-up. Condensation appearing on or inside a window indicates a number of problems, including seal failure, and may require replacing the glazing, if not the whole window. Comparing windows and their condensation resistance is another important step in making sure your windows work properly and are able to do so for a long time.
Steve Rumpf is founder and owner of Erin Isle Construction and has spent the last 25+ years working to improve local homes & commercial properties using expert design and installation techniques. Steve and the Erin Isle team are known for their delivery of quality siding, window and deck projects in Portland, Oregon and the surrounding area.