One of the most useful figures for quantifying the energy efficiency of windows is the U-factor. Also known as the U-value, this is a number that consumers can use to determine the energy efficiency of windows they are considering.
What Is U-Factor In Windows?
The U-factor is a rating system that was designed by the National Fenestration Rating Council, or NFRC, a nonprofit group responsible for administering independent and uniform ratings and labeling systems for the energy efficiency of various components of buildings.
It expresses the window’s insulation value, or how much heat passes through the windows. The US Department of Energy and the Environmental Protection Agency’s Energy Star program considers this important figure to be one of the factors for determining its window certification.
When it comes to the U-factor, less is more. Windows that boast the best resistance to the flow of heat and have the best insulating qualities are given a lower U-factor, while less-efficient windows that are not good at insulating have a higher U-factor. The U-factor is combined with a window’s solar heat gain coefficient, sunlight transmittance and air leakage to determine its overall energy efficiency level.
How Is U-Factor In Windows Determined?
The U-factor is given to the entire window assembly’s energy efficiency. This includes the window’s frame and spacers as well as its glazing. The performance rating of the glazing itself without the frame is a separate figure that is referred to as the center-of-glass U-factor. In most energy-efficient windows, the overall U-factor for the full assembly will be higher than the U-factor of the center of the glass.
The U-factor of a window is calculated using a formula that factors in British thermal units, feet and degrees Fahrenheit. The window must be tested and pass the verification and certification processes carried out by the NRFC before it can earn an NRFC sticker that lists performance ratings such as its U-factor.
This sticker will also provide other helpful energy efficiency information about the window, including its condensation resistance, or how well the window stops a buildup of water; its visible transmittance, or how much light is reflected into the home; its air leakage; and its solar heat gain coefficient, or how well it keeps out natural heat from the sun.
As a general rule, a top-of-the-line high-performance double pane window should have a U factor that is 0.30 or lower. Some triple pane windows may even boast U-factors as low as 0.15. Manufacturers are increasingly taking steps to bring the U-factor of their windows down to make their products more energy efficient, such as incorporating low-emissivity coatings and gas fillings between window panes.
How Is A U-Factor Are Affected by Climate
Although the U-factor can be boiled down to a simple rule of thumb that lower is always better, there are recommendations for the most effective U-factors depending on the climate zone in which you live.
For colder climates in the northern part of the country where heaters are relied on heavily in the winter months, the U-factor should be 0.30 or lower for windows and 0.55 or lower for skylights. For homes in this climate zone, having windows with a low U-factor is particularly important as minimizing heat loss is crucial for achieving maximum energy efficiency.
In mixed climates in the northern and Midwestern areas of the country that use both cooling and heating, the U-factor should be 0.32 or less for windows and 0.55 or less for skylights. Meanwhile, those in mixed climates in the central and southern regions of the country that use both heating and cooling should aim for windows with a U-factor of 0.35 or less and skylights with a U-factor of 0.57 or less.
The situation is a bit different for homes in the hotter climates of the south that are largely cooling-dominated. In these areas, homeowners should aim for a U-factor of 0.60 or less for Windows and 0.70 or less for skylights. However, a lower U-factor is still important here during colder times of the year when heaters may be needed.
Is the U-Factor the Same as the R-Value?
When it comes to home construction and renovation, another figure you may encounter is the R-value. Like the U-factor, this also refers to heat flow resistance and insulation, but it applies to the insulation of the roof, floors and walls rather than the windows.
Moreover, unlike the U-factor, higher figures indicate greater insulation when it comes to R-value. Nevertheless, both the U-factor and R-factor are useful numbers to take into account when determining how well insulated your overall property is.
Contact the Experienced Window Replacement Contractors
The right windows can help make your home more comfortable year-round, increase its curb appeal and lower your electricity bills. To learn more about low U-factor window options, contact us for residential window replacement and installation at Adelphia Exteriors.