U-Factor Ratings for Windows

by Nick Gromicko, CMI® 
When quantifying the energy efficiency of a window assembly, the rate of loss of non-solar heat can be expressed as its U-factor (or U-value).  Understanding the U-factors of windows is helpful for inspectors performing energy audits, as well as for consumers planning a new build or updating a house with energy-efficient windows. 
U-Factor or R-Value?
While windows are rated using both U-factors and R-values, the U-factor is used to express the insulative value specifically of windows, while the R-value is used primarily to rate the energy efficiency of insulation installed in other areas of the building envelope, such as beneath the roof, in the attic, behind the walls, and beneath the floors.  In order to translate a window's U-factor into its R-value, divide 1 by the U-factor.  For example, a window with a U-factor of 0.25 is calculated as 1 ÷ 0.25 = 4, so the same window has an R-value of 4.
What is the U-Factor?
The U-factor rating system was devised by the National Fenestration RatingAn example of an NFRC-certified product label Council (NFRC).  The NFRC is a non-profit group that administers a uniform, independent rating and labeling system for the energy efficiency of building components, including windows, doors, skylights and attachment products.  The U.S. Department of Energy and the Environmental Protection Agency’s Energy Star Program take the U-factor into account when evaluating the energy efficiency of windows for product certifications, and federal incentive and rebate programs.
Windows that have the best resistance to heat flow and, thus, the best insulating qualities, have a low U-factor.  Less efficient windows with poor insulating ability have a high U-factor.  The combination of a window’s U-factor, air leakage, sunlight transmittance, and solar heat-gain coefficient add up to determine its level of energy efficiency. 

The temperature difference between the interior and exterior of a building creates the non-solar heat flow that results in windows losing heat to the outside during the winter, and gaining heat from outside during the summer.  Compensating for this by cranking the thermostat or turning up the AC results in added energy needs and higher bills.  Greater energy efficiency calls for a closer examination of the individual building components to see how they can work individually and in relation to each other in more effective ways.  U-factor ratings can help in formulating standardized comparisons and objective evaluations.

Determining the U-Factor

The U-factor generally refers to the energy efficiency of the complete window assembly, which includes the glazing, window frame and spacer.  The spacer is the component of a window frame that separates the glazing panels, and often reduces the U-factor at the glazing edges.  The performance rating of the glazing alone, independent of the frame, is known as the center-of-glass U-factor, but use of this rating is less common.  For most energy-efficient windows, the U-factor for the entire window assembly is higher than the U-factor at the center of the glass.

The best, high-performance, double-pane windows may have a U-factor of 0.30 or lower, indicating that they are very energy-efficient.  Some triple-pane windows may have a U-factor as low as 0.15.  Manufacturers have started to incorporate low-emittance coatings and gas fills between panes in attempts to further decrease U-factors and provide an even more energy-efficient product.

U-Factors in Different Climates

While beneficial in cooling-dominated climates, a low U-factor is most important for windows in heating-dominated climates.  The following are recommendations for the most effective window U-factors based on the major climate zones in the United States.

  • In colder climates in the North that are heating-dominated, the U-factor should be less than or equal to 0.30 for windows, and less than or equal to 0.55 for skylights.  In areas where air-conditioning needs are minimal, windows that allow for solar heat gain during the day (a solar heat-gain coefficient of 0.40 or higher) can be considered energy-efficient with a U-factor as high as 0.32.  Low U-factor windows are most important and will be most effective in this colder climate area where minimizing heat loss is critical to energy efficiency.  

  • In mixed climates in the North and Midwest regions that use both heating and cooling, the U-factor should be less than or equal to 0.32 for windows, and less than or equal to 0.55 for skylights. Heating bills can help determine the importance of U-factors in this climate. Higher bills indicate the importance windows with a lower U-factor for added energy efficiency.

  • In mixed climates in the South and central regions that use both heating and cooling, the U-factor should be less than or equal to 0.35 for windows, and less than or equal to 0.57 for skylights.  In these climates, again, heating costs can determine if a lower U-factor could be beneficial and more energy-efficient.  If costs are high and a list of factors for heat loss is being addressed, window U-factor can be taken into consideration.  A low U-factor for windows can also be helpful during hotter seasons when it is important to keep heat out, though a low solar heat-gain coefficient is more important in such situations.

  • In hot climates in the South that are cooling-dominated, the U-factor can be less than or equal to 0.60 for windows, and less than or equal to 0.70 for skylights.  A lower U-factor is still useful during any cold times of the year when heating is needed in this climate.  Such low ratings can ensure that heat is kept out on hot days when combined with a low solar heat-gain coefficient, which is the most important consideration in this climate.
Understanding the function and rating criteria for U-factors is a helpful tool for inspectors who perform energy audits.  They can then pass this information along to their clients who may have questions about their windows and their home's overall energy efficiency.

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