U Factor and R-Value

Window Ratings do use both U values and R factors. The U-factor specifically represents the insulative value of a window, whereas the R-value is used most commonly to rate the efficiency of wall insulation installed throughout the structure. But, the U value can be expressed in terms of R-value:

To convert the U factor into the R-value for windows, you can divide 1.0 by the window’s U factor. Here’s an example of how the simple conversion calculation works:

You are viewing: What Is A Good U Factor For Windows

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If a window has a U factor of 0.20, then the calculation is: 1 / 0.20 = 5. That means that the window’s R value is 5. Note that the quite low U factor in this example represents a commensurately high R-value.

Determining the U Factor

Today’s state-of-the-art double-paned energy-efficient windows can have a U factor of about 0.30 or lower, which indicates that these are especially energy-efficient. High-end triple-pane windows can have a U factor of 0.15, indicating exceptional energy efficiency. Makers of high-efficiency windows have begun adding low-emittance (low-E) coatings and even krypton or argon gas fill between panes in attempts to reduce the U factors even further and produce a more energy-efficient product.

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The U-factor is the quantification of the level of energy efficiency of the entire assembled window, which includes:

• The Window Frame: The insulative value of the entire aluminum or vinyl window frame.
• The Glass Glazing: The energy-efficiency performance of just the glass glazing, not including the frame. This is called the center-of-glass U factor (a less commonly used efficiency rating). Normally, the U factor of the collective assembled parts of an energy-efficient window is higher than the U-factor measured only at the middle of the glass.
• The Spacer: This part of a window frame keeps the glazing panels separated, which can cause a reduction of the U factor along the edges of the glass glazing.

How a Low U Factor Increases Energy Efficiency

The difference in temperature between a building’s interior and exterior generates a (non-solar) flow of heat. This causes windows to lose heat from the inside to the outside in cold weather and gain heat from outside during hot weather. U factor ratings help us in forming standard comparisons and evaluations of energy efficiency levels. For example, windows allowing for the gain of solar heat in the daytime, which indicate a high solar heat gain coefficient, are energy-efficient when combined with a low U factor.

Of course, increasing the energy efficiency of a building requires examining all the construction components, to determine how each is working individually and together with others. So, a proper U factor analysis further evaluates potential changes to improve the U factor as well as other energy efficiency factors, to make the system more energy efficient.

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