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Understand Window Energy Ratings: U-Value, Low-E Glass & SHGC


Going GREEN is one of the hottest trends in America and having energy efficient windows in your home is quickly becoming a must-have for many people.

Not only is the “green” movement near the top of many political agendas, its presence is often seen throughout the mainstream and minor media. The bottom line is this: green sells and going green is cool.

This trend is positive. With energy costs reaching new heights, we need to be proactive and intentional. We should not only reduce the amount of energy we use, but also conserve the energy we don’t. And since nearly 15-20% of all home energy is lost via windows and doors, having energy efficient windows in your home should be a priority.

The focus of this hub is to help explain (without getting too technical) the current energy ratings scheduled to today’s energy efficient windows. These ratings are crucial in making informed decisions regarding the purchase of energy efficient windows for the new home construction or replacement.

With the green trend gaining momentum, an informed decision concerning one of the most critical areas of energy loss (or conservation) in your home is paramount.

Below are a few terms needing definition.

Two or more individual panes of glass separated by a specified spacer bar system and then sealed to be air and water tight. The “captured” airspace between the panes of glass forms the insulating barrier. The majority of modern energy efficient window systems utilize some type of insulated glass (IG) application.


Emissivity is the capability of a surface to emit heat radiation. A black surface is often used as a constant in measuring other surfaces against it.

For example, in measuring the emissivity of a particular IG unit, the IG unit is placed next to a solid black surface and subjected to an identical heat source. Measurements of heat radiated from each surface are then taken. The lower the number results in better heat-reflecting capability.

With relation to energy efficient window systems, lower emissivities are desired.


U-Value is the measure of a window’s ability to reduce heat loss during indirect radiation exposure; such as during the winter months in moderating climates. Lower U-values translate into less indirect heat lost from the interior of the home resulting in lower heating costs.

U-value is the inverse of R-value (a more common term used in the insulation business). To find a correlating R-value from a given U-value, simply divide the number 1 by the U-value. Lower U-values correlate to higher R-values. For example: 1 divided by a .50 U-value gives us an R-value of 2.00.

Lower U-values are important because many municipalities are adopting the 2006 version of the International Residential Code (IRC 06 for short). This code mandates all energy efficient window and exterior door units with IG to carry a minimum U-Value of .40, translating to an R-value of 2.5.

This may sound like a low insulating value but even the finest energy efficient windows today carry U-values hovering in the .22 through .30 ranges; thus correlating to R-values of 4.55 through 3.33.

And given the fact most exterior wall cavities are a minimum R-13 (with standard 2 x 4 framing) to R-19 (with 2 x 6 framing), the most energy efficient windows are 3 to 4 times less efficient than the wall they’re installed in; thus the significance to upgrade building codes with regard to window glazing applications.

So, even with the large disparity between the wall vs. window insulating factor, improving U-values greatly increases the energy efficiency of the home.


Solar Heat Gain Coefficient (SHGC) is a measure of a window’s ability to reduce heat gain during direct radiation exposure; such as during the summer months in warmer climates. A lower SHGC translates into less direct heat being pulled into the home resulting in lower cooling costs. SHGC and U-value are closely linked since the lowering of one directly affects the other.

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