Volume 2   Issue 3               Fall 2001

t h e   r a t e   d e b a t e

Hot Weather, Cool Savings
by Jim Benney

Hot town, summer in the city. That song rings true in the summer as the sun blazes down with its greatest intensity, blasting the earth with waves of energy. Those waves of heat move straight through windows, of course, unless you live in a home or work in a building equipped with high-performance windows that can block them. 

The glass industry has been working for decades with coatings that let sunlight in but not the heat that comes with it. These technologies, known as spectrally selective glasses, allow different percentages of the solar spectrum to pass.

Several different wavelengths of the sun’s energy are relevant to the way a window operates and performs. Because of their different lengths, the following waves have different characteristics:

• Ultraviolet (UV) rays have a relatively short wavelength and can fade furniture, drapes and other furnishings (as well as cause skin cancer);
• Visible light rays let us see the outside through the window and are of medium length;
• The long wavelengths of infrared (IR) energy bring heat into a home as much as they heat cars left out in the sun even on cold winter days.

Let in the Light, Keep Out the Heat

Typical clear glass always allows the same percentage of the sun’s energy through the window and into a room. This includes the heat waves associated with IR energy that re-radiates off nearby driveways and streets. A glass fabricator, however, can use thin films or coatings on glass that act like a filter—screening out more of the IR or UV wavelengths, while transmitting visible light. 

Spectrally selective glass and glass coatings have been used in commercial applications for years, where cooling loads typically dominate energy usage. Some of these products made inroads into the residential market, but were undesirable generally due to the coloring or tint. More recently, clear, low-solar-gain glass has been introduced (typically low-E coatings in sealed insulating glass units). 

IDEAL SPECTRAL CURVE FOR GLASS

uv|visible|ir


UV, visible light and IR waves all
 have different characteristics.

The amount of solar-heat gain allowed through a window is measured in terms of the solar-heat gain coefficient (SHGC). The lower the SHGC, the less solar-heat gain is admitted through a window. Products are rated for SHGC in accordance with the National Fenestration Rating Council’s (NFRC) standard 200—“Procedure for Determining Fenestration Product Solar Heat Gain and Visible Transmittance at Normal Incidence.” 

The availability of low-solar-gain glass products has made an impression on code developers in the South and West. California’s residential energy code now requires that windows meet or exceed a SHGC of 0.40 in 11 of its 16 climate zones. Other states following suit are Texas, North Carolina, South Carolina, Florida, Arizona and Georgia. 

How do you tell if your windows meet these requirements? Simply look for the NFRC label that provides a certified SHGC performance rating. Or look in the NFRC Certified Products Directory (which is now available online at www.nfrc.org). It is important to note that many manufacturers take advantage of the NFRC Specialty Products Table when obtaining certified SHGC ratings. The table offers a simplified, calculated approach for determining a product’s performance.

The use of NFRC-certified energy-efficient windows not only provides long-term returns on your investment from reduced energy bills, but it also increases the home’s comfort, beauty and value. 

Jim Benney is director of education at the National Fenestration Rating Council. You may e-mail him at 
jbenney@nfrc.org.


DWM
© Copyright 2001 Key Communications Inc. All rights reserved. No reproduction of any type without expressed written permission.