A profusion of gases, glazings, and gap sizes are among the factors that confound efforts to measure the energy performance of a window or skylight.
The increasing variety of efficiency-enhancing options for windows and their frames poses a formidable challenge to builders, utilities, code officials, and consumers. Fortunately, a new system for accurately rating and labeling these products promises to help demystify them and to foster nationwide improvements in energy efficiency.
Window trade groups have historically organized around specific materials or components (such as glass or frames), and energy has rarely been their focal point. This changed in 1989 with the formation of the National Fenestration Rating Council. One impetus behind the industry's collaboration with builders, utilities and regulators in establishing the NFRC was the emergence of disparate, mandatory state energy certification and labeling standards for windows. The specter of a national patchwork of nonuniform requirements prompted the industry trade groups to help form the NFRC and to help devise a single national system.
Environmental labeling is especially useful when an important attribute is not visible to the naked eye. These two cans of tuna appear identical, but for one it is certified that no dolphins died in the nets when the tuna were caught. Labels can also tell consumers how two windows that appear identical have very different energy performance.
The Energy Policy Act of 1992 charged NFRC with developing a national labeling and rating program. The Department of Energy and the Federal Trade Commission are empowered to step in and create standards in the absence of industry action. NFRC has already made progress: California, Washington, and Oregon now require windows to be rated and labeled using the NFRC method. Building codes in Idaho, Alaska, and Minnesota have adopted NFRC values, and a dozen other states are considering them or are in the final stages of adoption.
LBL's Windows and Daylighting Group played a large role in helping NFRC's technical committee establish credible methods for determining window properties and creating a low-cost rating procedure. The University of Massachusetts, the Florida Solar Energy Center, and Canada's Energy Mines and Resources also contributed to the technical work.
One LBL contribution to the NFRC process is a software package called WINDOW 4.1, the computational engine behind the NFRC labels. Based on the target window's physical properties, WINDOW 4.1 calculates the total U-value, solar heat gain coefficient, shading coefficient, and visible transmittance, accounting for complex heat-flow interactions. Correcting for factors such as heat loss through frames can, for example, reclassify a super-efficient R-8 glazing (U=0.125) in a poor frame to a whole window value of less than R-4 (U=0.25). The program models specific window types, such as picture, casement, or horizontal slider. Future versions will also model doors and skylights. In addition to being used for rating and labeling, WINDOW 4.1 is a powerful tool for designing prototypical windows from an electronic inventory of glasses, gases, gap widths, coatings, and frame materials.
The WINDOW 4.1 program (version 3.0 shown below) enables a window manufacturer to substitute expensive laboratory tests of thermal performance with computer simulations. A single window test can cost more than $1,000.
A clear benefit of the NFRC approach is that rating a window's optical and energy characteristics using a computer program is less expensive for manufacturers than laboratory testing. This makes it easier to perform "tests" on a diverse product line and eliminate uncertainties introduced by both errors or "noise" in test procedures and differences from one test lab to another. These complications previously prevented the reliable comparison of one window product to another. NFRC will conduct annual quality-control inspections of institutions that test windows or develop energy ratings.
The next research challenge is to extend labels from showing simple properties to include estimated energy and economic savings. This work will use LBL's DOE-2 program to differentiate among operating conditions that vary regionally, such as climate and energy costs.
The DOE sees NFRC's success as an important breakthrough, and the NFRC experience is now spawning other nonfederal labeling initiatives. Notably, the Home Energy Rating Systems council and the Council on Office Products Energy Efficiency are contemplating similar strategies.
International groups are coming to LBL to learn how to create window rating systems for their own countries modeled after the NFRC's. One recent example is Valery Tishenko, head of Building Standards at the Russian Construction Ministry (Gosstroy), who wrote DOE expressing interest in improving his country's certification of construction technologies, particularly windows. He asked for help in transferring the NFRC rating procedures and computer programs to Russia.
As the first step in this exchange, three visitors from Russia spent several weeks in February working with Dariush Arasteh, Charlie Huizenga, and other members of the Windows and Daylighting Group to translate LBL's WINDOW 4.1 computer program into Russian. WINDOW 4.1 is the basis for the U.S. window energy ratings system under development by the NFRC.
Alexander Spiridonov, the project leader, and programmers Vladimir Chernorutsky and Michael Vilinsky are from the Sol Company and the Gosstroy Institute. Their work is expected to form the basis of a window rating system for Russia. The visit was funded by the DOE's Office of Building Technologies. Meanwhile, LBL staff, acting as NFRC representatives, trained the Russian programmers as Certified Window Rating Simulators. NFRC plans to complete certification of the window test facilities at Gosstroy's Building Physics Research Institute later this spring.
In February, a meeting of the International Energy Agency at LBL examined window energy-efficiency and rating systems. Representatives of several European countries and Australia looked into adopting parts of WINDOW 4.1 and the NFRC process.
Windows and Daylighting Group
National Fenestration Rating Council
1300 Spring Street, Suite 120
Silver Spring, MD 20910
(301) 589-NFRC; (301) 588-0854 fax
WINDOW 4.1 is available from NFRC or Bostik Construction Products: (800) 523-6530.
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