This article was originally published in the July/August 1993 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.



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Home Energy Magazine Online July/August 1993



Trends in Energy is a bulletin of residential energy conservation issues. It covers items ranging from the latest policy issues to the newest energy technologies. If you have items that would be of interest, please send them to: Trends Department, Home Energy, 2124 Kittredge St., No. 95, Berkeley, CA 94704.



Out The Window with U-Guessed-It Values

Thermal performance in a window is expressed as a U-value, a measure of heat transfer through it--the greater the heat loss, the higher the U-value. Until recently, there were several ways to measure a given window's U-value. The problem was that many window manufacturers could present the most complimentary U-values to market their products, whether scientifically derived or not. Particularly contentious have been ways to measure the energy impact of frames. For any given window, consumers were left guessing its real cost-effectiveness and building inspectors pondering its impact on energy code compliance.

Now a uniform process for determining window thermal performance has been implemented in California. It fits into the state's stringent building energy compliance regulations written and updated by the California Energy Commission (CEC). To oversee the window rating and certification process, the CEC has chosen the National Fenestration Rating Council (NFRC), a national non-profit organization composed of window manufacturers, researchers, utilities, and state energy offices.

The CEC now requires window manufacturers and dealers to affix a label on each product declaring its U-value. In each case, a uniform set of procedures that cover fenestration products--windows, skylights, and glass doors--determine the content of the label. For manufacturers who chose to label their products, the validity of each label is backed by an NFRC certification process for testing and products.

Advances in energy-conserving windows make uniform ratings important. Factors like low-emissivity coatings, inert gas fills, vinyl frames, and thermal breaks in the frames have led to windows with lower U-values, which improve the overall energy performance of new homes. In the new world of uniform window ratings, a manufacturer can choose to have products tested and labeled, not tested but still labeled, or not labeled at all (see Table 1).

In the first case--labels with testing--the manufacturer sends window specifications and drawings to an NFRC-accredited simulation laboratory for modelling the thermal performance, then actual samples to an NFRC-accredited test laboratory to validate the simulated U-values. If the models and test results correspond with each other, the manufacturer receives simulated U-values for all windows in the product line. The maker then affixes both temporary and permanent labels displaying U-values and other information. (California already requires certification of maximum air infiltration.) This procedure makes the U-values of high performance windows stand out.

In the second case--labels, but no testing--the manufacturer compares basic design characteristics of a product line to a default table developed by the CEC. A temporary label affixed to the window displays a U-value corresponding to a figure from the table. The default table values are typically conservative to ensure valid building energy budget calculations by designers and compliance officials.

In the last case--no labeling--a designer may take the U-values needed for calculating an energy budget from a special default table--with the highest conceivable U-values--for windows whose makers are not interested in presenting the U-value. They may be manufacturers of the least expensive windows, or builders who make custom windows for new homes or alterations.

Ratings are advantageous to manufacturers who have sought a fair system for years. Distributors will find that windows with lower U-values will be in demand. Building design professionals will discover that ratings will hasten house plans through the building code compliance process. Labeling also helps building code enforcement agencies easily compare building specifications with what's installed.

Not all manufacturers have been pleased with the new system. NFRC is testing for only one aspect of energy performance, said Randall Ward of the Aluminum Window Association. The frame is a small part. Others factors are not recognized, like durability. Window thermal performance goes down as a window deteriorates. Other factors will balance thermal performance for rating windows.

Things were supposed to be up and running a year ago, added Bob Raymer of the California Building Industries Association. Manufacturers were supposed to be able to go through the process long before now. Window makers made capital outlays for a transition to new regulations that didn't begin when expected, he explained.

In response to both concerns, the CEC granted aluminum window manufacturers a six-month breathing space, ending in July, during which they were temporarily allowed to utilize more liberal U-values. They are now subject to the same parameters as other window manufacturers.

Dariush Arasteh, who is a leading window energy researcher at Lawrence Berkeley Laboratory, helped shape the NFRC procedures. When the consumer sees the NFRC label, he explained, the window will be as good or exact as the rigorous certification process behind it. That includes in-plant inspections, accreditations of labs, and third party verifications of testing. While many states are participating at some level in a national NFRC rating scheme, Arasteh sees universal adoption in the not-too-distant future. The U.S. Department of Housing and Urban Development is also considering referencing NFRC for manufactured housing standards, he added. The NFRC-based ratings will soon encompass factors other than heat transfer, such as durability, solar heat gain, visual transmittance, and air infiltration.

California's labeling program does not prohibit manufacturers from making any kind of window they want though there probably will not be a large market for low-performance units as a result of the program, said Nehemiah Stone of the CEC who was responsible for setting up the rating system.

Late in 1992, the CEC, in league with six California utilities, launched the window labeling program by offering cash rebates to manufacturers located in the state--up to $1,000 per product line for tests performed in California and a lesser amount for products tested by out-of-state laboratories. The incentives, totalling $1.2 million, were intended to ease the financial burden of testing for smaller companies.

CEC has a hotline for inquiries about its window rating and certification program: (916)654-5106. Readers can also contact the NFRC at 962 Wayne Ave., Suite 750, Silver Spring, MD 20910. Tel: (301)589-6372;

Fax: (301)588-0854.

-- Jim Obst

Labels for Windows in California If manufacturer Label information Temporary label required? Permanent label Benefits chooses ... determined ... required? and drawbacks certify ...for thermal Yes Yes * Distinguishes according performance, by higher- to NFRC (1) NFRC-accredited Displays Displays performance simulation and * certified U-value (3) * information windows. procedures testing needed to * Manufacturer ...for air * special characteristics replace window pays for infiltration, by affecting energy * certification rating. CEC-approved performance of maximum air * Product line table (2) * certification of maximum leakage rate testing is air leakage rate cheaper than testing each May be removed after energy product. compliance building inspection. label default Yes No * No values lower than without NFRC table from the tested types. process CEC Residential Displays * Table renders conservative Manual with * U-value values. generic U-values, * special characteristics * Rating, even though not as with adjustments affecting performance good as models tested by for some energy- (optional) NFRC, facilitates calculating saving * certification for maximum building energy budget. characteristics air leakage rate * No testing fees. May be removed after energy compliance building inspection. ...not to label No label, so No No * Thermal and infiltration * thermal characteristics are assumed performance to be much higher than products assumed according tested and those not tested to another default but rated. table to which * Attractiveness of low price features cannot may offset low efficiency be added in the marketplace. * air infiltration determined by CEC-approved table according to classification of window build custom No label, so thermal Optional No * Appropriate for sites prohibiting windows or performance assumed Affixed by builder manufactured windows. alterations according to another * Must meet certain caulking and (applies to CEC default table weatherstripping requirements. builders) * Assumed U-values are the poorest and unreliable for energy budget calculations. Notes: 1. National Fenestration Rating Council. 2. California Energy Commission. Air infiltration values derived from American Society for Testing and Materials test method. 3. A measure of a window's heat transfer properties. U-value = [Btu/hr-ft(2)(oFindoor-oFoutdoor)]

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