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January - February 2003

From A-Z
Everything Builders Need to Know about Selecting Energy-Efficient Windows
by Jim Bennney

We live in a world where the technology around us is changing constantly. New ideas, new products and new applications fill the showrooms with improved window, door and skylight products. Consumers become more informed and more demanding—demanding “the best.” It is in this complex world that builders must strive to provide their customers (home buyers) with fenestration products that are functional, cost-effective and energy efficient. How can a builder possibly keep up-to-date with the latest technology? How can the builder know how well a window system may actually perform? The following tips are designed to help builders answer these questions.

The National Fenestration Rating Council (NFRC) is a nonprofit organization whose goal is to provide uniform, accurate information about the energy performance of windows, doors and skylights. In addition to publisheding consensus standards (for consistent ratings), NFRC administers a third-party certification and labeling program to provide the builders with verified product information. The NFRC performance label provides credible information on the U-factor, solar heat gain and visible transmittance of each product. The NFRC label assures builders that they are getting the performance that has been specified for a building; either to assure energy efficiency or to meet the local code requirements. 

The two most important energy ratings are U-factor and Solar Heat Gain Coefficient (SHGC). By reviewing the label information, builders and other consumers make an informed choice about the product that is better for their individual situation.
What is U-factor?
U-factor is also known as thermal transmission. It is a measure of the rate of heat loss through a product, therefore, the lower the U-factor, the lower the amount of heat loss. In climates where heating bills are a major concern, choosing windows with lower U-factors will reduce the amount of heat that escapes through a window from inside a house. Typical energy code requirements in the United States reference U-factors from as low as 0.35 in the north to 0.75 in the south. 

What is Solar Heat Gain Coefficient?
The SHGC measures the rate of heat gain through a product. Therefore, the lower the SHGC, the lower the amount of solar heat gain. In climates where air-conditioning bills are a major concern, choosing windows with lower SHGCs will reduce the amount of heat that comes in through your windows from the outside. Energy codes reference a maximum SHGC of 0.40 in many Southern states. 

The U.S. Department of Energy and Environmental Protection Agency have developed an ENERGY STAR designation for products meeting certain energy performance criteria. Since the energy-efficiency performance of windows, doors and skylights can vary by climate, product recommendations are given for three climate zones: a mostly heating zone, a mixed heating and cooling zone; and a mostly cooling zone. Builders that require ENERGY STAR windows can be assured of getting energy-efficient fenestration products.
In addition to ENERGY STAR and NFRC ratings, there are a number of other considerations when choosing windows. This information includes: installation, air infiltration, water infiltration, structural performance, acoustical performance, security performance, product cost and warranty. Product cost and warranties are issues that should always be considered when choosing windows, doors and skylights, and this information should be available from any window distributor or manufacturer.

Proper installation is essential for windows, doors and skylights to function as designed. Be sure that the windows have been installed plumb, square and level; and that the proper flashing and sealing techniques have been followed, as recommended by the manufacturer. In addition, an ASTM standard has been approved for window installation.

Air infiltration measures the amount of air that leaks into your home from the outside. The lower the air leakage rate, the less air is exchanged between outside and inside the window. Typical energy codes set maximum air infiltration rates at 0.30 cfm/ft2. NFRC 4300 is one standard for determining air infiltration. In addition, there is an ASTM standard for testing air infiltration (E283). 
Water infiltration measures the amount of water and pressure that a window can resist to keep the water from leaking through it. The higher the water infiltration rating, the better the window is at restricting water leakage. The ASTM standard for determining the ability of a window to resist water infiltration is ASTM E547 (Test Method for Water Penetration of Exterior Windows, Skylights, Doors and Curtain Walls by Cyclic Static Air Pressure Difference).

Structural performance ratings measure the amount of air pressure (wind load) a window can resist before failing. The amount of structural pressure ratings required for windows in your area is often determined by local code requirements. The higher the structural performance ratings, the higher the wind load a window can resist. The ASTM test method for determining structural performance for windows is ASTM E330 (Test Method for Structural Performance of Exterior Windows, Skylights, Doors and Curtain Walls by Static Air Pressure Difference).

It should be noted that the International Residential Code references the use of AAMA/WDMA 101/I.S.2 for determining the structural performance of fenestration window systems.

Acoustical performance ratings measure the amount of sound transmission through a window. The higher the sound transmission rating, the better the product is at blocking noise from coming through the window. The ASTM standard for determining acoustical performance is ASTM E1425 (Practice for Determining the Acoustical Performance of Exterior Windows and Doors).
Security performance ratings measure the ability of a window to resist different types of forces. For example, there are burglar-resistant windows, fire-resistant windows, bullet-resistant windows, forced-entry resistance, wind-borne debris-resistant windows and many others. Many of these products have special uses for different building types and may be covered by local building code requirements. 

NFRC has additional information for selecting energy-efficient windows on its website at www.nfrc.org

Builder’s Resource Center

Whether homebuilders have a question about building issues, window and door standards, energy efficiency or glass options, there are a variety of resources available. Here are some of the resources on hand:

American Architectural 
Manufacturers Association
847/303-5664 (P) 

American Institute of Architects
800/242-3837 (P)

Canadian Window and Door Manufacturers Association
866/871-8885 (P)

Efficient Windows Collaborative
202/530-2231 (P)

ENERGY STAR®/U.S. Environmental Protection Agency 
888/782-7937 (P)

Glass Association of North America
785/271-0208 (P)

National Association of Home Builders
800/368-5242 (P)

National Fenestration Rating Council
301/589-1776 (P)

National Kitchen & Bath Association
877/652-2776 (P)

National Sash & Door Jobbers Association
727/372-3665 (P)

Northeast Window & Door Association
609/799-4900 (P)

Window & Door Manufacturers Association
847/299-5200 (P)

Also, for the most comprehensive information on glass and related products check out www.glass.com.

Jim Benney is director of education at the National Fenestration Rating Council, based in Silver Spring, Md. 

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