The Federal Energy Management Program (FEMP) provides acquisition guidance for residential windows, doors, and skylights, which are an ENERGY STAR-qualified product category. Federal laws and executive orders mandate that agencies meet these efficiency requirements in all procurement and acquisition actions that are not specifically exempted by law.
Most manufacturers display the ENERGY STAR label on complying models. For a model not displaying this label, check the manufacturer's literature to determine if it meets the efficiency requirements outlined by ENERGY STAR.
Performance Requirements for Federal Purchases
For the most up-to-date efficiency levels required by ENERGY STAR, look for the ENERGY STAR label or visit the ENERGY STAR Product Specifications website. For more information, contact the FEMP point of contact listed at the bottom of this page.
Buying Residential Windows, Doors, and Skylights
Fenestration products, such as windows, doors, and skylights, do not consume energy directly. They do, however, add to the heating and cooling loads of the buildings in which they are installed. Selecting and installing energy-efficient windows, doors, and skylights can minimize these additional space conditioning loads and reduce the amount of energy used by the building's heating and cooling system.
Unlike many products, the energy efficiency of fenestration products varies according to climate. Features that make windows energy efficient in one type of climate may offer little benefit in another. The energy efficiency of fenestration products is primarily a function of the U-factor and solar heat gain coefficient (SHGC). When choosing fenestration products, first identify the climate zone where they will be installed and then find products with U-factors and SHGCs that are less than or equal to those specified by ENERGY STAR.
All qualifying units must be certified and labeled by the National Fenestration Rating Council (NFRC). When buying fenestration products through commercial sources, choose those that are ENERGY STAR-qualified for the climate zone where they will be installed. When fenestration products are being provided as part of a construction or renovation contract, specify the U-factor and SHGC for the appropriate climate zone.
The Federal supply source for windows is the General Services Administration (GSA), which offers them through its Federal Supply Catalog "FSC 56 - Building Materials (563 24)." Not all windows sold by GSA are ENERGY STAR-qualified, and some products that do qualify may not be indicated as such. When buying windows from this source, check models against the ENERGY STAR website or the NFRC Certified Products Directory.
These requirements apply to all forms of procurement, including guide and project specifications; construction, renovation, repair, energy service, operation and maintenance (O&M) contracts; lease agreements; and solicitations for offers. Energy performance requirements should be included in all evaluations of solicitation responses. Buyers shall insert the standard clause from FAR section 52.223-15 into contracts and solicitations that deliver, acquire, furnish, or specify energy-consuming products for use in Federal facilities. Agencies can claim an exception to these requirements through a written finding that no ENERGY STAR-qualified or FEMP-designated product is life-cycle cost effective for a specific application.
To select or specify energy-efficient windows properly, Federal buyers must be familiar with the following terms and strategies.
U-Factor: The heat lost or gained through fenestration can have a substantial impact on the energy required to condition a building. Heat transfer through fenestration is a function of the difference between indoor and outdoor temperatures and the U-factor (thermal transmittance) of the glazing system. The U-factor is a measure of the rate of heat flow through glazing products; the lower a U-factor, the less heat will flow through the window. In the United States, U-Factors are reported in Btu/(h·ft?·°F), and typically range between 0.2 to 1.2 Btu/(h·ft?·°F).
Energy-saving features that reduce the thermal transmittance and lower the U-factor include increasing the number of panes to two or more, adding low emittance (low-e) coatings to glazing layers, filling the space between panes with either argon or krypton gas, and constructing the frames and sashes with non-thermally conductive materials such as wood, vinyl, or fiberglass. If the frame or sash component is mostly metal, it needs to have carefully integrated thermal breaks to minimize conductive heat flow.
Solar Heat Gain Coefficient (SHGC): Fenestrations can allow significant amount of solar irradiation, or heat, into buildings. SHGC is a measure of heat transmitted through glass and theoretically varies between 0 and 1, where the lower the number, the less heat is transmitted. The SHGC of actual fenestration products is typically between 0.20 and 0.80. Features impacting SHGC are low-e coatings and tints. Fenestration products with low-e coatings are available in low, moderate, and high solar gain. Low gain (less than 0.4) products are designed for hot climate zones whereas high gain products (greater than 0.55) are designed for cold climate zones. Although tinting can reduce solar heat gain, it also reduces the ability to see clearly through a glazing unit. Because of this, tinting is not recommended for residential buildings. Exterior overhangs or sunscreens and interior blinds or shades can also be used to effectively reduce undesirable solar heat gain.
Visible Transmittance (VT): Visible transmittance is a measure of the amount of energy within the visible spectrum (light) that passes through a glazing unit. Although similar to SHGC, VT indicates how much daylight is transmitted whereas SHGC is a measurement of the amount of heat transmitted. The NFRC Certified Products Directory and NFRC labels typically report VT, which is expressed as a value from 0 to 1. The VT of actual fenestration products, which includes opaque areas such as the frame and sash, varies between 0.10 and 0.80. A higher value indicates that a greater percentage of light is transmitted through the window. Typically, high values are preferred in residential applications. VT values below 0.5 noticeably reduce the amount of light transmitted. When purchasing windows with a low SHGC, verify that the VT is not too low.
Air Leakage: Although an air leakage rating is not required by ENERGY STAR, air leakage through fenestration can be a significant source of heat loss or gain in a building. The air infiltration of many fenestration products is reported in the NFRC Certified Products Directory and on NFRC labels. The air leakage rate is a measure of how much air leaks through cracks in a window under reestablished test temperatures and pressure differences. Although the air leakage is reported in cubic feet per minute per window area in the NFRC Certified Products Directory (cfm/ft?), some manufacturers report air infiltration in cubic feet per minute per linear foot of window edge (cfm/ft). A lower value means less air leakage. Minimizing the air infiltration through windows increases occupant comfort by reducing drafts and condensation.
Condensation Resistance: Condensation resistance ratings may also be provided by NFRC and are listed for some products in the NFRC Certified Products Directory. The condensation resistance value indicates how well a window, door, or skylight prevents moisture or frost from forming on its interior surface. The higher the value, the less condensation will occur when it is cold outside. Over time, excess condensation can damage window components and the surrounding wall surfaces, and promote the growth of unhealthy mold and mildew.
Cold Climate Strategy: In cold climates, where space-heating costs are a concern, buy or specify products with a low U-factor to keep heat loss to a minimum. High SHGC are preferred in these climates to utilize the sun's free heat. Reduce drafts and increase comfort by choosing products with a low air leakage rate. Specify fenestration with high condensation resistance ratings in extremely cold regions.
Hot Climate Strategy: In warm climates, where air conditioning costs are the concern, buy or specify products with low SHGCs to reduce unwanted heat gain. Windows with low-e coatings, especially spectrally selective coatings, are effective at reducing summer heat gain and air conditioning costs without significantly impacting visible light or color. Tinted windows also reduce solar heat gain but transmit less visible light and, therefore, often have an undesirable aesthetic impact. Although windows and skylights are often designed to be operable (i.e., open to allow air to pass), it is desirable to choose windows with low air leakage rates, especially if the building is air conditioned.
Good installation practices will minimize air infiltration around fenestration products. ASTM E 2112 provides guidance on the proper installation of fenestration products. To minimize air infiltration when outside temperatures are extreme, keep windows tightly closed with all latches locked because this saves energy as well as minimizes uncomfortable drafts. Proper use of interior shades and blinds can reduce unwanted solar heat gain and maximize daylight to offset the need for interior electric lighting.
The selection of a window, door, or skylight should also include considerations of other important factors that are not directly related to energy efficiency. Cost, aesthetics, structural capability, water resistance, durability, maintenance, sound control, ventilation, thermal comfort, fading, and glare control are all important criteria when selecting fenestrations.
Finding More Information
Federal supply sources are the U.S. General Services Administration (GSA) and Defense Logistics Agency (DLA). GSA sells products through its Multiple Awards Schedules and GSA Advantage! DLA offers products through the Defense Supply Center Philadelphia and DOD EMALL.
Lawrence Berkeley National Laboratory provided supporting analysis for this acquisition guidance.