Purchasing Energy-Efficient Residential Windows, Doors, and Skylights

You are here

Did you know?

The energy savings of fenestration products varies according to climate. When choosing fenestration products, first identify the climate zone where they will be installed and then find products with U-factors and solar heat gain coefficients (SHGCs) that are less than or equal to those specified by ENERGY STAR.

The Federal Energy Management Program (FEMP) provides acquisition guidance for residential windows, doors, and skylights, a product category covered by ENERGY STAR efficiency requirements. Federal laws and requirements mandate that agencies purchase ENERGY STAR-qualified products or FEMP-designated products in all product categories covered by these programs and in any acquisition actions that are not specifically exempted by law.

FEMP's acquisition guidance and associated ENERGY STAR efficiency requirements for residential windows, doors, and skylights are technology neutral, meaning that one technology is not favored over another. However, ENERGY STAR's product specification requirements are limited to products that meet the definition of a residential window, door, or skylight as specified in the product specification information.

All other residential windows, doors, and skylights types are excluded, including but not limited to products that are assembled on site, including but not limited to sash packs or sash kits; windows, doors, or skylights that are intended for installation in nonresidential buildings; and window, door, or skylight attachments that are not included in a product's National Fenestration Rating Council (NFRC)-certified rating.

This acquisition guidance was updated in January 2017.

Find Product Efficiency Requirements

The U.S. Environmental Protection Agency (EPA) provides residential window, door, and skylight efficiency levels and product specification information on its ENERGY STAR website. Manufacturers meeting these requirements are allowed to display the ENERGY STAR label on complying models. Get a list of ENERGY STAR-certified residential windows, doors, and skylights by climate zone in the NFRC's directory search.

Make a Cost-Effective Purchase: Save Up to $6 by Buying Energy Star

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 savings 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 (thermal transmittance) 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.

FEMP has calculated that the required ENERGY STAR-qualified residential windows, doors, and skylights save money if priced no more than $3 above less efficient models. The best available models save up to $6. Table 1 compares three types of product purchases and calculates the lifetime cost savings of purchasing efficient models. The results in the table are based on windows, which represent the largest market segment. Federal purchasers can assume products that meet ENERGY STAR efficiency requirements are life cycle cost-effective.

Table 1. Lifetime Savings for Efficient Residential Window Models
PerformanceBest AvailableENERGY STARLess Efficient
Annual Energy Use (kBtu/ft2)108137174
Annual Energy Cost$1$2$3
Lifetime Energy Cost$18$21$24
Lifetime Cost Savings$6$3======
View the Performance and Model Assumptions for Table 1
Performance Column

Annual Energy Use: Based on the Façade Design Tool developed by the University of Minnesota and Lawrence Berkeley National Lab, listed in kBtu/ft2.

Annual Energy Cost: Calculated based on an assumed electricity price of $0.09/kWh, which is the average electricity price at federal facilities, and the heating load energy cost is calculated based on an assumed natural gas price of $0.69/therm, which is the average natural gas price at federal facilities. Learn more about Federal Government Energy/Water Use and Emissions.

Lifetime Energy Cost: Calculated as the sum of the discounted value of the annual energy cost over the assumed product life of 10 years, from InterNACHI's Standard Estimated Life Expectancy Chart for Homes. Future electricity price trends and a 3% discount rate are from Energy Price Indices and Discount Factors for Life-Cycle Cost Analysis - 2016: Annual Supplement to NIST Handbook 135 and NBS Special Publication 709 (NISTIR 85-3273-31).

Lifetime Cost Savings: The difference between the lifetime energy cost of the less efficient model and the lifetime energy cost of the ENERGY STAR model or best available model.

Best Available Model Column

Calculated for triple-pane, low-emissivity, low visible transmittance, argon windows with a U-value of 0.12 and a solar heat gain coefficient (SHGC) of 0.21, from the Regents of the University of Minnesota, Center for Sustainable Building Research Design Guidance for Offices in Minneapolis, Minnesota. More efficient models may be introduced to the market after FEMP's acquisition guidance is posted.

Energy Star Model Column

Calculated based on December 2016 ENERGY STAR efficiency levels. Federal agencies must purchase products that meet or exceed ENERGY STAR efficiency levels.

Less Efficient Model Column

Calculated for double-pane clear windows with a U-value of 0.47 and SHGC of 0.7.

Determine When ENERGY STAR Products Are Cost-Effective

An efficient product is cost-effective when the lifetime energy savings (from avoided energy costs over the life of the product, discounted to present value) exceed the additional up-front cost (if any) compared to a less efficient option. ENERGY STAR considers up-front costs and lifetime energy savings when setting required efficiency levels. Federal purchasers can assume ENERGY STAR-qualified products and products that meet FEMP-designated efficiency requirements are life cycle cost-effective. In high-use applications or when energy rates are above the federal average, purchasers may save more if they specify products that exceed federal efficiency requirements (e.g., the best available model).

Claim an Exception to Federal Purchasing Requirements

Products meeting ENERGY STAR or FEMP-designated efficiency requirements may not be life cycle cost-effective in certain low-use applications or in locations with very low rates for electricity or natural gas. However, for most applications, purchasers will find that energy-efficient products have the lowest life cycle cost.

Agencies may claim an exception to federal purchasing requirements through a written finding that no FEMP-designated or ENERGY STAR-qualified product is available to meet functional requirements, or that no such product is life cycle cost-effective for the specific application. Learn more about federal product purchasing requirements.

Incorporate Federal Acquisition Regulation Language in Contracts

These mandatory requirements apply to all forms of procurement, including construction guide and project specifications; renovation, repair, energy service, and operation and maintenance (O&M) contracts; lease agreements; acquisitions made using purchase cards; and solicitations for offers. Federal Acquisition Regulation (FAR) Part 23.206 requires agencies to insert the clause at FAR section 52.223-15 into contracts and solicitations that deliver, acquire, furnish, or specify energy-consuming products for use in federal government facilities. To comply with FAR requirements, FEMP recommends that agencies incorporate efficiency requirements into technical specifications, the evaluation criteria of solicitations, and the evaluations of solicitation responses.

Find Federal Supply Sources

The federal supply sources for energy-efficient products are the General Services Administration (GSA) and the Defense Logistics Agency (DLA). GSA sells products through its Multiple Awards Schedules program and online shopping network, GSA Advantage!. DLA offers products through the Defense Supply Center Philadelphia and online through DOD EMALL. Products sold through DLA are codified with a 13-digit National Stock Number (NSN) and, in some cases, a two-letter Environmental Attribute Code (ENAC). The ENAC identifies items that have positive environmental characteristics and meet standards set by an approved third party, such as FEMP and ENERGY STAR.

The United Nations Standard Products and Services Code (UNSPSC) is a worldwide classification system for e-commerce. It contains more than 50,000 commodities, including many used in the federal sector, each with a unique eight-digit, four-level identification code. Manufacturers and vendors are beginning to adopt the UNSPSC classification convention and electronic procurement systems are beginning to include UNSPSC tracking in their software packages. UNSPSCs can help the federal acquisition community identify product categories covered by sustainable acquisition requirements, track purchases of products within those categories, and report on progress toward meeting sustainable acquisition goals. FEMP has developed a table of ENERGY STAR and FEMP-designated covered product categories and related UNSPSC numbers.

Residential Window, Door, and Skylight Schedules and Product Codes

GSA offers residential windows, doors, and skylights through Schedule 56 (Buildings and Building Materials/Industrial Services and Supplies). 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.

The DLA ENAC for residential window, door, and skylight models is "DZ".

The UNSPSCs for residential windows, doors, and skylights are shown in Table 2.

Table 2. Product Codes for Residential Windows, Doors, and Skylights
PerformanceBest Available
Glass Doors30171501
Wooden Doors30171504
Metal Doors30171505
Swing Doors30171511
Double Hung Windows30171604
Single Hung Windows30171605
Casement Windows30171606
Horizontal Slider Windows30171607
Tilt or Transom Slider Windows30171608
Fixed Windows30171609
Bay Windows30171610
Bow Windows30171611
Projected Windows30171612
French Windows30171615
Fixed Skylights30171801
Vented Skylights30171802
Tube Skylights30171803

 

Buyer Tips: Make Informed Product Purchases

All qualifying units must be certified and labeled by 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 provided as part of a construction or renovation contract, specify the U-factor and SHGC for the appropriate climate zone.

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 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·ft2·°F), and typically range between 0.2 to 1.2 Btu/(h·ft2·°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 nonthermally 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

Fenestrations can allow significant amounts 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

Visible transmittance (VT) 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/ft2), 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 SHGCs 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.

Some utilities offer rebates or other incentives for the purchase of ENERGY STAR-qualified products. Use the ENERGY STAR Rebate Finder to see if your local utility offers these incentives.

User Tips: Use Products More Efficiently

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.

Additional Considerations

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.

Lawrence Berkeley National Laboratory provided supporting analysis for this acquisition guidance.