Specific guidance describing the difference between the performance and prescriptive paths of ENERGY STAR is available in the ENERGY STAR National Program Requirements.
ENERGY STAR estimates that installing ENERGY STAR-rated windows rather than standard windows can save homeowners about 7% to 15% on their utility bills. ENERGY STAR windows typically consist of two or more panes of glass in a fiberglass, vinyl, wood, or combination frame. An odorless, colorless, nontoxic gas such as argon or krypton fills the space between the panes to provide better insulation than just air. Special low-emissivity (low-e) coatings on one or more of the glass surfaces reduce the infrared radiation from a warm pane of glass to a cooler pane, thereby making the window more energy efficient (lowering the U-factor). These coatings also reflect ultraviolet (UV) rays to minimize fading of furniture and drapes. A spacer keeps the panes of glass the right distance apart; some spacers are made of non-metal, insulating materials that also insulate the edges of the glass panes reducing heat transfer through the window.
ENERGY STAR windows are independently performance tested according to procedures established by the National Fenestration Rating Council (NFRC). The NFRC label shows performance ratings in five categories:
- U-Factor measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.25 to 1.25 and are measured in Btu/h·ft²·°F. The lower the U-factor, the better the window insulates.
- Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. The lower the SHGC, the less solar heat the window transmits.
- Visible Transmittance (VT) measures the amount of light the window lets through. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80. The higher the VT, the more light you see.
- Air Leakage (AL) measures the rate at which air passes through joints in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leaks through the window. Most industry standards and building codes require an AL of 0.3 cf·m/ft².
- Condensation Resistance measures how well the window resists water build-up. Condensation Resistance is scored on a scale from 0 to 100. The higher the condensation resistance factor, the less build-up the window allows.
Figure 1 - All ENERGY STAR-Qualified Windows Display the ENERGY STAR Label. The label will indicate the climate zones for which this product qualifies. The ENERGY STAR label should be located next to the National Fenestration Rating Council (NFRC) label or NFRC information may be included on the ENERGY STAR label.
ENERGY STAR establishes U-Factor and SHGC criteria for ENERGY STAR windows based on climate using a climate zone map developed by ENERGY STAR. The criteria show the highest U-factor permissible in each climate zone and the SHGC criteria in each climate zone, with higher SHGCs in colder climates and lower SHGCs in hotter climates.
Figure 2 - ENERGY STAR Climate Zone Map for Windows
Figure 3 - ENERGY STAR Climate-Specific Criteria for Windows and Skylights
How to Purchase and Install ENERGY STAR Windows
Determine your ENERGY STAR climate zone for ENERGY STAR window criteria. SEE the ENERGY STAR website
for a locator tool based on the builder’s county.
Find a retailer of ENERGY STAR-qualified windows. Click here for help locating a retailer
. If ordering from a showroom or building materials supplier, ask for a product that qualifies for ENERGY STAR in your climate zone.
Check with local utilities and check the ENERGY STAR website
for tax exemptions, tax credits, and rebates that may be available for purchasing ENERGY STAR-rated windows.
Follow the manufacturer’s installation instructions when installing the windows to avoid voiding the warranty. Some warranties require that you use an installer certified by the manufacturer.
For more instructions on properly installing windows to keep air and moisture out, see Fully Flashed Window and Door Openings.
For instructions on air sealing around window rough openings, see Window and Door Rough Openings.
When the performance path is followed, fenestration must meet or exceed requirements of the 2009 IECC as shown in Table 1.
Table 1. Fenestration Requirements (adapted from the 2009 IECC)
|Climate Zone ||Fenestration U-Factor (excluding skylights) ||Glazed Fenestration SHGC|
|1 ||1.2 ||0.30|
|2 ||0.65 ||0.30|
|3 ||0.50 ||0.30|
|4 except Marine ||0.35 ||No requirement|
|5 and Marine 4 ||0.35 ||No requirement|
|6 ||0.35 ||No requirement|
|7 and 8 ||0.35 ||No requirement|
How to Install Windows
Follow manufacturer’s installation instructions when installing windows to avoid voiding the warranty.
To maximize beneficial solar heat gain and minimize unwanted solar heat gain, consider window location and sizing when designing the house. Limit the number of west-facing windows, especially in hot climates, to limit late afternoon glare and solar heat gain. Locate south-facing windows under properly sized overhangs or covered porches to minimize heat gain from high summer sun and maximize gain from low winter sun. Locating deciduous shade trees to the south and west will also minimize summer solar gain and maximize winter solar gain. Consider selecting specific windows for different sides of the house (EPA 2013).