U.S. Department of Energy - Energy Efficiency and Renewable Energy

Building Technologies Program – Residential Buildings

Windows and Doors

Windows and doors are prominent features of any home. They can also help or hinder a home's energy efficiency. High-performance windows can help a home achieve ENERGY STAR® qualification. Quality, efficient windows will add cost, but they also provide tremendous value in comfort, durability, and energy savings. Lower heating and cooling bills are one benefit of high-performance windows. They may also lower peak loads for heating and cooling a home, which allows for smaller heating, ventilating, and air-conditioning equipment that costs less to purchase and operate. These types of savings go a long way to recovering the cost of installing energy-efficient windows.

How Windows Are Rated

A voluntary rating system developed by the National Fenestration Rating Council (NFRC) provides performance information for many available residential windows.

NFRC Window Label

Example of National Fenestration Rating Council (NFRC) label, including brand name, U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, and Air Leakage ratings.

The NFRC label contains ratings for the following features:

U-factor

The U-factor takes into account the entire window assembly and rates how well the window prevents heat from passing through the window. The U-factor is similar to R-values in insulation products with one important difference: with R-values, the higher the number, the greater its insulating ability, while with the U-factor, the lower the number, the better the window's ability to stop heat flow. U-factor values for windows generally fall between 0.15 for highly efficient triple-pane windows and 1.2. for older, single-pane windows.

Solar heat gain coefficient (SHGC) rating

SHGC is the solar heat gain coefficient, which measures how well the window blocks heat caused by sunlight. The lower the SHGC rating, the less solar heat the window transmits. This rating is expressed as a decimal between 0 and 1. The best SHGC rating for a window depends on the climate where the window will be installed (see Climate Considerations for Window Selection).

Visible transmittance (VT)

Visible transmittance (VT) measures how much light comes through a window. VT is expressed as a decimal between 0 and 1. The higher the VT value, the clearer the glass and the more visible light that is transmitted through the window.

Air leakage (AL)

Air leakage through a window assembly is included on most manufacturers' labels, but is not required. The AL rating is expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq. ft.) The lower the AL, the less the window leaks. A typical rating is 0.2.

Condensation resistance (CR)

Another optional rating is condensation resistance (CR), which measures the ability of a window to resist the formation of condensation on its interior surface. The higher the CR rating, the better that product resists condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various windows for condensation formation. CR is expressed as a number between 1 and 100, with a higher value representing more resistance to the formation of condensation.

Climate Considerations for Window Selection

For any window, it helps to have as low a U-factor as possible. A window that insulates well (low U-factor) works better in any climate. A low U-factor helps keep the summer heat out in warmer climates, and the winter cold out in colder climates. The air leakage rating also should be as low as possible.

The best solar heat gain coefficient (SHGC) rating can vary with a home's particular location and climate zone. In climates that are dominated by heating more than cooling needs, higher SHGC windows may be more appropriate. The higher rating means that more heat from sunlight will be transmitted into the home's interior and provide a measure of passive solar warming. This helps reduce the winter heating loads for homes in colder climates. Conversely, in cooling dominated climates, lower SHGC windows lessen the amount of heat from sunlight which reaches the home's interior. This helps lessen the cooling needs of homes in warmer climates. In mixed climates where hot summers and cold winters are the norm, a compromise between allowing heat from sunlight in during the winter and keeping it out during the summer is needed.

Specifying efficient windows controls solar energy gains and helps reduce heating and cooling loads. Building America experts recommend that, nationwide, windows have a U-factor of 0.35 or lower and a SHGC of 0.35 or less. The tradeoff in using a smaller SHGC is that the sun is blocked in both winter, when the sun helps to heat the house, and summer, when the sun works against air conditioning. Note that ENERGY STAR qualification can be met with windows at less stringent ratings.

Resources
Additional sources of information for selecting high-performance windows can be found at the links below:

ENERGY STAR
The ENERGY STAR program qualifies specific windows in addition to qualifying houses for energy efficiency. The label is one way to be sure a window meets energy efficiency guidelines. The ENERGY STAR Web site provides guidance on selecting the right windows for different geographic areas along with tax incentive information for installing energy efficient windows.

Efficient Windows Collaborative
The Efficient Windows Collaborative operates a Web site that can help builders, designers, and consumers choose windows. It includes a tool that allows users to analyze energy costs and savings for windows with different ratings, and fact sheets with comparisons for each state.

Consumer's Guides to Energy Efficiency and Renewable Energy
The Department of Energy's Office of Energy Efficiency and Renewable Energy produces Consumer's Guides to selecting new energy-efficient windows and exterior doors.

Upgrade to Energy Star: Annual Savings: Map of U.S. separated into regions, and showing estimated annual savings on energy costs from installing Energy Star qualified windows. Savings from upgrading from single paned windows range from $126 in California to $465 in New England, and savings from upgrading from double paned windows range from $27 in California to $111 in Florida.
What Makes a Window Energy Efficient: energy efficient technologies; including improved frame materials, such as wood composites, vinyl, and fiberglass that reduce heat transfer and help insulate better, low-E glass that uses special coatings to reflect infared light, keeping heat inside in winter and outside in summer, as well as reflecting damaging ultraviolet light, which helps protect interior furnishings from fading, gas fills, which may include argon, krypton, or other gases between the panes, and are odorless, colorless, non-toxic, and insulate better than regular air, multiple panes, which insulate better, especially with gas space in between them, and also provide impact resistance and sound insulation, and warm edge spacers, which help keep the glass panes the correct distance apart and reduce heat flow and prevent condensation.
What Makes a Door Energy Efficient: energy efficient technologies; including multiple glass panes to reduce heat flow, improved core materials such as fiberglass, wood cladding, and steel with polyeurethane foam core, and tighter fit and improved weather stripping such as a magnetic strip to create a tighter seal that reduces air leakage around the edges.
What Makes a Skylight Energy Efficient: For traditional skylights, the same technologies as windows, but even more valuably due to increased reception of direct sunlight and greater temperature differentials in winter. For lighting closets, bathrooms, hallways, or other spaces that typically do not have access to sunlight, tubular daylighting devices may be used, which gather sunlight at the roof and transmit it down to a diffusing lens mounted in an interior surface, usually a ceiling.