ENERGY STAR Window Ratings Explained (2024)

By Amanda Lutz Updated July 15, 2024

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Roughly 30% of a home’s total energy use is wasted due to heat lost through inefficient windows. To help consumers find the most energy-efficient windows, the U.S. Environmental Protection Agency issues an ENERGY STAR rating on products that meet high efficiency standards. In the guide below, we’ll explain this rating and what homeowners can expect from ENERGY STAR windows.


What Is ENERGY STAR?

The ENERGY STAR program has aimed to reduce energy costs, lower greenhouse gas emissions, and preserve the climate since its 1992 founding. It gives a stamp of approval to materials that increase energy efficiency, minimize environmental impact, and prevent the waste of electricity and fossil fuels.

To receive an ENERGY STAR rating on a window, door, or skylight, a manufacturer must first send the product to the National Fenestration Rating Council for certification. An EPA-recognized lab will determine how successfully the item insulates and prevents heat from passing through or air from leaking. If the test ratings fall into the acceptable range to meet ENERGY STAR standards, the product can display a blue ENERGY STAR on the label.


Overview of ENERGY STAR Window Ratings

The rating process for windows includes three energy performance metrics: air leakage, Solar Heat Gain Coefficient, and U-factor. Here’s what those metrics mean in practical terms.

Air Leakage

Air leakage is the rate at which air escapes through a window. Windows that allow heated or cooled air to leak outside are less efficient. A low air leakage rating is desirable no matter what climate you live in, and an ENERGY STAR-rated window should have air leakage of less than 0.3 cubic feet per minute per square foot. Hire the best window installers in your area to reduce the risk of air leakage.

Solar Heat Gain Coefficient (SHGC)

The SHGC number signifies how much solar radiation the window lets into a home. A window with a low SHGC number allows relatively little heat from sunlight inside. SHGC can be lowered by applying a low-e, or low-emissivity, coating to the glass to reflect the sun’s UV radiation while blocking a minimum of visible light.

A window with low SHGC can substantially reduce cooling costs in warm climates by preventing sunlight from heating indoor air. Windows in Southern and South-Central climates should have an SHGC of less than 0.25, but SHGC is less important in cold, Northern climates. Blocking solar heat also means less natural light, so homeowners in cold climates may want to opt for windows with a higher SHGC.

Some homeowners may benefit from having both high and low SHGC windows to make use of passive solar heating. Combining high SHGC south-facing windows with shading features such as trees and overhangs that block heat during the summer makes for an energy-efficient home. 

U-Factor

A window’s U-factor is a measure of the rate of non-solar heat transference (that is, how fast heat is gained or lost through the window). An energy-efficient window should prevent the transfer of heat. The lower the U-factor number, the more energy efficient the window. The U-factor provided by NFRC testing applies to the whole window assembly, but the glass may have its own U-factor.

The acceptable range to achieve an ENERGY STAR rating differs by climate. The U-factor must be less than 0.27 in Northern climate zones, but it only needs to be less than 0.40 in warmer Southern climate zones. You’ll see greater savings on your utility bills with a lower U-factor window in a cold climate than in a warm climate.


Other Performance Ratings

Visible transmittance (VT) is a measure of how much visible light can pass through the window while still blocking UV rays and is expressed as a number between 0 and 1. A higher VT means more light passes through and signifies a less efficient SHGC.

Condensation resistance is a measure of how likely condensation is to form on or around the window and is rated as a number between 1 and 100. A higher number means less condensation and better energy efficiency. Condensation resistance is more important in cold climates, but it is helpful everywhere.


ENERGY STAR Criteria by Climate Zone

The ENERGY STAR program includes four climate zones within the United States. You can find your location on this map. We outline the four climate zones below:

Here are the recommended ENERGY STAR criteria based on climate zone:

U-factorSHGCAir Leakage

Southern

≤0.40

≤0.25

≤0.3 cfm/ft2

South-Central

≤0.30

≤0.25

≤0.3 cfm/ft2

North-Central

≤0.30

≤0.40

≤0.3 cfm/ft2

Northern

≤0.27

any

≤0.3 cfm/ft2


Reading the NFRC Energy Performance Label

ENERGY STAR-rated windows list their metrics on a rectangular NFRC label. The brand, make, and model of the window will typically be at the top, and an NFRC-certified logo will be at the top left. Beneath it will be the U-factor and SHGC, and beneath those will be the air leakage and VT ratings. Condensation resistance may also be listed toward the bottom.


Are ENERGY STAR Windows Worth It?

Energy-efficient windows reduce heating and cooling costs and lower your carbon footprint. ENERGY STAR windows also reduce sound transference and block UV radiation, which protects your belongings from sun damage.

The only major drawback to ENERGY STAR windows is the price. These windows can cost 10% to 15% more than comparable, non-ENERGY STAR-rated windows, and you won’t get the benefits unless you replace most or all of your old windows. This makes overall window replacement costs substantially higher. 

The EPA says homeowners can save an average of 12% on their energy bills nationwide with ENERGY STAR windows, but your actual savings will depend on the climate of the area in which you live and the state of your current windows. Homeowners in extreme climates who haven’t replaced their windows in a long time will see the most significant savings.


Tips for Choosing the Most Efficient Windows

Here are some tips to keep in mind as you look for the best windows for your home:


Our Recommendation

Many top window brands now offer ENERGY STAR-certified windows. Make sure you’re looking at the best windows for the climate in which you live, and consult a reputable window contractor if you have any questions. We recommend getting at least three quotes from local window installers before making your choice.


ENERGY STAR Window Ratings FAQ

What U-factor is best for cold climates?

Homeowners in cold Northern climates should look for a window with a U-factor of 0.27 or less.

How is SHGC different from U-Factor?

A window’s U-factor measures how much nonsolar heat can transfer through the entire window assembly, while its SHGC is a measure of the amount of solar heat that passes through the glass.

Can I increase efficiency without replacing windows?

Yes, you can increase your home’s energy efficiency without replacing your windows. Seal up all cracks with caulk or weatherstripping, and apply solar window film to the glass. You can also install shutters or curtains to block heat and drafts.

How much do ENERGY STAR windows reduce bills?

ENERGY STAR-rated windows reduce heating and cooling costs by an average of 12%, according to the EPA. This usually represents a savings of $101 to $583 per year.

Do ENERGY STAR windows block noise?

Yes, ENERGY STAR windows block noise. A more insulated window is a quieter one, so a double-paned window with very little air leakage will block far more noise than a single-paned, leaky window.