Everything You Need to Know About Replacement Window Energy Ratings

A window is not just a window. There are a variety of different window styles, types of glass, frames, and additional features. Today an increasingly important one is windows that have high energy ratings.

replacement window energy ratings

The energy rating can tell important performance factors that influence how effective windows, doors, and skylights are at gaining or losing heat. It can also reveal other important aspects like how much sunlight is transmitted.

Every new window is assessed these days for its energy rating. New windows have a label placed on the glass or frame to notify you of its performance. Replacement window energy ratings are a little confusing if you don’t know where to look and what certain terms mean.

Therefore, we’ve put together this guide to explain everything you need to know regarding window energy ratings and performance.

National Fenestration Rating Council (NFRC)

The National Fenestration Rating Council (NFRC) is a voluntary program that oversees tests related to the certification and labeling of windows, doors, and skylights regarding their energy performance ratings.

The NFRC label is the best way to guarantee the energy properties of the window are energy efficient. It is also a great method for comparing one product to another with actual, tangible analytics.

The NFRC label is located on all Energy Star windows, doors, and skylight products. It is a good third-party assessor for U-factor and solar heat gain coefficient ratings.

How Do Windows Gain and Lose Heat?

Windows are like magnets for the sun. They draw its light which has a byproduct of also producing heat. Windows gain or lose heat for a number of reasons:

Regardless of the cause, windows gain and lose heat that sours home performance. When you have warm air that you would like to keep inside yet constantly escapes, or have cool air that’s getting overwhelmed by exterior warm air then it’s time to reconsider your windows.

Properties of Energy Efficient Replacement Windows

What characteristics does the NFRC consider when they gauge new windows for energy efficiency? The following properties are the most important factors:


The U-factor is the rate at which a window conducts non-solar heat flow. The U-factor can get really scientific as it uses a complex equation to figure out the rating. What you as a consumer need to understand is that the U-factor can refer to just the glass or the entire window performance. Bottom line the lower the U-factor the more energy efficient the window is with heat gain or loss.

Solar Heat Gain Coefficient (SHGC)

The SHGC is the fraction of solar radiation admitted through a window. The radiation can either get transmitted directly or absorbed through another material. Low SHGC windows don’t transmit a lot of solar heat which is ideal for summer months and places with a notorious hot climate.

solar heat piercing through a window

However, colder climates may actually want a high SHGC window rating so it collects more heat during the bitter cold season. Therefore, no ‘right’ answer exists only the one that’s more suitable for the home’s climate, orientation, and external shading properties.

Air Leakage

The third most important factor in determining energy efficiency with new windows is air leakage. The term is fairly self explanatory. It covers the rate of air movement around the window in the presence of specific amounts of pressure.

Once again a complex mathematical equation makes the determining rating. The best replacement windows have low air leakage ratings. However, some of this can also get influenced by human error as poor window installation may lead to high air leakage even on a window that was rated low in performance tests.

Visible Transmittance (VT)

The VT is a fraction of the visible spectrum of sunlight (studied in amounts from 380 to 720 nanometers) that are transmitted through the glaze of a window. A product with a high VT will provide more natural light. Reducing the VT to zero will produce less sunlight but also not as much interior glare.

Light to Solar Gain (LSG)

The LSG is the ratio between the SHGC and VT. The light to solar gain can help determine the overall efficiency of the window in relation to heat gain and loss. The higher the rating the more light is transmitted without producing extra heat. Unlike the other characteristics of window energy ratings, the LSG is not reported on every window label.

NFRC Label

Consequently, the NFRC label featured on every new window should provide ratings for the U-factor, SHGC, and VT. Manufacturers have the option of reporting air leakage ratings as well as condensation resistance. Light to solar gain also doesn’t have to get labeled on every new window.

Window Energy Ratings by State

Did you know that every state in the nation has its own standards and recommended ratings regarding energy performance on windows? By visiting the state fact sheets you can get important information and recommended performance based on the climate of the region.

The state breaks energy performance into two categories: New Construction and Existing Construction. As the names suggest new construction guidelines are dedicated to brand new home installations. Existing construction is related to old home renovations receiving replacement windows.

The energy performance of all Energy Star qualified windows must get independently tested, certified, and then verified in order to receive an official National Fenestration Rating Council (NFRC) review.

Window Performance and Energy Efficiency

The NFRC label will tell you everything you need to know about the performance of a window and how energy efficient the replacement option is for your home.

Once you learn the different criteria studied in independent tests, as well as recommendations based on the state you live in, you can make the most appropriate choice for your window replacements.

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Window Resource started with one goal in mind - to help people become up-to-date on all things windows. Windows play a major role in your home's looks, value and energy efficiency so it's important to make the right decision when getting new ones installed. We'll cover just about every topic there is so you can make the best decision. Still don't see what you're looking for? Contact us and we'll answer any questions you may have.