Understanding Replacement Windows Ratings Terminology

Have you ever wondered how replacement windows are rated by NFRC? The terminology can be a bit confusing which is why we put together this quick list. Whether you are looking to buy replacement windows in Escondido, CA or somewhere with a colder climate, ratings can help you make a better decision.

Escondido California Window Replacement

Below, you’ll find more detailed information about the categories used by the NFRC when rating window performance.


Also known as U-value, this describes how quickly a window loses heat. A lower U-factor rating means higher window performance. If you’re looking at window replacement options, you should focus on the windows U-value rather than its R-value.

R-value is designed to measure the effectiveness of insulation. It’s normally used when describing wall insulation, not windows. A window with a lower U-factor will be more resistant to heat flow. This means that the window will provide better insulation overall.

Solar Heat Gain Coefficient

This term is designed to measure how much incident solar radiation a window allows through. When calculating this, it looks at the amount of radiation that the window absorbs and releases as well as how much radiation passes through into a home. This is measured via a number that falls anywhere from 0 and 1. When a window as a lower SHGC rating, it transmits less solar heat.

This definition if fairly complex, which can make it difficult for the average person to parse. A simpler way of putting things is to say that solar heat gain is similar to what you feel if you spend time in direct sunlight. When you’re in the sun, your body will absorb its radiant heat. This will cause you to feel warmer. Once you’ve stood in the sun for a while, you’ve basically gained heat from the sun. This can cause you to become hotter, which means you’ll want to remove yourself from direct sunlight.

You can apply this principle to your home’s windows. When your windows are in direct sunlight, those windows will start to absorb radiant heat from the sun. If your window has a higher SHGC rating, more of that heat will make its way into your home. This can cause the temperature of your home to rise.

When your window has a lower SHGC rating, less radiant heat will be able to enter your home. This means that it will be easier to maintain a cold temperature during the warmer months of the year. The sun is more intense in Southern climates, which is why this is a particularly important consideration if you live in the south.

Visible Transmittance

Visible Transmittance (VT) measures how much visible light is transmitted through a home’s windows. When the NFRC is calculating VT, it looks at the entire window, including the frame. The value should call somewhere from 0 to 1. In most cases, a window’s VT will be somewhere between 0.3 and 0.8. When a window has a higher VT, there is more light being transmitted through the window. If you want to take full advantage of natural lighting, you’ll want to look for windows that have a higher VT.

Choosing windows with a high VT can reduce your reliance on electric lighting.

Air Leakage

Even a properly assembled window has some cracks. These cracks can cause a home to lose or gain heat. This is something that is measured by a window’s air leakage (AL) rating. It measures air leaks in cubic feet, looking at the square footage of the area around the window. When a window has a low AL, less air will be able to pass through these cracks.

Currently, AL ratings are not mandatory. With that said, when you’re selecting replacement windows, you should aim to find windows that have a lower AL rating. When less air is able to infiltrate the home, it will be easier for you to keep your house warm or cool. A window that has a high AL will be less energy efficient. A window like this may feel drafty. Ideally, you should look for a window with an AL rating at or below 0.30.

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