If you've shopped for replacement windows, you've probably seen the National Fenestration Rating Council (NFRC) label, complete with numbers and ratings for different specs about energy efficiency and performance. Sometimes you'll see an Energy Star Logo alongside it, too.

The certification from the NFRC is created with data from third-party labs that test windows for certain attributes, such as visible transmittance, solar heat gain coefficient, and more. Past tests by Consumer Reports found NFRC ratings to be accurate and reliable.

That doesn't mean that the labels ratings are easy to understand. Below, we break down the meaning of each figure on the NFRC sticker.

NFRC Label

1. Product and Glass Information

You will find a frame type, glass coating, and type of gas that fill the cavities between the glass panes; all contribute to the overall energy efficiency of the window.

2. U-Factor

This number represents the heat transfer coefficient, or how much heat the windows coatings keep inside the home. U-factor ranges from 0.2 to 1.2. The lower the number, the more heat the product is keeping in.

3. Solar Heat Gain Coefficient

This number measures how well a product can resist unwanted heat gain, which is especially important during summer cooling season. The lower the number, the less you’ll spend on cooling. If you live in a hot, sunny climate, you will want a low number. If you live in a balanced climate with cold winters and warm summers, you'll want a rating of around 0.3.

4. Visible Transmittance (VT)

This is how much visible light will come in through the window during daytime. The range is from 0 to 1. The higher the number, the more visible light passes through. Clear glass without any coating has the highest VT rating. Coatings added to improve energy efficiency can sacrifice some visible light.

5. Air Leakage

This indicates the amount of outside air that will come through, according to the NFRC's testing. The lower the number, the better.

6. Condensation Resistance

Not every label will carry this optional rating. If you live in a humid climate or are concerned with mold growth, you may want to look at the condensation resistance rating. The higher the number, the better the window is at resisting condensation on a scale of 1-100.