NFRC stands for the National Fenestration Rating Council. They are a non-profit organization that establishes objective window, door, & skylight energy performance ratings to help you compare products & make informed purchase decisions.
In the first box on the top left, shows the NFRC logo. In the top right box on the label, is the information about the window. It includes the manufacturer, series, & window options. Below those boxes are the four performance ratings required. There are some additional optional ratings that can be listed but are not required (for example the condensation rating). The four required are U-Factor, Solar Heat Gain Coefficient (SHGC), Visible Transmittance (VT), & Air Leakage.
The U-Factor measures how well the window slows heat transfer. Think of this like the insulation in your walls. The better the insulation is, the less heat transfer occurs. This has a range of 0.19-1.20 with the lower the number; the less heat is being transferred.
The SHGC measures how well the window resists unwanted heat gain. This is usually achieved by reflecting the UV rays out in the summer & reflecting the heat in the home in in the winter. This has a range of 0 – 1 with the lower the number, the higher the resistance.
The VT measures the light coming through the window. This has a range of 0 – 1 with the lower the number the less light is coming in & the darker your home will be.
The Air Leakage measures how much air is coming through the product. Any operable window will have some air leakage while your fixed windows should not leak air. The NFRC requires that the air leakage be equal to or less than 0.3. The lower the number the less air is coming through the product.