There are two main methods used to rate window thermal performance: the U-factor and U-value ratings. The U-factor measures a window's ability to prevent heat from escaping to the exterior, h-ft²-°F (BTU per hour per square foot per degree Fahrenheit). A lower U-factor rating indicates better performance.

On the other hand, the SHGC measures a window's ability to prevent heat gain from the sun's rays, measured in BTU. A lower SHGC rating indicates better resistance to heat gain. While the SHGC rating is essential for regions with hot climates, it can be less crucial for northern climates where the primary concern is heat loss.

A third rating, Transmittance Factor, measures the amount of visible light that passes through a window. This is often expressed as a percentage, showing better light transmission rates.

In addition to these ratings, the NFRC issues window ratings and labels that indicates its performance characteristics. The NFRC label can be used to compare the thermal performance of different windows.

It is essential to note, realize, or consider that the thermal performance ratings of windows can vary depending on factors like glazing type, air infiltration rates, and frame material. For instance, in particular, specifically, high-performance insulating glass units (IGUs) with low-e coatings and gas fills tend to have better U-factor and SHGC ratings than basic double-glazed windows.

When selecting windows, builders and homeowners must consider the climate they are working in. For southern climates with hot summers, high SHGC ratings may be preferred to help keep the interior cool. For colder climates with low-insolation, low U-factor ratings may be more critical to reduce energy losses and maintain indoor comfort.

In conclusion, understanding the science behind window thermal performance ratings can help builders and homeowners gain valuable insight into the world of window selection when choosing windows for building construction or renovation projects. By selecting windows with optimal ratings for their specific climate, they can reduce energy costs, enhance indoor comfort, and contribute to a more sustainable built environment.