Euro vs U.S. Window Rating Procedures

Ajit Naik, VP, Director of Building Performance Analytics, PE, BEMP, CCP

< 1 mins
Window Standards Comparison Graph
< 1 min read

Understanding window U-value is critical to achieving a high-performance building and avoiding condensation indoors, but did you know that all window rating procedures aren’t equal? In the U.S., façade experts use NFRC-100 to calculate window assembly U-values, while in Europe, the industry relies on ISO 10077-2. However, these two rating procedures can determine two U-values up to 18% different–for the exact same double-pane window assembly! This has a major impact on façade design details like the space between windowpanes…the two systems can indicate optimum gaps that differ as much as a quarter inch!

To complicate things further, there’s no consistent relationship between the two systems’ results for a given window assembly–sometimes NFRC predicts a lower U-value than ISO, and sometimes vice versa. This is due to differences between the NFRC and ISO algorithms, indoor and outdoor design temperatures, design solar radiation intensity, and surface film coefficients. Neither method is better–just different. When evaluating European and North American window products, it’s important to understand how the rating method will impact your project–it’s not as simple as comparing the rated numbers.

Need help understanding how your envelope design will perform before it goes out to bid? Our team of building enclosure analysis and energy experts can help give you the feedback you need to ensure your façade details will deliver. Contact us at

Euro vs U.S. Window Rating Procedures