Energy Performance of Windows: Navigating North American and European Window Standards (Paper)

An important element in the design of low energy buildings is the use of high performance windows coupled with passive solar design strategies. As energy efficiency becomes more important, North American designers sometimes look to Europe for energy efficient standards and technologies, such as the Passive House approach. However, comparing European and North American windows is complicated by the significant differences between European and North American energy performance rating standards. The different evaluation methods and boundary conditions also affect product design, as European and North American windows are optimized to achieve the lowest U-values under their respective rating systems.

This paper presents a review of the primary fenestration rating systems in North America and Europe, including those developed by the National Fenestration Rating Council (NFRC), the Canadian Standards Association (CSA), the International Organization for Standardization (ISO), and the Passive House Institute (PHI). A literature review is presented to highlight the primary differences between the rating systems. These differences include boundary conditions (temperature, surface film coefficient and incident solar radiation), algorithms for calculating centre of glass heat transfer, methods of accounting for edge of glass effects, window sizes, standard material properties, and treatment of sloped glazing. Computer modeling is used to show how the different rating systems evaluate calculated U-value and solar heat gain performance characteristics for several window configurations. Modeled results show that European and North American U-values can vary by up to 25% for a single frame and glass configuration. Center of glass solar heat gain values were shown to differ as much as 8%. The insights gained from the literature review and simulations are summarized to highlight important considerations for designers, specifiers, and fenestration product manufacturers in both North America and Europe. This paper will help interested parties to understand the differences, to make informed product selection choices, and, if desired, to optimize product performance for North American or European regulatory regimes as needed.

This paper was presented at the 2014 Canadian Conference on Building Science and Technology.