NFRC 100 offers three approaches to determine the U-factor values of a fenestration product or a combination of products:
1. A U-factor value based on prescriptive methods
2. A U-factor value based on project-specific conditions
3. A range of multiple U-factor values that determine the thermal performance limits of the product(s); also known as Linear Energy Analysis For Fenestration (LEAFF)
The top two approaches to obtaining U-factor values have been used for many years, and will continue to be, within the progressive evolution of NFRC 100’s procedures. The third approach was more recently introduced by NFRC to ease the use and understanding of data. The data from these approaches are neither used at the same stages of a project’s development or for the same types of projects.
Nevertheless, all three of these approaches generates data that are in compliance with NFRC 100, but the various data from these different approaches should never be compared, or extrapolated for comparison – such manipulations have been proven invalid.
Specification professionals are challenged to stay up to date on these various approaches and, based on their knowledge, to critically review the U-factor values for curtainwall and other fenestration systems. NFRC 100 and 102 are available free for download. NFRC also offers regular training and webinars on energy efficiency topics.
The prescriptive approach leads to a standardized U-factor value appropriate for fenestration products and considers specific criteria defined by NFRC 100. This approach allows for similar product type comparisons and is a good starting point for preliminary selections. It also works well for basic projects that follow the NECB’s prescriptive path.
The project-specific approach leads to a unique U-factor value based on the performance of a fenestration product, possibly in combination with other fenestration systems, to serve a particular project’s requirements. This approach should be considered for any project that exceeds NECB’s prescriptive requirements, or that has special design conditions, such as historically significant buildings.
The project-specific approach can be obtained by either a simulation or a physical test. As shown in Figure 2, numerous aspects can be modified to suit the project’s requirements, including a fenestration product specimen’s dimensions, assemblies, glazing types, interior and exterior temperatures, and more. This allows the U-factor value to be determined according to the project-specific conditions. This unique U-factor is necessary when calculating the building’s energy consumption, and ultimately, for meeting energy codes’ conformity by demonstration paths.