NFRC Calculations Now Accepted for PHIUS Verified Window Performance Data Program

Graham S. Wright, Senior Scientist & Product Program Manager

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PHIUS is pleased to announce the addition of a new path to performance data verification within our PHIUS Verified Window Performance Data Program. Based on the calculation standards of the US-based National Fenestration Rating Council (NFRC), this new compliance path offers an accurate and low-cost solution for manufacturers.

Previously in the PHIUS window program, window performance (U-value and solar heat gain coefficient) was only calculated based on European standards (EN/ ISO 10077-2 for frames/spacers, EN 673 for center-of-glass U-value, and EN 410 for solar heat gain.) Since the technical details of the EN standards differ from those of NFRC (which also have an ISO designation, ISO 15099), the performance numbers from the two methods could not be fairly compared. Although the technical sophistication of the EN and NFRC methods is similar, the actual NFRC labels give only whole-window properties for standard sizes and omit the component-level performance data needed by passive house modelers.

Now under the new program, domestic manufacturers who have already (or are currently pursuing) an NFRC rating can pursue this new calculation method in order to save time and money by avoiding additional calculation costs. The NFRC calculation method also allows performance numbers from North American products to be compared to those of European imports, thus giving passive house consultants and energy modelers performance numbers in a format that can be plugged directly into passive building modeling software WUFI Passive and the Passive House Planning Package (PHPP).

The “EN mode” of calculation will continue to be supported in the PHIUS window program. The EN method, referred to as “Orange Path”, is signified by an orange data label and noted in the border at the top of the label. As in the current program, pre-existing NFRC data files (for THERM and WINDOW) can serve as time-saving starting points for the EN-based calculations, but a significant amount of rework is needed due to the differences in method.

Orange Path data label, based on EN standards

Orange Path data label, based on EN standards

The label for the new “NFRC mode”, referred to as “Blue Path”, will have the same format and types of data listed as the EN mode label, but is signified by a blue data label. This path applies a conversion factor to the component-level data from the NFRC calculations.

Blue Path data label, based on NFRC standards

Blue Path data label, based on NFRC standards

PHIUS and NFRC jointly developed a program to train qualified NFRC simulators to run the proper conversion to produce the Blue Path data labels as an extension of their regular work on NFRC ratings. The conversion has two aspects – adjusting for the difference in standard window sizes, and for the different treatment of the extra heat loss at the edge of the glass due to the spacers. More details about the glass-edge translation can be found in a 2014 report entitled “NFRC and PHIUS U-factor Calculation Comparison” by Jeff Baker of WESTLab and NFRC. Verified product performance values are then provided to PHIUS by the NFRC modeler and published to the PHIUS Verified Window Performance Database.

PHIUS calculates the center-of-glass properties in a climate-dependent way, rather than using fixed environmental conditions, and incorporates climate-dependent recommendations on the data label. Since higher performance is required to get recommendations in more extreme climates, this provides “bragging rights” for manufacturers in addition to providing numbers for comparison-shopping and numbers for energy modeling.

For more information and to download the Program Overview and Program Application & Instructions in PDF, visit the PHIUS Verified Window Performance Data Program site. Find a qualified NFRC simulator here and submit your product for data verification today.

If you have any questions, please contact Graham Wright, Senior Scientist and Product Program Manager, at