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A Closer Look at the Data Label

Once a product has successfully completed the Verified Window Performance Data Program process, the result is a data label containing all of the thermal performance information for the product. It is important to note that these performance values are based upon calculation results only, and not upon physical testing of the product.

Two options are available when selecting the calculation protocol:

  • Orange Path: Based on EN standards, this protocol calculates center-of-glass (COG) U-value per EN 673, center-of-glass solar gain per EN 410, and frame/spacer performance per EN 10077-2.
  • Blue Path: This protocol uses calculations by qualified NFRC Simulators, per ISO 15099.

The example data labels below show the information that is included for each calculation protocol: the Orange Path based on EN (European Standards), and the Blue Path based on National Fenestration Rating Council (NFRC) standards. The text border at the top of each label notes the corresponding calculation standards. The data label shows detailed data used for energy modeling and provides performance summary information for comparison-shopping. Checkmarks appear on each data label indicating the particular climate zones for which the window is recommended.


Figure 1: Orange Path data label, based on EN standards


Figure 2: Blue Path data label, based on NFRC standards


At the upper right of each table are listed COG properties, which are climate-dependent. There is confusion in the market stemming from the fact that windows imported from Europe are calculated to European standards with fixed environmental conditions at 0°C (32°F) on the exterior, while domestic windows are calculated to NFRC standards at -18°C (0°F) on the exterior. This protocol difference allows European manufacturers to use wider gaps between panes to eke out a bit lower (better) U-value under the milder standard conditions, and thus complaints about unfair comparison with NFRC-rated windows arise.[1]

Top center are the whole-window U-values for a standard size window, correspondingly climate-dependent. The whole-window U-value includes the effect of a nominal installation thermal bridge coefficient of 0.026 W/m.K (0.015 Btu/h.ft.F).

At top left the recommendation criteria are applied. If the window meets the recommendation criteria, it receives a green check mark for the zone, south-facing or off-south as appropriate. Note that there are green check marks but no red “X” marks because windows could well be suitable in other zones depending on the details of the building design.

At the bottom of the table are the frame / spacer properties. These are calculated at fixed environmental conditions because the frame properties are not as temperature-dependent as the glazing.

At bottom right the “psi-opaque” value combines the frame and edge-of-glass heat loss into a single figure of merit. This enables fair comparison of frames of different widths, and comparison of different frame-spacer combinations. These are assigned letter grades as follows:

Calculating the glass U-value over a range of environmental conditions is intended to bring some clarity to the issue – if wider gaps are used, the performance can be better in moderate climate zones, but drops off in the colder zones where it is most wanted. If narrower gaps are used, performance may not be as good in the mild or moderate zones but holds steadier in the colder conditions. Energy modeling consultants are advised to use the glass properties for the zone where the project is located.

For PHIUS calculations, COG properties are calculated using WINDOW 7 and the International Glazing Database (IGDB) from Lawrence Berkeley National Lab (LBNL).

PHIUS can provide manufacturers with instructions for calculating the zone-by-zone COG properties per PHIUS protocols to assist with selecting glazing for the window data verification program. Several suggested examples of glazing packages are presented below.

The Find and Compare Windows page includes a summary table with the recommendation checkmarks and frame/spacer grades for all rated windows, with corresponding links to the full data labels. PHIUS window data will also be published periodically as a database suitable for import to WUFI® Passive.


[1] For more background on the differing protocols between Europe and the US, see RDH (2014) and Ebanks (2014)

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