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Submit a Window for Data Verification

To submit a product for review under the Verified Window Performance Data Program, please refer to the PHIUS Verified Window Performance Data Program Application & Instructions document available for download in the right sidebar.


Data Verification Protocols

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 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.

To pursue the Blue Path data verification method, contacting a qualified NFRC simulator is the first step. To pursue the Orange Path, please contact Graham Wright, Senior Scientist and Product Program Manager, at


Introducing the New “NFRC Mode” Data Label

As noted in the top border of the example Orange Path data label (based on EN standards) shown below, the PHIUS window rating performance calculations have up until this point been based on the European CEN (European Committee for Standardization) standards: EN 673 (for U-value of the glass), EN 410 (for solar gain of the glass), and EN ISO 10077-2 (for U-value of the frames).

But in North America most windows are calculated to NFRC standards, which are also referenced in building energy code language, including 2012 IECC (2012 International Energy Conservation Code), clause R303.1.3, and ASHRAE 90.1, clause As noted above, there are a number of technical differences between CEN and NFRC standards, such as different standard environmental conditions.

PHIUS now offers a second “mode” of window data verification based on NFRC standards, in addition to the original “CEN mode” verification. An example data label is shown as the Blue Path data label (based on NFRC standards) below.

The NFRC mode contains all the same kinds of information as the CEN mode data label, so it can be used the same way for building energy modeling, that is, it supports entering windows at their actual size and configuration.[1] However because of the technical differences in the calculation methods between the two approaches, the numbers should not, strictly speaking, be compared directly to CEN-based data. Therefore, the two kinds of labels are different colors – the CEN-mode label is orange and the NFRC-mode label is blue. Again the calculation standard is noted in the top border – NFRC’s methods are also internationally recognized and designated ISO 15099.


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


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


As with the CEN mode labels, an embellishment of the standard methods is made to calculate center-of-glass properties with outside conditions that vary by climate zone. For window manufacturers, the advantages of the Orange Path label are that the CEN calculation procedure inherently produces the desired component level performance data (frame, glass, spacer), and that the data can be compared to performance numbers from any European competitors who also report data based on CEN methods.

The main advantage of the Blue Path label is a lower cost of data verification for the manufacturer. Manufacturers who already have, or are pursuing, a NFRC rating can now also have their data verified by PHIUS, allowing them to serve the passive building community for less additional cost than a full recalculation to CEN standards. PHIUS then charges only a listing fee for web publication and inclusion in the WUFI® Passive window database. A second advantage is that the ISO 15099 method for frames is actually more accurate, as recognized in ISO 10077-2.

PHIUS has partnered with NFRC to train their qualified simulators and license them to produce the Blue Path labels as an optional additional service for interested manufacturers. A list of qualified simulators can be found on the Find a Qualified Blue Path Simulator page.

The reason that the NFRC calculation mode is possible, from technical standpoint, is that while the standard NFRC label reports only whole-window performance, the calculations performed by NFRC simulators also produce component-level data. The main difference between the CEN and NFRC approaches is the treatment of the extra heat loss at the edge of glass. The CEN approach is to model the window twice; once with a piece of rigid foam in the frame to determine a frame U-value, and once with the actual glazing and spacers. The difference between the two is used to calculate a linear thermal bridge coefficient “psi-spacer”. The NFRC approach is to model the window once and pick off separate U-values for the frame, the center-of-glass (COG), and an edge-of-glass region 2.5 inches wide.

In a 2012 PHIUS Tech Corner report entitled “Calculating Window Performance Parameters for Passive House Energy Modeling,” the PHIUS Technical Committee suggested that a conversion could be done in which the “extra” edge-of-glass heat loss from an NFRC calculation is converted into an equivalent linear coefficient to get the data into the same format as the CEN results and proposed a corresponding conversion formula.

In a 2014 report entitled “NFRC and PHIUS U-factor Calculation Comparison”, Jeff Baker of WESTLab and NFRC derived a different formula and demonstrated that, if all other things were equal, this edge-conversion procedure in and of itself would introduce very little error.[2] [3] [4] The additional work for an NFRC simulator to produce a Blue Path label thus comprises mainly:

  • Running the COG properties through the climate zones.
  • Retagging the THERM models for two-lite products to get separate U-values for the two halves of the meeting rail or stile (to support modeling each lite at its actual size in the building energy model).
  • Using the workbook that calculates the edge conversion and produces the data label.

The process described above should be significantly less work than a full calculation to CEN standards, produce results equally useful to passive building consultants, and offer the same kind of recognition and exposure for high-performance products.


Additional Window Performance Properties

The following properties and metrics are helpful to keep in mind when evaluating the performance of a window assembly.

Air-tightness is important in passive building construction as well as the windows used in passive buildings. ASTM E283 - 04 (2012) provides a standard test method for air leakage through windows, doors, skylights, and curtain walls. While it is currently optional, manufacturers are encouraged to submit any such air-tightness data they have and those reports are also linked on the summary table.

Anecdotally, windows leak more in the field than in the lab tests. As such, the reported numbers can be used for relative comparison, however it is not advisable to try to use these values to calculate “air leakage budgets” for building certification. Also, because ASTM E283 is a short-term test, it may not distinguish very well between more and less durable air-sealing designs.

Energy modeling for building certification also involves entering window installation thermal bridge parameters (“psi-install” for short) for each window. The psi-install depends on the window installation detail design. Conservative defaults suffice for most projects, e.g. 0.04-0.05 W/m.K (0.023-0.029 Btu/h.ft.F). However in some cases it may be appropriate to do the 2D finite element calculations to determine psi-install values more precisely.


Calculating the Psi-Install Value

PHIUS supports consultants in performing psi-install calculations for their project. Manufacturers are asked to choose at least one of the following three options to supply the data to PHIUS necessary to perform the psi-install calculation:

  1. Allow PHIUS to publish the THERM models of the frame sections so that the designer can use THERM to do their own calculations. (preferred option)
  2. Provide psi-install calculations as an in-house tech-support service.
  3. Like Option 2, but the calculation is outsourced to PHIUS.

There is also the option to pre-calculate installation details, however this option has not been popular. As in the building project certification program, this program does not prescribe any particular wall assemblies, but the installation details are reviewed for soundness-of-concept with regard to air-barrier continuity and water control. 

For Option 1, zip files containing the THERM models are linked to in the summary table of the Find and Compare Windows page.


Suggested Glazings

In order to generate recommendation checkmarks for both warm and cold zones, both a high-gain option and a low-gain option for the glazing is needed. The following suggested packages are constructed out of the Cardinal Glass catalog.

If verifying the performance values with just two glazings, it is suggested to use two triple-pane systems with Argon fill in 17 mm gaps and 3mm glass layers (43 mm overall thickness):

  • High-gain triple: low-e-180#2 / Clear / low-e-180#5
  • Low-gain triple: low-e-366#2 / Clear / low-e-180#5

Windows with these glazings can receive checkmarks up to Zone 5, or possibly 6, with a very good frame:

  • High-gain: can cover Zone 4C, 4, 5, and 6 south-facing
  • Low-gain: can cover Zone 2, 3, 3C south-facing and 2-6 off-south

If a third glazing is included, it is suggested to use a double-pane option for the warm zones, also with Argon fill in 17 mm gaps (23 mm overall thickness):

  • Low-gain double: low-e-366#2 / i89#4

For a fourth and fifth glazing it is suggested to use four-pane options for Zone 7 and 8 with 22% Argon and 66% Krypton in 12 mm gaps:

  • High-gain quadruple: low-e-180#2 / Clear / low-e-180#5 / low-e-180#7
  • Low-gain quadruple: substitute low-e-272#2 on the first (outer) pane


For More Information

For more information about the program, or if you have any questions, please contact Graham Wright, Senior Scientist and Product Program Manager, at


[1] Up to this point, the use of NFRC data for passive house modeling has been hampered by the fact that the standard NFRC data label reports only whole-window performance at standard sizes. 

[2] Baker’s formula properly accounts for the window corners, whereas the 2012 proposal would have double-counted them.

[3] Note that for NFRC mode the psi-spacer value will likely be generally lower and the U-frame generally higher.

[4] Note that for the whole-window U-value and checkmark criteria application, the window is resized from NFRC standard size to PHIUS standard size.

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