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PHIUS Verified Window Performance Data Program Overview

Passive House Institute US (PHIUS) developed the voluntary Verified Window Performance Data Program to make verified performance data for windows, doors, and skylights readily available to passive building practitioners. Manufacturers who have their window performance data verified by PHIUS are able to better position their products in the marketplace by providing practitioners with the information they need to specify high-performance products in their projects. Manufacturers seeking this verification supply the required product and testing data to PHIUS, from which a custom performance data set is generated.

Verified products receive a program-specific PHIUS product performance data verification mark upon completion of the verification process. The verified product performance values are then published to the PHIUS Verified Window Performance Database. These verified performance values are also periodically released in database files suitable for import to the WUFI® Passive building simulation modeling tool.

The PHIUS Verified Window Performance Data Program is a voluntary program intended to verify the accuracy and appropriateness of the energy performance data for windows and their components needed to accurately model passive building energy balances. For best accuracy in building energy modeling, window performance data (e.g. U-values) are needed at the component level, that is, for the frame and glass separately, so that the performance of the windows can be calculated for the exact sizes and configurations planned in the project. The verification process follows the appropriate window simulation protocols and conditions for high-performance windows, doors, and skylights.


New NFRC Compliance Path Cuts Costs for Data Verification

PHIUS is pleased to announce the addition of an exciting new path to performance data verification within the PHIUS Verified Window Performance Data Program. 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 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.

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. Verified product performance values are then provided to PHIUS by the NFRC modeler and published to the PHIUS Verified Window Performance Database.

For more information on the program, read the full blog post announcement here, and visit the “Take a Closer Look at the Data Label” page. Find a qualified NFRC simulator here and submit your product for data verification today.


Window Performance Data Verification Program vs. Building Certification Program

The PHIUS Verified Window Performance Data Program is separate from the PHIUS+ Passive Building Standard certification program. PHIUS’ building certification program does not require the use of windows, doors, or skylights for which performance data has been verified through the Verified Window Performance Data Program. While the PHIUS building certification program does not have a “hard requirement” on maximum window U-value, the window performance does have an impact on interior surface temperatures and thermal comfort. The PHIUS+ Passive Building Standard however does have maximums the building’s peak heating load, and because the window U-value strongly influences peak heat load, the peak load limit indirectly limits the window U-values. However the designer still does have flexibility in meeting the peak load limits by for instance reducing the window area or using a better ventilator instead of increasing the window performance.

It is important to note that the data verification program performs calculations according to set criteria in order to produce the recommendation checkmarks for which assemblies are recommended for use in which climate zones. That is, the calculation data have the force of recommendations as far as building project teams or building certification is concerned, but they have the force of requirements as far as window manufacturers are concerned. The products must meet the criteria in order to get recommendation checkmarks for the various climate zones. Since higher performance is required to get recommendations for the colder zones, the data label serves as a selling point for manufacturers looking to position their products in passive building applications.[1] Here are listed the performance criteria by climate zone.

The PHIUS building certification program does have a requirement that the window must be good enough to avoid outright condensation, but this is not usually a limiting factor – a window which gets any checkmarks at all under the PHIUS Verified Window Performance Data Program will almost certainly be low-risk for condensation in all climate zones. For reference, a window condensation calculator is linked to here.

PHIUS consultants are trained on how to do basic interior-surface-temperature calculations. If planned window U-values are significantly above those recommended for the climate, but the building overall meets the peak heat load criterion (more likely to happen for larger buildings), it would be appropriate to locate heating devices under the windows. Presently, there are still not many choices for windows better than R-7, and thus recommendable in Zones 6-8, so designers may need to resort to under-window heating to prevent cold air pooling there.[2] [3]

The PHIUS building certification program does require that windows are entered into the building energy model in their actual size and configuration (every lite), with the performance of the framing members and the glass broken out separately (as opposed to using whole-window properties at a standard size). PHIUS’ window data label contains the information needed to model the windows at that level of detail, but if a project team wants to use windows not (yet) rated by PHIUS, there are workarounds to obtain usable data from other (third party) sources and fill in the gaps with conservative assumptions. Reach out to the PHIUS Certification team at for more information.


High-Performance Windows: Reliable Information Sources

High-performance windows are an important part of a passive or high-performance building project. Some reliable sources of information about windows include the following:

  • Efficient Windows Collaborative (EWC)
    • Focuses on information on how to choose energy-efficient residential windows. A good place to start for DIY home designers and homeowners considering window replacement.
  • National Fenestration Rating Council (NFRC)
    • Rates residential and commercial windows, doors, skylights, storefronts, and curtain wall assemblies mainly for energy (thermal) performance.
    • Has a large online database of rated products, with several hundred manufacturers participating in its programs.
  • American Architectural Manufacturers Association (AAMA)
    • Rates and certifies windows for resistance to air leakage, water penetration, and wind pressure – particularly relevant in hurricane country.


[1] Currently, to get any checkmarks at all, the whole-window installed R-value must be at least 4.5 (IP units), and to be recommended for Zone 5 at least R-7. Most types of windows are rated at 1.23 x 1.48 m size.

[2] Some additional discussion of window comfort and condensation appeared in this November 2015 post on the PHIUS blog.

[3] Note it is possible to design an R-9 window that would meet the recommendations for Zone 8 without using any exotic components, as demonstrated in this hypothetical example.

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