Does Your Job Require You to Choose or Spec Windows? We Want to Hear From You!

Michael FrancoIn this week’s blog post, Phius Product Certification Manager Michael Franco breaks down the challenges of choosing the right windows for passive house projects and invites you to our upcoming roundtable event to discuss what data points are most important when selecting windows.

A sample color infrared result from LBNL THERM software. This software tool allows practitioners to model 2D heat transfer between building components (in this case, how a window frame, its spacer, and glazing package interact as seen via a cross-section at the sill area). The left-hand side of the image represents the outside-facing portion of the window, while the right-hand side is the indoor portion. For the purposes of this visualization, red is warm, purple is cold. Heat is shown moving through this window assembly in the direction of red to purple (from inside the building toward the outside area).

A sample color infrared result from LBNL THERM software. This software tool allows practitioners to model 2D heat transfer between building components (in this case, how a window frame, its spacer, and glazing package interact as seen via a cross-section at the sill area). The left-hand side of the image represents the outside-facing portion of the window, while the right-hand side is the indoor portion. For the purposes of this visualization, red is warm, purple is cold. Heat is shown moving through this window assembly in the direction of red to purple (from inside the building toward the outside area).

Windows are critical to the function of any building, especially a passive one. We as building occupants and passive building practitioners have come to expect a lot from these transparent feats of engineering.

Our windows must let in light, but also the right amount of heat from the sun. Depending on the climate of residence, the “right amount of heat” may be as little as possible. Windows in climates with long cooling seasons are often asked to reflect a significant portion of the heat that is packaged together with the sunlight we need.

We also expect windows and doors to keep outside air outside, except for when we change our minds, enter, or leave the building. Whether they swing, slide, tilt or turn, these operable envelope components must find a way to properly seal against outside air. On top of this task, windows and doors must mitigate energy losses via thermal bridging despite their often more complex frame profiles – quite a tall order since we cannot sacrifice outside air or egress on demand.


All of these considerations factor into performance. For example, in order to meet the demands of a passive building energy model, a CPHC
® (Phius Certified Passive House Consultant) or energy modeler must understand a window’s thermal performance at the component level. To accurately predict how a fenestration product will perform, the practitioner needs specific thermal performance values for the product’s frame, spacer, and glass in order to calculate how much heat will escape annually via the glass itself and the contact points between the wall, frame, spacer, and glass when the product is installed at actual size.

This requirement is why center-of-glass performance values alone or whole-window performance values at a standard size are insufficient to understand a window’s thermal performance in an actual product. A window’s performance does not scale simply or linearly if someone were to take a whole-window performance value at a standard size and attempt to scale it up or down to the size their project requires. The ratio of frame to glass heavily affects a product’s performance, and it’s not possible to accurately measure the change in performance from standard size to actual size without component-level data.

Passive building practitioners are acutely aware of all of the above requirements (and more), as they navigate a sea of data to find windows, doors, and skylights that are the right fit for their project. Fortunately, Phius Certified Windows contain all the component-level data necessary to properly model the thermal performance of a window, door or skylight.

Here is where Phius would like to contribute even further – we plan to release an all new, updated version of our Certified Window Database that offers a robust, easily-navigable list of Phius Certified windows. The goal for this database is to offer practitioners these key features:

  • Photo by Max Lapthorne during a tour of the 425 Grand Concourse project

    Photo by Max Lapthorne during a tour of the 425 Grand Concourse project

    A comprehensive list of Phius Certified windows

  • Data critical to energy modeling and building performance for each Phius Certified window
  • Searching, sorting, and filtering the database to find the right Phius Certified window for a job

While we at Phius think we have a good understanding of what window data is important to passive building practitioners, we also want to make sure that what we produce will be as useful as possible to the community. In order to ensure that we create a worthy tool, we humbly ask that you help us help you.

Are you a CPHC, architect, or engineer? Do you regularly source or model windows for projects? We would love to hear from you. Your input will help us ascertain which data practitioners consider most critical in their search for windows, doors, and skylights.
Please join us for a focus group on Jan. 11 hosted by the Phius Product Certification team (Michael Franco and Graham Wright) and help us understand what you need to find the right transparent envelope components for your projects. Please RSVP here via this link. If you have questions or cannot attend the focus group on the date above and would still like to offer input, email Michael Franco (mfranco@phius.org).

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