With Phius 2027 on the horizon, PhiusCon Phoenix in full swing, and celebrating a full decade (plus a little extra) since the release of Phius’ groundbreaking Climate-Specific Passive Building Standards in 2015, it's high time we take a moment to reflect on where we started, where we've been, and where we’re going from here.
The Phius (r)Evolution
By now, most in our community should know that Phius regularly updates and re-launches a new version of its standards on a three-year cycle. This includes a new version of the Phius Certification Guidebook, which saw the most significant round of updates yet in 2024. In fact, a new version was just released earlier this week (v26.1.0), aligning with our tradition of mid-cycle updates, that builds upon the significant improvements we made for 2024.
Most notably, this mid-cycle version now includes modeling protocol for the long-anticipated METr energy modeling software. This new cloud-based software was developed by C3RRO® in conjunction with Phius to bring the technical aspects of WUFI Passive into the modern age. It features improved geometry workflows and model navigation, new Phius databases for building materials, assemblies, and windows, and built-in calculations that aim to retire our external Excel-based calculators and simplify the modeling, documentation, and submission processes.
Okay, enough rambling about what's new this week (can you tell it's all I've been working on for the past few months?). Let's go back to the beginning and take a look at the “whys” and “hows” behind the evolution of the Phius standard.
We'll start in 2015, when Phius first launched the world's first climate-specific passive building standard. This was a major shift from the Passive House Institute (PHI) standard developed in Europe, whose performance targets are the same for all projects regardless of location or climate.
For this first pass, Phius developed a new target optimization approach using BEopt (Building Energy Optimization), which was initially developed by researchers at the National Laboratory of the Rockies (NLR, formerly NREL: National Renewable Energy Laboratory). One iteration of a single-family home was studied in 300 different climate locations across the United States. The results of the study were then curve-fitted for 1,000+ climate locations, which established static space conditioning targets for each climate location represented in the form of the PHIUS+ 2015 targets map (above).
Regarding the intersection between maximum performance and cost, Phius went slightly beyond the cost optimal point when establishing the targets for the 2015 standard. The cooling demand target included credit for natural ventilation, which relies on occupants opening and closing windows when appropriate to facilitate passive cooling. This led to more stringent cooling targets compared to later versions of the standard.
In addition to optimizing targets based on climate specificity, Phius also changed the metric for air leakage from air changes per hour (ACH) to air leakage per square foot of enclosure area (cfm50/sf env). This is generally a better metric of envelope performance than ACH because air leakage occurs “at the surface, not in the volume” (to quote Allison Bailes). Bailes talks more about this concept in several articles:
Infiltration Occurs at the Surface, Not in the Volume - Energy Vanguard
It's the Hole - Understanding What a Blower Door Is for - Energy Vanguard
How Much Air Leakage in Your Home Is Too Much? - GreenBuildingAdvisor
All-in-all, as is with any shiny new thing, there were some unforeseen limitations and challenges to meeting the PHIUS+ 2015 criteria:
Static source energy target was a very stringent per-person limit based on “fair share” of CO2 (per 2,000W Society) that proved difficult to meet without PV, especially considering the 3.16 site-to-source energy factor
PHIUS+ 2015 Source Zero saw a temporary increase of the targets to address this, but still required 100% source energy offset with PV if the project didn’t meet its target without it
Restrictive target criteria required simple (yet highly efficient) geometry, thick assemblies, and over-engineered design solutions
Local builders were unfamiliar with new air sealing metric, leading to difficulty with implementation and ability to price the work
Three years later, PHIUS+ 2018 was launched and aimed to address these shortcomings. A new optimization study was run with the scope expanded to include:
Small single family
Larger single family
Townhouses
Midrise (four-story) multifamily
Highrise (10-story) multifamily
Across these new typologies, multiple occupant and unit densities were studied. As it turns out, occupant density and form factor impact the performance of a building. Who knew!?
To make the standard more accessible from a cost perspective, the true cost-optimal points along the BEopt curves were selected to establish the targets, which generally relaxed them compared to the 2015 standard. Credit for natural ventilation was removed from the study to focus the standard more on the performance of the building and less on occupant behavior (certifying buildings, not people). This generally made the cooling targets more realistic and achievable, especially for multifamily projects. The source energy structure matured and introduced the CORE and ZERO tiers which allowed project teams to utilize on-site PV as well as off-site renewable energy (e.g., renewable energy credits (RECs), virtual power purchase agreements) to offset the source energy and meet the still-static targets without achieving full net-zero.
Overall, these fundamental changes allowed for increased design flexibility in the form of tradeoffs between geometric efficiency and assembly R-values. However, some projects, namely tiny houses and ADUs, were still struggling to meet the targets due to their high envelope-to-floor area ratio.
To address this, Phius added tiny homes to the optimization study for the 2021 standard. But 2021 included other major structural changes that once again aimed to make the Phius standard more accessible to projects of all shapes and sizes.
In regard to source energy, the limits for projects shifted from a static "macro" target to dynamic, project-specific targets. The key drivers of establishing these targets were occupant and unit density and, just like the space conditioning targets, were derived from the BEopt optimization study curve fits. Another notable change was the shift in source energy factor by way of using a future-looking scenario to align with the national grid trajectory. You can read more about the methodology in this Tech Corner article.
Since Phius finally had a firm handle on single family projects by this point, 2021 also saw the debut of the Prescriptive Path for single family, duplex, and townhouse projects. This new certification path eliminated WUFI Passive as the compliance tool, replacing it with the Phius CORE Prescriptive Checklist, which both expanded the accessibility of the Phius standard to those without energy modeling experience, and streamlined the design and certification processes and timelines for seasoned Phius professionals.
As noted previously, 2024 saw the biggest change in regard to the Certification Guidebook, but the core standards (for new construction projects, that is), saw the fewest updates in regard to target-influencing changes. Generally, small corrections were made to the optimization studies due to prior BEopt limitations as well as updates in anticipation of the adoption of new 2021-era IMC requirements. Additional details can be found in this blog post.
Non-Residential Source energy targets aligned with percentage improvement over ASHRAE 100 curve fit
Curves for high-rise projects re-optimized to correct issues stemming from modeling workarounds in BEopt
Appliances previously undercounted as a result of workarounds
Ventilation rates increased for kitchens and bathrooms to align with IMC 2021 minimum requirements
The major standard update for 2024 was the release of the REVIVE Pilot. This brand new standard aims to address one of the most important project typologies when it comes to sustainability: retrofits. It focuses on resilience, decarbonization, and passive survivability as we face increasingly extreme weather events and potential power outages. While submissions to this certification path are currently closed, there are three pilot projects pursuing this certification path acting as case studies to inform future development and improvements.
This brings us to the present day, where Phius has now certified 11.2 million square feet worth of projects across 42 states and three countries (US, Canada & Japan). So, where do we go from here?
Phius has been, and will continue to be, committed to listening to feedback from our constituents and adapting to the times in an effort to make our standards mainstream. The current list of items to evaluate for next year includes:
Optimization and re-evaluation of non-residential projects
Simplification of the prescriptive path
Viability for a multifamily prescriptive path
Updates to default modeling parameters to better align with monitored data and actual occupant behavior (i.e. heating/cooling setpoint temperatures)
Source energy target optimization by climate location/grid region
Continued development of the REVIVE (retrofit) Pilot Standard
…and more! Stay tuned on the Phius website and social media platforms for ongoing standard updates, events, trainings, resources, and more. Most importantly, keep building good buildings so we can keep learning and evolving the Phius standards.
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Note: The top image is of 425 Grand Concourse. To learn more about the project, see our coverage here.