Resilient ADUs: A Passive Survivability Modeling Study

Al MitchellIn this week’s blog post, Phius Technical Staff member Al Mitchell presents a follow up study on the resilience of accessory dwelling units.

As noted in a previous blog post from last July, the City of Chicago’s building code has recently been changed to allow for the construction of accessory dwelling units – also known as ADUs, coach houses, or granny-flats.

In the previous study, WUFI modeling showed an annual energy savings of 35% by designing one of these small backyard apartments to the 2021 Phius CORE Prescriptive path. The assembly performance values associated with Chicago are noted in the table below.

Case Wall R Roof R Slab R Window-U Airtightness CFM50/sf
IECC 2018 18.4 44.0 10.6 0.3 0.31
2021 Phius CORE Prescriptive 40.0 71.0 21.6 0.16 0.04

As demonstrated by the aftermath of a severe power grid failure in February of 2021 in Texas, buildings need to perform better during inclement weather events and/or power outages. To respond to this need, Phius has put considerable effort into analyzing building resilience and passive survivability. Other institutions have also pushed in this direction. For this first exploration, heating resilience was evaluated.

Those who are familiar with passive building design strategies are aware that these techniques are well suited for reducing the need for space conditioning, especially in cold climates. This comes in handy for heating resilience and survivability. 

The super insulation and airtight building envelope retain heat gains in the space, but is it enough when the heat gains are reduced to just the occupants during a power outage? In a cold climate such as Chicago, can this method of building save a life in a severe event? The same single-story coach house from the previous blog post was put through a dynamic modeling study to respond to these questions.


First, it is important to establish the metrics for the study. 

To quantify the thermal performance during a power outage in the coldest typical week in Chicago, two key metrics were used. The first is a simple graph of the space temperature, where it is easy to see the rate at which heat escapes the building, and whether or not the building temperature stabilizes around a balance point. 

The second metric is Standard Effective Temperature (SET) hours. Developed from an ASHRAE research project in 1986, the SET range is based on the Pierce Two-Node comfort model, which is calculated in a similar way to predicted mean vote (PMV). Using this to assess passive survivability, it is quick to evaluate a single number of SET Hours, or degree-hours in which the building zone temperature is outside the SET range, typically from 54°F to 86°F.

The same geometry and assemblies from the ADU study in WUFI were used, and the building was modeled in EnergyPlus, in order to perform accurate hourly calculations. To find a stress case for a power outage, a rolling average was used to find the coldest average week from the hourly TMY3 weather file. For Chicago, MDW Airport, that week is January 29th through February 4th.

In the simulation, the power to all systems other than the ERV was cut out. Based on preliminary work, it was determined that airtight buildings need to have some form of ventilation to keep CO2 concentrations down, and opening a window can let in too much untempered air. Therefore, especially in cold climates, ERVs should be provided on battery backup. The occupants are assumed to stay home 24 hours a day during this time period.  

Infiltration was adjusted between the two cases, and for the IECC 2018, this provided enough air exchange to not require a ventilation system to maintain CO2 concentrations. Hourly temperatures were plotted on the below graph, and SET temperatures were collected.

ADU Chart


The IECC 2019 case had 2006 SET hours with a maximum duration of 167 hours below the 54°F.  The Phius Prescriptive Case had 164 SET hours with a maximum duration of 56 hours.  

Looking at the above graph, it is clear that in the code-built building, the zone temperature drops quickly and reaches a minimum temperature below 10°F. In contrast, the Phius building maintains its temperature more, with a minimum interior temperature of 40°F, which visually seems to hit a balance point temperature. The Phius space will ultimately be cold, however significantly warmer than that of the code built building, and more survivable.  

This ADU is a pilot study for Phius, as more resilience studies are done. The aim is to provide guidance and simulation protocol to encourage the testing of building resilience, and to determine a way for it to be integrated into Phius certification programs and projects going forward.

Making Inroads: Phius 2021 Policy Wrap-up

isaac picIn this week’s blog, Phius Policy Specialist Isaac Elnecave outlines the various incentive programs, QAPs and energy codes that Phius was included in this past year.

The surge of Phius-related policies continued in 2021 with the inclusion of Phius in utility incentive programs, Qualified Allocation Plans and energy codes.

Over the last few years, it has become clear that well-designed policies can significantly spur the construction of projects meeting the Phius standard. In 2021 alone, there have been 47 projects submitted in Massachusetts, 29 projects in New York State and 27 projects in Connecticut (all states with existing Phius-related policies). 

With the calendar turning to 2022, we wanted to take this opportunity to provide a rundown of some of our policy successes from this past year. Below you will find a list of new Phius-related policies (including a number of advancements in Phius’ home state of Illinois).

Utility Incentives


ComEd in Illinois has established a pilot incentive program providing money for both pre-construction and upon receiving final certification. ComEd will be taking applications for the program through 2022.  The applications will be reviewed on a competitive basis and will be chosen based on criteria such as size, financial status and timeline. Utilities in Massachusetts (through the MassSave program) and Connecticut (through energizeCT) offer similar programs.

Low-Income Housing Tax Credit


The award-winning Finch Cambridge project was funded in part by a Massachusetts incentive program.

The award-winning Finch Cambridge project was funded in part by a Massachusetts incentive program.

The Illinois Housing Development Authority (IHDA) completed the 2022-2023 Qualified Allocation Plan (QAP). The QAP is the document the IHDA uses to evaluate which potential projects will receive tax credits. The evaluation process consists of using a points system (projects with higher point totals are more likely to receive funding). One of the criteria for evaluation is sustainability and/or energy efficiency. In the new QAP, projects receiving a Phius CORE certification will receive 10 points (Phius Zero projects receive an additional three points) out of a possible 100 points. 


The Michigan State Housing Development Authority included Phius in its QAP for the first time. Projects meeting the Phius standard will receive four points; most other standards will receive three points or fewer. 


While Phius projects have been included in previous versions of the Connecticut QAP, the updated version gives Phius projects the largest number of points (only projects meeting the Living Futures Standard receive the same number of points). 

Energy Codes


The Illinois legislature enacted – and the Governor signed – the Clean Energy Jobs Act (CEJA). Among the requirements included in the 800-page piece of legislation, is that the Illinois Capital Development Board (the agency in charge of developing the state energy code) must include Phius as an alternative compliance path. This means that projects certified by Phius automatically meet code in Illinois.

We at Phius are proud of the inroads we and our community made in the realm of policies, incentives and codes this past year, and we look forward to carrying that momentum into 2022.

New York City Takes Significant Step Toward Electrification

New York City is about to be one step closer to total electrification.

The New York City Council has enacted a bill, which current Mayor DeBlasio will sign, that will ban the use of natural gas hookups on a substantial number of new buildings in the coming years. The ban will apply to all new construction seven stories and shorter starting in December 2023 and for buildings taller than seven stories starting in 2027. 

New York City as seen from 425 Grand Concourse, a Phius project in the Bronx, New York

New York City as seen from 425 Grand Concourse — a Phius project in the Bronx

There are exceptions to the new law, including: multifamily buildings in which more than 50 percent of residents are low-income, some manufacturing facilities, laundromats, crematoriums, hospitals, and commercial kitchens. 

Despite the exceptions, this law will apply to a substantial number of new buildings. From a Phius perspective, this new law is in line with the new prescriptive path for the Phius CORE standard and the Phius ZERO standard which prohibits the use of fossil fuels. Even for projects that do not choose the above standards (projects pursuing the performance path under Phius CORE can use fossil fuel combustion appliances), an increasing number are opting to go all-electric. New York City joins other major cities including Seattle, Sacramento and San Jose in enacting policies restricting the use of natural gas in the built environment. 

Finally, the legislation also mandates two studies: one on the use of heat pumps and a second on the legislation’s impact on the electrical grid. The legislation, however, does not apply to existing buildings.

This new law is part of a larger electrification effort within both the City and State. Members of the New York State Assembly have introduced legislation that would prohibit towns and cities across the state from allowing new natural gas hookups. Exceptions to this requirement would require a project to show that an all-electric approach is either physically or technically infeasible (detailed rules determine infeasibility). Moreover, New York City Housing Authority (NYCHA), New York State Energy Research and Development Authority (NYSERDA) and the New York Power Authority (NYPA) are funding a challenge to space heating manufacturers ($263 million) to develop new products designed for existing multifamily buildings with the aim to spur the electrification market in this important sector. This effort moves in parallel with the heat pump study requirement in the New York City law. 

As we all work together to decarbonize the built environment, we look forward to reporting on many more similar policies in the coming months and years.

Why Your Project Should be Phius Certified

Why should I certify my project?

That is one of the most common questions we get asked by project teams and their clients. It’s a valid question that can be answered in a number of ways. And while we could go on and on about the advantages of Phius certification, we thought it would be helpful to put together a concise guide to help answer this question. 

As a Klingenblog reader, you may already be familiar with the topics discussed below, but we invite you to use this as a tool should anyone ask you why they should get their project Phius certified. 

Why Certify?

When properly harnessed, passive house principles help create buildings that are efficient, resilient, healthy, durable and comfortable. But if your project is not Phius certified, you have no assurance that it meets any of those criteria.

Simply “incorporating passive house principles” is not enough. Phius certification is the only way to guarantee the quality and performance of a passive house project — like insurance on your investment. Think of certification as a risk management tool. 

That is the Phius Difference.

How does the Phius Certification process guarantee results and benefit project teams and clients? We’re glad you asked.

  1. Park Ave GreenIt provides comprehensive design review and consultation throughout the entire building delivery process Our technical staff works with teams during the design phase to optimize energy and cost efficiency, and to identify and solve potential problems early on.
  2. It mandates third-party verification — This ensures the building is designed and constructed to meet the high-performance standards for energy use and that critical systems are commissioned into proper operation. Third-party verification is also typically required by the incentive programs of utilities and governments to insure their investments. It mitigates risk for all stakeholders in a project.
  3. It introduces risk management — Passive building requires special attention to moisture control and ventilation. Phius Certification staff and third-party QA/QC professionals can identify problem areas at the design stage before they become real-world problems.
  4. It builds and shares knowledge — As more scenarios, project types and solutions pass through the certification process, Phius pays it forward to future project teams through direct feedback, building the public Certified Project Database and ongoing updates to the Guidebook.

So, what is the certification process like?

The Phius Project Certification process has two main components: design review and final construction review. The design review is an iterative feedback process, often exchanging detailed feedback back and forth with the submitter three times before design certification is awarded.

Phius’ feedback isn’t just a yes/no response. We want your projects to be successful and therefore offer insight when red flags arise and are as committed to taking your project to the finish line as you are.

To learn more about submitting a project for certification, visit our Project Certification web page or email

Giving Thanks to the Phius Community

We at Phius have plenty to be thankful for this year.

First and foremost, we are thankful for our dedicated staff, and all the loyal members of the Phius community. If it weren’t for all of you, we would not be well on our way to making Phius the mainstream building standard.

Cups ConferenceWe are also grateful to have had the opportunity to connect in person with so many of you during PhiusCon 2021. More than 400 people joined us in Tarrytown, New York, for this year’s conference, with about 100 more joining us virtually as part of our Bootleg Series. No amount of Zoom calls or virtual meet-ups can replace an event like that, and we are grateful to have been able to host such an informative, (hopefully) entertaining and most importantly, safe event.

Thanks also to the ambitious project teams who participated in the Annual Design Competition. Each year we are inspired by the exemplary entries.

The inroads being made in policy in a number of areas is a crucial step forward for our organization and the passive house movement as a whole. We are appreciative of our hard-working team as well as the policymakers who are willing to work with us to create healthier, more efficient buildings.

We would be remiss not to mention our ever-growing number of project certifications. We have already nearly doubled our submitted project count from 2020 and are well on our way to having our first ever year with 200+ submitted projects. And as we continue seeing an influx of multifamily projects, we’re thrilled to have more than 1.8 million square feet of pre-certified and fully certified Phius projects in 2021. Our Certification Team has been plenty busy this year, and we’re thankful for all their hard work.

Expanding to the greater Phius community, we are of course thankful for our engaged and enthusiastic Phius Alliance members and chapters. We have been able to add a number of new chapters and members, and we are grateful to support such a wonderful community and be able to bring them dynamic programming such as webinars and office hours.

There is much work to be done, but we are most thankful for all the people and organizations who have helped us make great strides toward our goals.