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.

Emissions Down, Scale Up: Phius 2021 Year in Review

This past year was the start of something special at Phius.

We began 2021 by declaring a commitment to “Emissions Down, Scale Up!” and we did just that. From the Phius staff to the inaugural PhiusCon (formerly North American Passive House Conference) to the greater passive house community, we have witnessed plenty of growth over the last 12 months.

New Brand Same Phius GraphicTo commemorate the year that was, we have compiled some of the accomplishments from 2021 that we and dedicated professionals like you have accomplished together. 

  • We safely hosted a sold-out PhiusCon in Tarrytown, New York, which created a substantial wave of momentum throughout the industry as we charge into 2022. We also announced PhiusCon 2022 will be taking place Oct. 25-28 in Chicago.
  • We successfully launched an Intro to THERM Workshop and WUFI Passive for Beginners training in 2021. We plan to launch several more to continue spreading knowledge in 2022.
  • We welcomed five new members to the Phius team last year, adding to the tremendous and ongoing growth of the Phius staff. We’re thrilled to have John, Tony, Max, Josh and David on board!
  • We began the transformation of the Phius brand as we work to scale up. There is much more to come in 2022 with the launch of our brand new website and Phius portal, both of which will help us continue to grow passive house.
  • The Phius Alliance activated the Southern region, including the addition of our brand new Houston Chapter! This movement fueled our decision to host PhiusCon 2023 in Houston, so get ready for Phius to head south next year.
  • A number of bills were passed aimed at reducing the use of natural gas in the built environment, including a significant piece of legislation recently in New York City.
  • We went global as we hosted the first-ever Phius CPHC training and certification exam in Japan in partnership with PHIJP! We have also created a Japanese version of our criteria calculator to provide those professionals with all the tools they will need.
  • We celebrated the top passive house projects in our annual Phius Passive Projects Design Competition! We are always grateful to the teams who submit their wonderful projects, and we were once again blown away by the ingenuity of all the submissions. The projects were so impressive this year, that our distinguished panel of judges named two winners of Best Overall Project: Solis and McQuesten Lofts.
  • We had more than 1.8 million square feet of submitted projects in 2021, and we look forward to blowing past the 2 million mark in 2022.

We are proud of what we have accomplished, but we are nowhere near satisfied. There is so much work to be done, and we look forward to rolling up our sleeves and working hard to ensure that this list is twice as long next year.