When the Power Goes Out, Will Your House Keep You Safe?

In February 2021, a polar vortex swept through the Midwestern United States. At the Iowa Nest Residence, a recently completed house in rural southeast Iowa, the homeowners had left their heating system off to see how the home would fare with only its passive systems. Outside, temperatures dropped below freezing for over a week, dipping as low as -13 degF (-25 degC). But inside, the home remained between 45 and 55 degF (7.2 to 12.8 degC)—with no active heating whatsoever. [1]

The home is a Passive House, and was carefully designed to be not only beautiful, livable, and extremely energy efficient, but also to be thermally resilient: to maintain habitable temperatures when the power goes out—even for extended periods.

This concept, known as both “thermal resilience” or “passive survivability,” has become increasingly relevant as severe weather becomes both more extreme and more frequent.
 

The Bondurant Passive House

We did a deep dive into Passive Survivability for the design of the Bondurant Passive House, a retrofit and addition to an existing home outside of Des Moines, Iowa. The homeowners wanted to prioritize resilience—particularly in light of the increasingly severe weather Iowa is already experiencing, which is only projected to grow more acute in the coming decades. They wanted a home that would stay safe and livable, even if the power went out for days.

We asked three big questions:

1. How much would improving the home’s insulation and air-tightness help to maintain habitable temperatures during an extended power outage?
2. Was there an optimum level between upgrading the home to a current “code minimum” envelope, and upgrading it to a full Passive House envelope?
3. How would the answers to the first two questions change if we also took into account predicted future climate conditions in the region?

To find out, we studied four different potential retrofit scenarios with increasing levels of insulation and air-tightness, from “Code Minimum” to “Full Passive House.” For each, we ran a simulation to determine the number of hours that would be comfortable, too hot or cold, and dangerously hot or cold without mechanical heating and cooling. We ran the simulation twice for each option: once using industry-standard historical weather files, which are based on the past 30 years of weather data, and again with an approximation of the future weather conditions for the home’s location, based on research conducted by the University of Maryland Center for Environmental Science. Our hypothesis was that higher levels of insulation would be less beneficial in the future climate scenario due to reduced heating needs.

The results of our study are shown here. The image above is a summary chart, showing the portion of hours in each temperature bin. The image below is a detailed view showing each hour of the year.

The results were clear: the Passive House design eliminated the “dangerously hot” and “dangerously cold” hours in both historical and future climate scenarios, whereas the Code Minimum design had a small but potentially dangerous number of hours in these categories.

Passive House as a Life Safety Standard

These findings are not an outlier. In 2020, Rocky Mountain Institute released a report that concluded that homes built to the Passive House standard significantly increase the “hours of safety” compared to other types of construction – even compared to the above-code “Net Zero Energy Ready Home” certification from the U.S. Dept. of Energy [2].

In 2022, Phius conducted a similar study, finding that Passive House level retrofits significantly reduced dangerous temperatures during an extended power outage [3]. This study, in part, led to the new Phius standard for retrofits, Phius REVIVE, launched in 2024, whose requirements focus largely on thermal resilience.

And in 2023, the Pacific Northwest National Laboratory (PNNL) issued even starker findings in the report “Enhancing Resilience in Buildings.” PNNL looked at the impact of passive design on passive habitability (“days of safety”) and loss of life (“excess mortality”) during a coincident power outage and extreme weather, for a range of housing types and for multiple U.S. climate zones. They found that Passive House significantly improved days of safety and lives saved across all climate zones [4].

These studies show that the same strategies that provide a super-efficient, healthy, and comfortable home can also help to keep occupants safe in extreme weather.
 

Innovating Toward a Resilient Future

At Sol, we integrate climate resilience analysis into all of our architectural design projects. Our team is helping clients integrate Phius standards in both retrofit and new construction projects, incorporate FORTIFIED designations to protect homes against severe weather, and explore new avenues of resilience, such as expanding local food systems, decentralizing water and waste infrastructure, and fostering mutual aid through design for caring and commoning.

We will continue to investigate how we can build in ways that not only help to mitigate climate change, but also adapt to its inevitable impacts.


REFERENCES

1. Iowa Nest Residence, https://www.iowanest.com/
2. Rocky Mountain Institute, "Hours of Safety in Cold Weather," 2020, https://rmi.org/insight/hours-...
3. Graham Wright and Al Mitchell, "Passive Building, Thermal Resilience, and Retrofits," August 2022, https://passivehouseaccelerato...
4. Pacific Northwest National Laboratory, "Enhancing Resilience in Buildings," July 2023, https://www.energycodes.gov/si...