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Severe storms highlight the importance of building resiliency.

On Sept 23, 2022, Hurricane Fiona made landfall on PEI as a Category 1 hurricane with winds up to 111 km/hr. Immediately following the storm there were over 86,000 homes without power. With temps now dropping to 5 deg Celsius overnight, how long can one stay in their home safely before being adversely affected by the cold. How well a home can perform during extended blackouts is what is described as building resiliency.

Passive Houses again prove themselves to offer significantly better advantages in terms of building resiliency. The Passive house design ensures the home heats up during the day from the low-lying winter sun. This combined with the home's high thermal insulation helps retain the heat during nighttime and in turn stay warmer for significantly longer without any form of active heating.


Winter storms are increasing in severity because of climate change. Warmer ocean temperatures can lead to heavier snowfall, and extreme cold temperatures can travel further south because of the warmer and weaker arctic air jet streams. When winter storms are in full blast, millions of people take refuge inside to stay warm. But what happens to indoor temperatures if there is a power outage or if a furnace stops working? Homes vary widely in their ability to maintain comfort during these events. High-performance homes with more insulation, better airtightness, and better windows can outlast the cold, making it possible for people to comfortably “shelter in place” until power is returned. Improving our homes to withstand extreme weather events is an essential strategy for climate change adaptation (while providing lower utility bills and other benefits)—but to this point, we haven’t had a way to tangibly quantify the resilience benefit yielded by these improvements or to compare the extreme weather performance between homes.

This insight brief is intended to address this knowledge gap by outlining the concept of hours of safety. The concept attempts to define the duration of time that homes can be expected to provide safe temperatures when the power goes out based on key building characteristics (e.g., insulation levels). This metric can be used to quantify the amount of time people are exposed to extremely hot or cold temperatures indoors, information that can be used to effectively guide weatherization efforts, emergency response measures, and more. This study modeled the interior conditions for five representative buildings during a simulated power outage in extreme cold conditions in Duluth, Minnesota. These five buildings represent the range of conditions seen in our current building stock: a typical home built in the 1950s, a typical home built in the 1980s, a home that meets the 2009 IECC Code, a net-zero energy ready (NZER) home, and a house that meets Passive House (PH) standards. This research characterizes how well each building typology retained safe indoor temperatures once power was lost.


The risks of extreme cold and their contributing factors are influenced by age, physical health, clothing, duration of event, humidity and other climactic factors, and more. This variability makes it challenging to quantify a single threshold for cold weather safety that applies to all people and all situations, which in turn makes it difficult to provide actionable information about building resilience in extreme cold events. We developed a cold stress scale based on a literature review, but more modeling and work with healthcare professionals is needed to define a widely applicable single threshold for safety from cold exposure. This section summarizes the impacts of cold stress and describes the scale that was used in this analysis.

Cold stress can cause a wide range of health challenges

Cold stress, as defined by the US Navy Environmental Health Center, is when the net heat balance at a given activity level with typical clothing results in heat loss unless the body compensates by thermoregulatory mechanisms. But the health impacts from cold weather can either be acute and happen in minutes (falling into cold water) or chronic and happen in weeks or months (commuting to work every day in cold weather). Acute exposure can result in serious health conditions that can potentially be life-threatening such as hypothermia, when the core body temperature drops below its usual temperature; frostbite, when a body part becomes injured by the cold; and chilblains, ulcers formed by damaged small blood vessels in the skin. Other health risks include pneumonia, flu, cardiac arrhythmias, cerebral insults, ischemic stroke, amnesia, and breathing difficulties. The impacts of cold exposure increase for vulnerable populations. As we age, we lose the ability to effectively thermoregulate. For example, at the age of 80, metabolic heat production is about 20 percent less than that at age 20, so people in their eighties may prefer temperatures about 3°F warmer than people in their twenties. In addition to age, people with certain diseases like diabetes and people acclimated to living in warmer environments can be more susceptible to extreme cold.

Providing a starting point for a cold safety threshold

Because windspeed is not a consideration in indoor environments, hours of safety for cold weather events should be largely defined by temperature and metabolic rate. While more work needs to be done to effectively correlate the dangers of cold stress to these factors, available research can provide a starting point. The temperature ranges shown in Exhibit 1 were selected to roughly categorize cold stress based on a review of over a dozen data points from ASHRAE, the World Health Organization, the National Institutes of Health, and more.


In an extreme cold event, a community’s most poorly built (and often oldest) homes can become dangerously cold within the first 12 hours of a power outage. Exhibit 2 shows how each type of building performed during a power outage. As seen in Exhibit 3, it took between 8 and 152 hours for the indoor air temperature to fall below 40°F in each representative home. During the first day, residents of the modeled Passive House would experience some discomfort as temperatures drop to 64°F. However, they fared much better than those occupying the most poorly insulated and older homes, where the indoor temperature drops below freezing within 12 hours. Even 2009 code-compliant homes drop to 56°F on the first day, a temperature where residents, especially vulnerable populations, may start to experience health issues. During the second day, 1980s homes and code-compliant 2009 homes drop below 40°F, while Passive House residents experience just a 5°F degree temperature change throughout the day. The fact that older buildings quickly drop to unsafe temperatures compared with newer and more efficient housing is an equity concern—low-income residents or seniors may be more likely to live in older housing, which can expose them to life-threatening conditions in winter storm-triggered outages.

City and state governments can use hours of safety as a metric to overcome these equity and safety concerns:

• Retrofits: Weatherization programs such as California’s Low-Income Weatherization Program and New York’s Retrofit New York can use hours of safety to more comprehensively define the value proposition of energy efficiency in building envelope upgrades. This metric can help support incentives offered by state energy agencies and utility companies and potentially from other sources interested in health, safety, and resilience.

• City and state emergency response plans: Hours of safety can be used as a metric to help identify neighborhoods or buildings that are particularly vulnerable to extreme weather, and to identify priorities for support or evacuation. It can also be used to identify safe places to “shelter in place.” Eventually this metric could be incorporated into geospatial models of cities to identify where government agencies should prioritize retrofit programs and can inform program design, evaluation, and metrics. Homeowners can also use hours of safety to justify purchasing decisions. Today, many homeowners in cold climates purchase gas or diesel backup generators to survive grid outages, but these generators are costly, inefficient, and can fail without notice. Providing clear and actionable insight on the resilience benefit of envelope improvements could help many homeowners avoid the cost of that generator by instead investing in efficiency measures such as increased insulation and improved windows that provide additional cost savings and comfort benefits.


As expected, the Passive House building was able to buffer the cold temperatures for the longest period during the outage. As shown in Exhibit 4, there was a 90 percent reduction in severe cold stress hours between the 1950s model and the Passive House model. Exhibit 5 shows that Passive House buildings performed over 7 percent better than the next most efficient building type, the net-zero energy ready building. Thus, the more we improve the insulation, thermal bridging, and air sealing of homes, the more hours of safety the home will provide, with dramatic differences from current code up to Passive House standards. Considering hours of safety improves the value proposition for Passive House buildings and other super-efficient buildings by quantifying the resilience benefit of greater investments in high-performance building envelopes, particularly in areas prone to extreme weather events.


This research suggests that there is value in further developing an hours-of-safety metric that can be used by home performance contractors, policymakers, and homeowners to make informed decisions about resilience, safety, and comfort. Once defined, the data necessary for quantifying hours of safety could be collected through typical construction inspections, energy auditing, commissioning, and building certification practices. This research suggests that there is an opportunity to factor hours of safety into policies and business models in the following ways:

1. Increased data availability: Hours of safety is one metric that can be used to provide transparency and data to homeowners, the performance contracting industry, and policymakers interested in resilience planning. This can be driven through local policies such as residential energy disclosure policies, which require sellers or landlords to make home energy performance information available to prospective buyers and/or renters. This type of information enables cities to drive more energy efficiency upgrades, protect consumers and help them make informed decisions, create jobs and invest in local businesses, and reduce energy burdens for low-income households. Increased data availability can also be driven by the private real estate sector in mortgage underwriting algorithms to help insurance agencies mitigate risks.

2. A tool for resilience education and public health campaigns: The hours-of-safety concept can help inform a consumer-facing education campaign to highlight the health and safety benefits of energy efficiency in building envelope construction and retrofits. This can help seniors and other vulnerable residents take measures to improve their homes to protect against extreme hot or cold weather events.

3. A metric to include in future code development and improvements: Hours of safety can be incorporated into future code cycles to inform new construction and retrofits and help create market incentives for efficiency upgrades. The metric could be particularly useful for jurisdictions adopting energy efficiency standards for rental properties, given that these homes are often the most poorly insulated and renters are less likely to own a backup generator for grid outage events.


Efficient building envelope components provide a significant added value during extreme weather events. Society needs a way to consider that value alongside energy cost savings. The hours-of-safety concept can help value resiliency and further the value proposition for energy efficient building envelopes, helping us understand if our buildings are prepared for the extreme weather events that are unfortunately becoming more frequent. Future actions for refining this concept include:

1. Refining and updating specifications for a threshold for unsafe temperatures in both extreme heat and cold weather events. The scale used in this analysis was developed from a preliminary review of research on thermal comfort and stress that can be used as a starting point for a more rigorous clinically tested metric.

2. Performing energy modeling for both hot and cold weather scenarios to determine which building characteristics can be used to accurately predict approximate hours of safety in a cost-effective manner. This modeling should consider characteristics such as window-to-wall ratios and infiltration.

3. Creating a standard hours-of-safety metric that the industry can adopt.

4. Crafting guidelines for simplified adoption of the hours-of-safety metric to make it easier to integrate into the industry.

Contact us to find out how we can help make your building more resilient.


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