Looking at the energy performance of building envelopes through a life-safety lens fundamentally changes the conversation about energy cost payback and could drive more stringent fenestration performance requirements. Conversations linking life safety with the stringency of building envelope requirements in energy codes are starting at the governmental level.
A new study commissioned by the U.S. Department of Energy (DOE) provides a framework for monetizing resilience. It also provides a mechanism for including these resilience benefits and the social cost of carbon as part of a cost-effectiveness framework for code development. This work could be significant for driving state-wide model code adoption.
Passive Survivability and Building Envelope Performance
One of the first studies linking building envelope thermal and solar control performance to passive survivability was commissioned by Urban Green and completed by Atelier Ten. Aptly named “Baby, It’s Cold Inside,” it was published this month 10 years ago. It clearly showed the inability of buildings built to the model energy codes (at that time) to sustain a survivable environment for humans in a heat wave or a cold snap during a prolonged power outage.
Since then, awareness of thermal resilience has been growing, but slowly. In 2018, the U.S. Green Building Council’s (USGBC’s) LEED program introduced a pilot passive survivability credit, and in 2019, Ted Kesik of the University of Toronto published the Thermal Resilience Design Guide. A 2023 report by the Building Energy Codes Working Group of the International Energy Agency identifies building energy efficiency as a key strategy for climate resilience. They identify several jurisdictions, many of them in North America, which have started to tackle resilience in combination with energy codes:
- New York City’s Climate Resiliency Design Guidelines;
- British Columbia’s Energy Step Code Design Guide Supplement S3 on Overheating and Air Quality; and
- Energy stretch codes in Massachusetts and Vancouver, which include resilience requirements.
However, quantifying the life-safety benefits of energy code stringency (especially related to the building envelope) is still not mainstream. The issue has certainly not yet reached the energy code development process. Envelope stringency improvements are based solely on the cost-effectiveness of energy cost savings. They do not consider benefits from saving lives, reducing property damage, improving occupants’ well-being or reducing carbon emissions.
The DOE Resilience Study
The National Laboratories completed the DOE’s resilience study and created a methodology to quantify the resilience benefits of building energy efficiency. These benefits include reducing deaths and property damage and providing shelter-in-place during extreme heat and cold weather events. Using this methodology, the study estimates the cost-benefit of adopting higher stringency energy codes in single-family homes (SFH) and multi-family residential buildings (MFRB), incorporating resilience, carbon emissions, and energy efficiency.
The Study Details
The DOE’s study simulated buildings constructed to three different performance levels in both hot and cold weather extremes for six cities in climate zones 2 through 6:
- Current building stock for existing buildings or historic code for new buildings;
- Current model code: 2021 IECC-R for SFHs and ASHRAE 90.1-2019 for MFRBs; and
- Above code: The Passive House Institute U.S. (PHIUS) 2021 standard.
The PHIUS standard focuses on delivering an air-tight, thermal bridge-free, high-thermal performance envelope tailored for U.S. climate zones. It requires a substantially higher envelope performance than the current code in each climate zone.
In addition to SFHs and MFRBs, the study analyzed the performance of an assisted living facility (ALF) during the 2021 extreme cold in Texas. Also, it simulated the impact of a heat wave. The findings further isolated the impact of different building envelope strategies.
Days of Safety
The study by the DOE estimated the “days of safety” in buildings built to the three performance standards (current stock, current code, above code) during a seven-day period of extended hot and cold extremes without power (see table below for an example for SFHs). The results showed that improving the building envelope performance increases the number of safety days in all locations except Los Angeles and has the most impact in Portland and Atlanta. For example, in Portland, an existing SFH would be safe to live safely in for only 1.1 days in a cold snap and 4.7 days in a heat wave. A current code-compliant home would be safe for 2.4 and 5.5 days, respectively, and a median PHIUS-built home would be safe for the whole week.
Better Building Envelopes Save Lives
The DOE’s study also estimates the number of lives saved per hot or cold climate event, although they identify several challenges with the methodology that likely result in an underestimate. For example, in Houston, existing MFRBs upgraded to PHIUS performance could save an estimated 48 and 49 lives during an extreme cold and heat event, respectively, compared to the installed base performance. Compared with bringing existing buildings up to the current code, MFRBs built to PHIUS performance could save 17 more lives in extreme cold and seven more lives in extreme heat events. The table below summarizes the DOE’s estimates for other climate zones.
Financial Cost-Benefit Analysis
The DOE’s study provides a model for monetizing the estimated excess deaths ($10 million per life!) and property damage caused by these extreme weather events. This model also considers the likelihood of these hot and cold events happening and the potential for power outages coinciding with them. The model then overlays mortality and property damage quantification to provide for cost-annualization.
Savings due to regular energy costs and additional reductions in the social cost of carbon are also included to create a full view of energy, carbon and resilience monetized benefits compared to upfront costs.
Takeaways
SFH: In all climate zones, the DOE study found a financial benefit to going above code to adopt additional passive envelope design strategies in new homes. Despite some locations’ low probability/risk-adjusted mortality benefits, the social cost of carbon and energy savings are high.
The estimated financial benefits of upgrading existing homes to code are between 10% and 300% higher than those for new construction because the baseline is worse. Energy, carbon savings and mortality improvements drive these benefits. Mortality improvements become even more significant when using PHIUS performance across all climate zones. However, the exceptionally high (estimated) initial costs for retrofitting outweigh the financial benefits of upgrading to the current or the above code. This leads to a negative cost-benefit despite the mortality benefits.
MFRB: The study also showed a clear financial benefit to adopting the current model code in new MFRBs across all climate zones studied. The results indicate that benefits outweigh upfront costs by three-fold to almost eight-fold. It also indicates that PHIUS envelope performance is cost-effective for warmer climates like Houston, Atlanta and Los Angeles. Passive house envelope performance did not show a positive cost-benefit in Portland, Detroit and Minneapolis. This is primarily because of high initial costs compared to other climate zones and a low probability of power outages coinciding with extreme hot/cold climate events, resulting in a lower mortality contribution.
Risk-based mortality helps drive the cost-effectiveness of better envelopes in mild climates: Because Houston and Los Angeles are at the highest risk of a heat wave coinciding with a power outage, reducing mortality contributes significantly to the total financial benefits of improved building envelope performance in those areas (see table below). It is often difficult to show cost-effectiveness from energy alone when improving envelope performance in these climates – especially in Los Angeles, which is typically considered mild. The DOE’s study concludes: “The findings make a compelling case for including resilience considerations when considering efficiency investments in locations like Los Angeles, that have high risk coupled with low building energy use and carbon emissions due to a mild climate.”
Operable windows are important for passive survivability: For an ALF in Houston, natural ventilation was the most effective passive strategy to support habitability during power outages coinciding with heat waves. Natural ventilation offsets the negative impact of low air leakage in those situations. Low air leakage is critical because it is the most effective measure in cold snaps. Natural ventilation is also important for MFRBs in heat events. In some scenarios, estimated mortality increased slightly with better thermal envelopes during heat waves in Portland and Detroit, mostly on the middle floors, because of overheating. Operable windows help address this while maintaining high envelope thermal performance since the latter is needed for life preservation during cold snaps.
Window shades are important: While the second-most effective strategy in the ALF during a heat event is reducing solar heat gain through windows, this does not help in the cold snap. To manage this conflict, the study’s authors suggest operable window shades to prevent heat from entering during heatwaves and escaping in cold snaps.
The study demonstrates the effectiveness of a balanced “envelope package” that can be effective in both extremes.
This DOE study is a big deal: Firstly, it makes a compelling case that highly energy-efficient building envelopes provide a safer environment for occupants. Secondly, it provides a way of monetizing the value of resilience benefits (although it warns of underestimation) and the social cost of carbon and regular energy savings.
The implications are meaningful: The methodology could be adopted in the energy code development process as a more expansive way of evaluating the cost-effectiveness of building envelope improvements. Its use could move the needle on building envelope provisions in our model codes and put them on a faster path to net zero.
More significantly, this information could propel the many states that have been historically slow to adopt the newest model codes because it aligns with their priorities for resilience and public health rather than just energy efficiency. Introducing this data to your elected officials at the state level could be impactful.