[Paper Reading] Ten questions concerning thermal resilience of buildings and occupants for climate adaptation

Publication year: 2023
Authors: Tianzhen Hong, Jeetika Malik, Amanda Krelling, William O’Brien, Kaiyu Sun, Roberto Lamberts, and Max Wei

Introduction

• These events, such as heat waves, cold snaps, wildfires, floods or hurricanes are often coincident with grid power outages or high energy prices, thereby making it difficult to maintain habitable indoor conditions for building occupants. A recent report by the World Meteorological Organization stated that about 12,000 extreme events across the globe have occurred over the past 50 years, resulting in over 2 million deaths and over $4.3 trillion of economic losses. The catastrophic effects of extreme events on human health, lives and the economy, along with the projections that the intensity and severity of climate-change related events will continue to increase, trigger an urgent need to adapt buildings to cope with and adapt to the changing world.
• The global building decarbonization effort, aiming to mitigate the impacts of climate change through energy efficiency upgrades and end-use electrification to reduce energy demand and carbon emissions, presents new opportunities and challenges for building operations, energy flexibility and resilience. For instance, the increased electricity demand due to electrification and severe weather conditions requires more electricity use, and particularly higher peak electricity demand, to run the heating, ventilation and air-conditioning (HVAC) systems to provide safe and habitable indoor conditions. This increased demand can heavily strain the power grid and necessitates an improved understanding of the design and operation of climate-resilient buildings to provide a safe and comfortable indoor environment to occupants.
• Resilience refers to the ability of a building to prepare and plan for, absorb, recover from, and more successfully adapt to adverse events. There exists a variety of dimensions to resilience in buildings, such as structural resilience, fire resilience, or seismic resilience, but here we focus on thermal resilience, which is a building’s ability to maintain a comfortable and safe indoor thermal environment for its occupants throughout its lifetime; particularly during extreme weather events arising from climate change or building system disruptions due to technical failure or power outages. Thermal resilience in buildings has gained a lot of attention during recent years within the scientific literature and industry practices, owing to the increased frequency and intensity of extreme weather events and their growing widespread impacts on infrastructure, human health and economics.
• Within the building energy modeling domain, significant advances have been made in approaches to simulate a building’s dynamic thermal response during extreme weather scenarios, including the projection of its performance under future weather events.
• The lack of standardized procedures is also reflected in the missing requirements in building codes for thermal resilience, unlike other resilience dimensions (e.g., earthquake, fire) addressed through comprehensive risk management standards.
• This paper aims to provide a deeper understanding of thermal resilience of buildings, particularly occupant-driven buildings such as residences or offices, from the perspectives of designers and policy makers focusing on occupant health and thermal safety. It also identifies future research directions and provides policy recommendations.

Ten questions (and answers) concerning thermal resilience of buildings

Q1. Who are the stakeholders and decision-makers, and why do they care about thermal resilience of buildings?

• Thermal resilience of buildings can be of value to a variety of stakeholders and decision-makers across the building life cycle (Fig. 2). During the design stages, thermal resilience analysis can help architects and engineers in designing buildings, systems, or spaces to optimize building performance and occupant comfort (e.g., passive design strategies).

Q2. What are the key factors that influence thermal resilience of buildings?

• (1) Outdoor environment - climate trend, urban microclimate, urban heat island effects, local weather conditions, weather hazards of the building location; (2) Building characteristics - envelope, HVAC system, onsite power generation, energy storage, energy demand, operation and controls; (3) Occupant characteristics - social, demographics, and health condition of the building occupants, and climate acclimatization; and (4) Reliability and resilience of the power grid serving the building neighborhood. A brief description on how these factors influence the thermal resilience of buildings is presented below.
  1. Building characteristics
  • The HVAC system design and operation, such as a decentralized system or even availability of controls, can help reduce energy demand, thereby minimizing the building’s reliance on grid power.
  • Additionally, the availability of onsite power generation, such as solar photovoltaic, energy storage or backup power may also improve a building’s ability to enhance thermal resilience by meeting the demand for critical loads (e.g., HVAC, medical devices, phone charging, internet devices), particularly during power outages coupled with extreme temperature conditions when passive measures may not suffice. 3. Occupant characteristics
  • Thermal comfort requirements and preferences vary greatly among occupants of different age groups, income or health vulnerabilities.
  • Other factors such as perceived ease of use of controls, shared or private spaces, or external factors such as utility costs may also influence the decisions occupants make to improve their indoor thermal environment. 4. Reliability and resilience of the power grid
  • The operating reliability of the grid—i.e., its ability to withstand sudden disturbances such as system losses or failure and its adequacy to meet the energy demand of the buildings at all times through integration of renewable energy generation sources or demand response programs is crucial.

Q3. What approaches can be used to assess thermal resilience of buildings?

1. On-site measurements
   • One approach to assess thermal resilience of buildings is to monitor the outdoor weather, the indoor thermal environment, and occupant comfort. This could encompass sensing and monitoring techniques such as temperature and humidity sensors, IoT based sensors, or even wearable sensors for monitoring occupant heart rate or skin temperature.
   • The approach is best suited for use cases concerning existing buildings. For example, architects or engineers may find it useful for evaluating the actual indoor thermal environment using data-driven approaches to develop effective retrofit strategies.
2. Computational modeling and simulation
   • While building performance simulation (BPS) has been traditionally used for energy and comfort related applications, the past few years has witnessed increasing use of BPS for thermal resilience assessment. BPS enables thermal resilience analysis for both new and existing buildings considering different scenarios at the required spatial and temporal scale.
   • This method may also be useful for assessing thermal performance of different building design options, risk assessment analysis for property insurance or to inform retrofit decision-making. However, particular attention must be given to the simulation parameters, performance metrics to report, the model inputs and their resolution, and the modeling approach adopted for analysis to ensure accurate and meaningful results.
3. Qualitative approaches
   • Interviews and surveys to collect occupant experiences, feedback, preferences or contextual factors that may influence their interaction with buildings can prove valuable for thermal resilience assessment of existing buildings, particularly when complemented with on-site measurements or walk-through observations.
   • However, utmost attention is required in designing surveys or the interview process to ensure reliable and representative results. An associated challenge would be to gather meaningful responses from occupants such as their possible behaviors during extreme events when they have not encountered any such events in the past. These qualitative approaches often require significantly more time and effort, as well as the need to address human subject and privacy issues, and thus must be adopted with caution.

     Q4. What are the available metrics for assessing thermal resilience of buildings?

   • Part of the challenge of defining such metrics is that the term “resilience” does not have a common definition, nor can it be directly measured.
   • Occupant vulnerability metrics may be either qualitative or quantitative. They are used to identify populations with higher propensity to be affected by extreme events.
   • These vulnerability metrics can also be used to dive into a buildings’ thermal dynamics to quantify its capability of maintaining adequate indoor thermal conditions.
   • These metrics are usually calculated from outputs of building performance simulation or from field measurements. For example, metrics can be calculated from hourly values of indoor environmental parameters including air drybulb temperature, humidity, air velocity and surface temperatures.
   • (1) the Standard Effective Temperature (SET) degree-hours for both hot and cold events, (2) the Heat Index for hot events throughout a period of time, and (3) the Hours of Safety for cold events. These metrics are used to quantitatively evaluate the thermal resilience of the baseline building conditions, as well as to identify improvements to thermal resilience for the efficiency upgrade scenarios.
   • SET is a temperature parameter that considers indoor air dry-bulb temperature, relative humidity, mean surface radiant temperature and air velocity, as well as the activity rate and clothing levels of occupants.
   • The SET degree-hours metric is more complex to calculate but considers six thermal comfort parameters and the accumulated severity of the ther mal stress duringextreme weather events. The metric is hard to measure directly in indoor environments but can be easily calculated using building simulation tools such as EnergyPlus.
   • Heat Index (HI) combines air temperature and relative humidity to measure the human-perceived equivalent temperature. There are four levels of heat stress based on HI: Caution, Extreme Caution, Danger, and Extreme Danger. HI is easy to measure, as it only requires the indoor air temperature and humidity.
   • Hours of Safety is a metric developed by the U.S. Environmental Protection Agency (EPA) and the Rocky Mountain Institute as a measure of the duration of time a building is able to maintain safe conditions above a predefined temperature threshold during a cold event.
   • The metric of Hours of Safety is simple to understand and easy to calculate via simulations or measurements. It aims to serve as a potential resilience score of buildings.
   • Quantitative occupant vulnerability metrics can be divided into four types, depending on what type of information they provide about the indoor thermal environment and its consequences to occupants.
     • Frequency metrics are those that describe how often certain conditions occur.
     • An intensity indicator usually describes extreme thermal conditions within the period, like the annual maximum operative temperature.
     • Duration indicates the length of time to reach or recover from certain conditions.
     • An indicator of severity combines both frequency and intensity, like the degree hours, and the SET degree-hours used to determine the passive survivability.
     • System vulnerability metrics may help mechanical and civil engi neers to future-proof building technical systems, as well as guide building operators in responding to extreme events.
     • Financial metrics are those associated with the cost of either investing in thermal resilience measures or dealing with consequences of not being resilient. The value of a statistical life is an example of a financial metric that estimates the value of saving lives through mitigation measures
   • As practitioners do not want to focus on thermal resilience at the expense of other indicators, energy performance metrics can also be evaluated as a means to consider energy efficiency in tandem with resilience.
   • A resilience analysis should include a comprehensive set of metrics to compose a thermal resilience assessment that will ultimately serve for evaluation, comparison and decision-making.

Q5. What is a reasonable workflow to model thermal resilience using building performance simulation?

• The minimum capabilities expected from a BPS tool to model resilience include the following:
  • Ability to run full-year or partial-year analyses
  • Comprehensive consideration of weather variables as input
  • Capacity to model failure events
  • Ability to model the occupant behavior and their adaptive measures
  • Ability to model detailed zoning, including multiple floors and rooms
  • Capacity to model natural ventilation, shading effect, and other strategies and technologies
• The fundamental definition of resilience is associated with how buildings respond and recover from a shock, such as heat waves, cold snaps, and power outages. Thus, unlike conventional BPS that consider buildings under typical meteorological and normal operational conditions, a thermal resilience analysis should account for multiple scenarios that may impact a building ’s coping capability.
• After running simulations that consider multiple scenarios, metrics are calculated from the outputs. They should quantify occupant’s vulnerabilities, as well as the necessary energy used to operate the building and maintain thermal comfort and safety.-

Q6. What scenarios are needed to consider for robust thermal resilience modeling?

• Unlike conventional BPS, thermal resilience modeling requires accounting for an integrated set of scenarios considering various sources of hazard that can disrupt buildings in a geographic region.
• These events may be modeled directly or indirectly in the BPS, with possible approaches listed in the right column.
• When a building does not need to be evacuated, earthquakes and flooding can damage structures and technical systems, which also affect building operation and system performance, limiting the capacity to respond to hazards.
• The COVID-19 pandemic recently demonstrated how building occupation and operation patterns can deeply change, consequently impacting building performance
• When analyzing the effectiveness of phase change materials in residential buildings, Baniassadi et al. verified that the severity of overheating highly depended on the time of day that the air-conditioning system lost power.
• Sengupta et al. evaluated the impact of multiple types of shock on resilience to overheating in a nearly-zero energy educational building and concluded that the impact of heat waves was significantly higher than any system failure, with a future heat wave being the most extreme shock.
• The source of weather data is fundamental to enable an accurate evaluation. As extreme scenarios are often intensified by local urban characteristics (e.g., urban heat islands), local weather data are preferred.
• A prominent initiative to generate future weather files can be found in the works of the International Energy Agency Annex 80 which provided future Typical Meteorological Years (TMY) and Heat Wave Years (HWY)for multiple cities worldwide.
• Available techniques to generate future weather data are based on downscaling general circulation models. Examples are time series adjustment (morphing), interpolation, stochastic weather generation, and dynamic downscaling
• Weather data can be provided for building performance simulation in different types of weather files, depending on the application. Heat wave years represent actual years in which at least one heat wave has been detected.
• A standard definition of a heat wave is still absent, and detection methods differ in literature and practice. 0 Thus, multiple heat waves can be identified within a period, allowing users to select the longest heat wave, the most intense, and the most severe. However, the minimum number of scenarios to be consideredin a robustresilience analysis remains a research gap.
• For buildings in mixed climates, e.g., requiring cooling in summer and heating in winter, it is important to include both the extreme hot and cold events in the modeling and evaluation of thermal resilience.

Q7. What technologies and design strategies can be used to achieve resilient buildings?

1. Passive solutions
   • Passive solutions do not require power supply to function, so they can be particularly helpful during power outages. The first line of defense against extreme temperatures is the design of the building itself.
   • Passive designs that can effectively reduce unwanted heat gain through the envelope during extremely hot events include (1) thermal insulation, mainly in walls and roofs; (2) window measures, such as high-performance windows, interior and exterior shading devices (e.g., blinds, overhangs, awnings), and solar control window films; (3) solar reflective materials, such as cool roofs, cool walls, and radiant barriers; (4) evaporative envelope surfaces, such as green roofs, green facades, and roof ponds
   • The properties and performance of the above passive designs are static throughout the year. In some cases this might cause conflicting impacts between extreme hot and cold events
   • Natural ventilation can provide free cooling when the outdoor environment is cooler than the occupied space. For buildings with operable windows of reasonable size and orientation ,natural ventilation is a very effective passive measure to decrease indoor temperature during heat waves, particularly for top floors
   • Thermal mass can be an effective passive strategy. It refers to the ability of a material to absorb, store, and later release heat, acting as a thermal buffer. Materials with high thermal mass (such as concrete, brick, or stone) can absorb heat during the day when the temperature is high and release it slowly at night when the temperature drops.
   • Passive solar heating and cooling systems, such as Trombe walls, can further enhance the building’s resilience
2. Active solutions
   • Active measures, backup power, and/or energy storage are needed to provide cooling/heating to maintain safe conditions for occupants. Active solutions need power supply to function, either from the grid or from bat teries or on-site backup power systems.
   • During extreme temperature events, this may cause failure of the HVAC equipment to provide sufficient cooling/heating to maintain thermal safety in the buildings.
   • However, there are two caveats with the typical active solution:
   • HVAC systems, especially whole-building central types, consume large amounts of energy.
   • If a power outage did happen, the HVAC system could not run without a large capacity backup generator or large capacity battery due to its high energy demand, which would require significant investment. Therefore, low-energy active solutions are highly preferred, as well as active solutions that are based on optimal control strategies. - Personal comfort systems (PCS) are another attractive low-energy active solution. PCS are devices to heat/cool individual occupants directly or heat/cool the localized thermal environment of an individual occupant, under the control of the occupant without significantly affecting the thermal environment of other occupants. - Optimal control methods can enhance a building’s thermal resilience during heat waves by optimizing the building load profile. A good example is pre-cooling.
3. Backup power and energy storage
   • Backup power and energy storage technologies ensure the contin uous operation of active solutions during power outages or periods of high demand. Solar photovoltaic (PV) systems, backup generators (e.g., wind, diesel), and batteries can provide reliable power, while thermal energy storage systems using water, ice or phase change material store excess thermal energy for later use.
4. Occupant behavioral strategies
   • These include self-dousing, foot immersion, misting fans, ice towels, ingesting cold water, adjusting activity levels, and adding or removing clothing layers. Such adaptive behaviors, complementing the technological solutions, can significantly contribute to enhancing the overall thermal resilience of buildings.

Q8. What are essential human factors to consider in achieving thermal resilient buildings?

• First, most buildings serve the explicit purpose of protecting occupants from outdoor conditions and often—particularly for conditioned buildings and in developed countries—rely on an un interrupted supply of external energy inputs and active building systems to provide a comfortable and healthy indoor environment.
• Second, occupants often play an active role in improving building performance in the absence of such active energy systems (e.g., opening windows to provide fresh air)—particularly for buildings that are not tightly controlled and automated (e.g., naturally ventilated).
• In this case, the same health condition caused the occupants to both be insensitive to indoor thermal conditions and have a limited ability to adapt.
• This suggests a lack of user knowledge, but also highlights the importance of education and usability of such devices.

1. Acceptable indoor conditions
   • The literature on building thermal resilience has developed numerous definitions for the indoor conditions of a building’s thermal environment, with two main perspectives: the occupant’s or the objective indoor conditions.
   • The duration of exposure to extreme thermal conditions also needs to be considered for defining indoor thermal conditions.
   • The literature largely focuses on overheating; however, cold conditions also can be a concern (e.g., coincidence of freezing rain that causes power outages and extreme cold conditions).
2. Occupant behavioral response to extreme events
   • Occupants’ opportunities to adapt to uncomfortable or extreme conditions depend greatly on the building design, but may include operable windows, moveable shading devices, clothing, and relocating to other parts of the building (or outdoors).
   • The ability of occupants to act depends on their physical abilities, while the awareness that they should act and how they should act depends on their cognitive abilities, as indicated by the BC example above.
   • If occupants are able to adapt to extreme events, the question remains on how effective occupant actions are and how predictive the occupants are.
   • Occupants’ knowledge about strategies and familiarity with the thermal dynamics of the building tend to be best in naturally ventilated or unconditioned buildings, whereby they take an active role in improving the indoor environment during normal circumstances.
   • While we can and should train occupants to act in ways to sustain their well-being during extreme events (e.g., as we conduct fire drills), a priority is to design buildings with systems that are resilient and allow occupants to help themselves in the first place

Q9. How can thermal resilience of buildings be incorporated into climate adaptation and building decarbonization plans?

• These decarbonization approaches often have impacts and trade-offs with thermal resilience approaches, and hence it is important to recognize the opportunity to harmonize and synergize decarbonization efforts with thermal resilience efforts.
• Even though the electrification of HVAC systems is at the forefront of decarbonization, the access to heat pumps is associated with challenges such as cost, availability and adoption.
• Occupant-centric building controls to minimize overheating or cooling, and personalized cooling systems such as personal fans and cooling chairs during periods of grid stress may offer improved thermal resilience.
• To achieve synergies in improving thermal resilience and use of renewable generation sources, technological innovations are needed to replace fossil-fuel based backup power options such as diesel generators with “passive survival” technologies for low cost emergency use such as low cost HVAC with integrated storage and/or direct DC-coupling to rooftop solar PV.
• Demand flexibility, similar to the other three decarbonization approaches, has a strong interaction with thermal resilience to the extent that it can achieve a more reliable grid.
• Intelligent and automated building control systems to optimize thermal comfort and energy use also may prove beneficial.
• Currently, there is a siloed approach towards climate adaptation plans for buildings, where the focus is mainly on extreme heat (such as California’s Extreme Heat Action Plan), while hazards such as snowstorms or wildfires are not accounted for

Q10. How can building energy codes and standards, building performance rating systems, and policies be adapted to support the design and operation of thermal resilient buildings?

• Building codes regulating general building design and construction requirements relating to fire and life safety, structural safety, and access compliance
• Building code energy efficiency requirements typically for new construction in residential and nonresidential buildings and for re models and additions
• Building code for existing buildings
• “Green building codes” that cover broader building sustainability areas
• Housing law regulations including minimum building habitability standards for health and safety
• green building rating systems applied to new construction, the most widely used of which is the Leadership in Energy and Environmental Design (LEED) rating
• Key opportunities
  • a starting point would be to building upon existing housing law habitability requirements (Title 25, Division 1) to cover extreme heat and maximum indoor temperatures and to extend the existing coverage of these laws to existing single-family home buildings in addition to apartment buildings
  • a newer class of regulation for thermal resilience with greater focus on ensuring inhabitant comfort and safety during emergency situations, acute events, and cascading and/or concurrent emergency events and to ensure passive survivability
  • A starting point for this in new construction is in building reach or “stretch” codes. These codes go beyond minimum acceptable performance standards and may give the option of a tiered or stepped series of enhanced measures for comfort or safety.
  • These can be implemented in a similar way to traditional codes but provide additional design and performance options beyond what codes currently prescribe. Similarly, Green Building Rating Systems could be updated and extended to provide more credits for thermal resilience measures.
  • It is still necessary to set the foundation of a resilience analysis into codes and standards, establishing a standardized procedure to assess thermal resilience considering multiple sources of disruption. Among these disruptions, heat waves and future climate projections should be considered when revising performance parameters, threshold values, and recommendations related to technologies in policies. Comprehensive metrics, data sharing, and labeling systems need to be established to quantify resilience and allow benchmarking and communication across different audiences.
• Challenges and barriers
  • While the performance path approaches in building codes could accommodate a simulation-based assessment of resilience, enforcing resilience using a prescriptive path is more challenging. This is because the resilience of a building depends on how design features and systems work together, rather than any individual building feature.
  • One broad challenge is that with additional resilience generally comes additional cost, and the question becomes, exactly how much resilience is required, where, and at what cost? Another challenge for heat resilience is that planning should be made to ensure that passive and low-energy or low-carbon active cooling measures are deployed to the maximum extent possible to ensure that air conditioning demand is minimized, to reduce investment costs, to constrain utility bills in creases, and to reduce stress to the grid during heat waves.
  • To the extent that a cost/benefit framework and cost effectiveness is a requirement for building code updates, resilience measures have multiple challenges.
  • The current practice of building modeling and characterization of building measures does not adequately handle the risks associated with both summer and winter extreme climate events of increasing frequency, duration and intensity, and needs to be updated to fully encompass future climate risks.

Summary and future perspectives

• There remains a need to develop a practical standardized methodology for assessing thermal vulnerability and evaluate benefits of passive and active technologies, and occupant behavioral strategies in improving thermal resilience. The assessment methodology should include a well defined set of thermal resilience metrics that can be quantified through measurements or building performance simulation. Standardized definitions and datasets of extreme temperature events (heat wave and cold snaps) covering major global cities are also needed.
• People living in disadvantaged communities tend to be more vulnerable to extreme heat due to limited resources for adaptation, therefore climate equity issues deserve more research.
• With the global trend to decarbonize the building sector for meeting economy-wide carbon neutrality in the next 30 years, there is an unprecedented opportunity to do this right—not only for reducing energy use and carbon emissions of buildings but also for improving their climate resilience for human health and thermal safety at the same time.