A Dual Approach to Heat Resistance: New NSF Study Reshapes Home Defense via PCM Walls and Natural Ventilation

Extreme heat can trigger health concerns for many people. Dangerous and prolonged heat waves often induce area-wide power outages as energy use soars from the significant demands for relief using traditional air conditioning systems. Recently published research in Science and Technology for the Built Environment studied how the make-up of a home’s exterior walls, coupled with natural ventilation at night, might better protect homes from the heat, lower energy usage, reduce cooling costs, and provide safer environments for residents. This study was conducted by researchers in the U.S. National Science Foundation (NSF) Environmentally Applied Refrigerant Technology Hub (EARTH). 

The researchers developed computer-simulated homes in 16 different climate zones that stretch from Alaska to Hawaii and across the continental US. Each simulated home had phase change material (PCM) integrated into the exterior walls to evaluate the impacts on energy performance and thermal resilience. 

“PCMs come in different forms from encapsulated liquid to a powder that can be integrated into insulation or panels,” said Xu Han, assistant professor in the School of Architecture and a concurrent assistant professor in the College of Engineering at the University of Notre Dame. “These phase change materials have thermal storage capacity, which means they are able to absorb and store heat as temperatures rise, reducing heat transfer into indoor spaces and keeping interiors cooler. The stored heat is later released into the air when temperatures drop.”

Separately, natural ventilation, such as opening windows during cooler nighttime hours, brings cooler outdoor air into the home and helps the PCMs to release stored heat, allowing them to recharge for the next day.

“The PCM technology combined with natural ventilation can lower indoor temperatures, even during a power outage,” said Han. “It can also potentially reduce electricity demand and contribute to lower peak power loads during prolonged heat waves.”

The researchers conducted a simulation-based study using US Department of Energy prototype residential building models. These standardized computer models featured fixed geometry, building systems, schedules, and construction characteristics tailored to different climate zones. By using a consistent modeling framework, the researchers were able to isolate and evaluate the effects of PCMs, nighttime natural ventilation, outdoor weather conditions, and power outage scenarios across the full range of US climates. 

“For the computer-simulated home, we selected a single-family house that includes a basement, HVAC system, and a heat pump,” said Youmin Xu, a recent Ph.D. graduate from the University of Kansas and lead researcher for this study. “And each prototype home was designed with building materials representative of its specific climate zone, as cold climate zone homes are constructed differently from those in warm regions. Finally, we integrated PCMs into the exterior walls of all the homes to evaluate the performance under different climate conditions.”  

Using these simulated homes, the researchers evaluated four scenarios. Cases 1 and 2 used typical meteorological year weather data to assess energy performance and utility cost savings under normal climate conditions. Cases 3 and 4 used extreme warm year weather data to evaluate how PCM-enhanced homes perform during prolonged heat waves and power outages, providing insight into the potential to improve thermal resilience under extreme weather conditions.

Case 1: Annual energy savings with PCM integration, exclusively

Different thicknesses of PCM layers were tested across all 16 climate zones, and energy savings increased as the thicknesses increased. The highest energy savings occurred in the San Diego prototype home at a 36.5 percent reduction. In warmer climates that have cooler nighttime temperatures, PCMs are able to absorb the higher temperatures and benefit from the cooler evenings even without the use of natural ventilation. 

Case 2: Cooling energy savings from PCM and natural ventilation combined

Using a uniform 3 cm thickness of PCMs across all simulated homes, natural ventilation was conducted during nighttime’s cooler hours, which yielded significant energy savings across all climate zones. The greatest savings occurred in Port Angeles, WA, with a 96 percent cooling energy reduction. The lowest reduction was in Miami, with 21 percent lower cooling energy consumption—still a significant savings. This variation is attributed to the differences in nighttime temperatures across different climates. In warmer climates, temperatures at night remain relatively high, so the use of natural ventilation does not yield the same results as homes in cooler climates.

Case 3: Improvement of thermal resilience with PCM, exclusively

Cases 3 and 4 looked at the impact of heat waves and associated power outages, the length of which varied across different climate zones (four to ten days, based on historical data of such occurrences in the respective climates). As heat waves are uncommon in International Falls, MN, and Fairbanks, AK, prototype homes in these climates were not studied in Cases 3 and 4.

Thicknesses of PCMs were studied in these heat wave scenarios, and it was found that the maximum thickness of 3 cm provided the greatest improvement in thermal resilience across the remaining 14 climate zones. Further, it was found that PCMs alone were effective in the early stages of heat waves, but this diminished after several days as PCMs were not able to effectively recharge overnight under prolonged high temperatures. 

Case 4: Thermal resilience improvement when combining PCM and natural ventilation

Introducing nighttime natural ventilation during heat wave scenarios reduced indoor temperatures in simulated homes across 13 of the 14 climate zones, with the highest temperature reduction of 4.4 ℃ (7.92 ℉). 

For example, in the Port Angeles, WA, simulated home, the PCM-only (Case 3) scenario started with an average indoor temperature of 81.3 ℉ on the first day of the heat wave and rose to 88.7 ℉ on the sixth day. With added nighttime natural ventilation, the average indoor temperature began at 78.3 ℉ and increased to just 82.8 ℉ on the sixth day. The cooler nighttime air helped the PCMs release stored heat and recharge, allowing them to continue moderating indoor temperatures throughout the heat wave.

“We chose to focus on residential homes, because occupants are often more vulnerable to the health impacts of extreme heat,” said Han. “Commercial buildings typically have more sophisticated HVAC systems and may have back-up generators. They can also temporarily close during extreme weather events. But residents have to be in their homes, making thermal resilience especially important.”

He continued, “One of the biggest challenges to broader adoption is that relatively few PCM products are currently available on the market. While the technology is advancing, widespread implementation is still in its early stages.”

“The use of PCMs in homes and buildings is a rapidly growing area of research,” Xu said. “Although some companies have begun exploring PCM-based applications, many are still in early stages of development. In our simulation-based study, we can see that using the PCMs in the exterior walls can achieve energy savings and improvements in thermal resilience. These findings suggest significant potential for future applications.” 

“This research on phase change materials and passive cooling strategies provides a promising solution for energy savings in buildings and offers thermal resilience during heat waves and power outages," said Mark B. Shiflett, Foundation Distinguished Professor of Chemical Engineering at the University of Kansas and Director of the NSF EARTH Engineering Research Center. "NSF EARTH is focused on developing sustainable, accessible, and equitable refrigerant technologies, and we are looking at novel materials that can increase energy efficiencies in the life cycle of HVACR systems.”

This research was supported by the New Faculty Research Development Awards at the University of Kansas and by the NSF. To learn more about NSF EARTH, please visit https://erc-earth.ku.edu

Please direct queries regarding this study to Xu Han, xhan7@nd.edu.

EARTH Engineering Research Center

The Environmentally Applied Refrigerant Technology Hub (EARTH) is an NSF- and corporate-funded Engineering Research Center (ERC). EARTH is dedicated to revolutionizing how refrigerants are formulated, manufactured, applied, monitored, and recycled to dramatically reduce the environmental footprint of the global cooling sector. Led by the University of Kansas, this consortium of partner research universities includes the University of Notre Dame, University of Maryland, Lehigh University, University of South Dakota, and the University of Hawai′i.