The Evaluation of the Temperature Reduction Effects of Cool Roofs and Cool Pavements as Urban Heatwave Mitigation Strategies

Abstract

Various types of damage, including health risks, caused by abnormal climate conditions have gradually increased in recent years, particularly in cities with a high degree of complexity. In particular, it is important to address heatwave damage risk from both technical and policy perspectives because it poses risks to older adults and/or disadvantaged individuals. This study was performed to assess the quantitative effects of two heatwave mitigation measures, cool roofs and cool pavements, in the Jangyumugye district of Gimhae, Republic of Korea. Thermal imaging cameras were used to measure surface temperatures after the installation of these measures; after the installation of cool roofs, indoor temperatures were also measured using temperature measurement data loggers. Then, a survey was conducted to obtain feedback from users of the associated facilities. From August to October 2019, surface temperatures were significantly reduced following the installation of cool roofs and cool pavements. Cool roofs constructed using slabs and panels provided average temperature reductions of 15.5 °C and 11.6 °C, respectively. Cool pavements showed a temperature reduction of ≥4 °C in both parking lot and alley sites. More than 30% of users noticed a reduction in temperature as a result of heatwave measure installation. These results may be used by the city to make informed policy decisions regarding the implementation of heatwave mitigation measures.

1. Introduction

Each year, the direct and indirect effects of climate change become more apparent, including sea level rise, along with increases in the frequency and severity of storms and natural disasters [1]. Temperature increases are expected to accelerate in the future, resulting in greater numbers of natural disasters such as heatwaves [2]. Therefore, mitigation policies in response to climate change are the focus of discussions worldwide [3]; such policies include short-, mid-, and long-term plans to minimize damage at national and regional scales [4].

Unlike typhoons and heavy rains, the impacts of heatwaves are not limited to a single location [5] and can be associated with various other problems such as sharp declines in agricultural production, increases in livestock deaths, and higher numbers of accidents related to increasing demands for electricity [6]. Additionally, outdoor workers, older adults, low-income earners, and chronically ill individuals are particularly vulnerable to heatwave effects [7]. Thus, national and local governments are primarily focused on social measures aimed at protecting the most vulnerable populations and providing emergency support during heatwaves [8]. To protect the health of many citizens during a heatwave, proactive measures should be implemented to reduce the temperatures of large buildings and public spaces in cities [9]. These steps are essential for reducing the number of casualties caused by heat-related illnesses among socially vulnerable groups, particularly older adults and low-income earners.

Human-made materials in urban environments, such as pavements and roofing, tend to reflect less solar energy, and absorb and emit more heat than trees, vegetation, and other natural surfaces; the transition from natural to artificial materials produces a gradual increase in city temperatures, which results in a heat island effect that impacts citizens [10]. Urban infrastructure is increasingly modified to minimize its negative environmental effects [11] through the introduction of green facilities to improve shock absorption and recovery after heatwaves [12]. The resulting heatwave mitigation systems include the application of cooling materials that dissipate thermal energy [13]. For example, cool roofs [14] and cool pavements [15] have been installed on urban roof surfaces and roads or parking lot surfaces, respectively, using reflective materials; studies are being conducted in order to reduce urban heat islands [16].

Heatwave mitigation measures are primarily installed as physical features in urban spaces. Because such technologies require significant expenditures, it is important to determine whether they will be fully effective [17]. To ensure the implementation of heatwave mitigation technologies such as cooling fog, cool pavements, and cool roofs by local governments, the effectiveness of these technologies during heatwaves should be quantified in advance [18]. To improve the sustainability of infrastructure under heatwaves, it may be appropriate to establish heatwave mitigation systems in downtown areas with relatively high surface temperatures.

A previous study analyzed and compared cool roof effects in different regions [19,20]. Cool roofs reportedly reduce both internal and external temperatures in an office building [21]. Compared with a normal roof, the surface of a cool roof is cooled by an average of 3.3 °C during summer [22]. A study was conducted in order to examine the possibility of reducing the suction air temperature of heat pump external units generated by cool roofs [23]. A long-term accessibility study showed that a cool roof reduced summer peak temperatures by up to 4.7 °C [24]. An urban heatwave response project estimated temperature reduction rates using a microclimate analysis modeling approach [25]. A thermal infrared image acquired by a drone was analyzed to determine the effects of heat island reduction techniques during a heatwave [26]. The solar reflectance index (SRI) is also used to evaluate cool roofs [27].

Reflective concrete pavement has been reported to lower temperatures by 0.2–0.4 °C [28], resulting in a 60% higher albedo, an indicator of the amount of light reflected by an object, for the coated area and a 40% lower surface temperature for the painted surface, compared with an uncoated surface [29]. In Rome, cool pavements installed to mitigate high urban temperatures resulted in a significant temperature decrease that was closely associated with solar reflection [30]. Cool pavements have been compared with existing asphalt in alleys to determine their effects on temperature [31]. Multiple studies have also examined the economics of the urban heat island effect and the impact of cooling pavements on climate change [32]. In some studies, the heat island effect was simulated using weather research and forecasting urban canopy models [33], or actual measurements are analyzed using high-precision GPS data [34].

Furthermore, previous studies have demonstrated that packaging materials significantly affect surface temperatures [15]. Although various heatwave mitigation technologies are available, their proper applications have not been thoroughly investigated. To understand how the installation of heatwave mitigation measures affects adjacent buildings or pedestrians in a complex city, the interactions of those measures with elements of the city must be examined. In particular, empirical studies are needed to understand how heat stress directly affects building users and pedestrians during a heatwave [35].

In this study, we quantified the effects of cool roofs and cool pavements on surface temperatures; we also conducted a survey of the users of these systems to determine their perceptions of the temperature effects of these heatwave mitigation measures. A heatwave mitigation facility was installed in the Jangyumugye district of Gimhae, Republic of Korea, from August to October 2019. In terms of the recording period, Republic of Korea is characterized by frequent rainy days during the rainy season in July and August, which is why measurements were conducted between late August and October. To examine temperature differences after heatwave mitigation measure installation, we measured temperatures at the mitigation facility and at control sites. A flowchart of the procedures followed in this study is provided in Figure 1. Our quantitative analysis of the results will be useful for establishing future heatwave policies using the examined technologies.

2. Study Area

Gimhae comprises a mountainous northern section and a wide plain along the Nakdong River to the south. The Jangyumugye district has flat topography; it developed around an ancient town along the Daecheong River, a tributary of the Nakdong River. The study area encompasses approximately 210,000 m² within Jangyumugye, near the new towns Jangyu and Yulha and a national highway that connects Gimhae and Changwon (Figure 2). Thus, the study area represents a hub connecting various other cities and regions.

Despite a slight decline in temperature over the past decade, the average annual temperature in Gimhae is 1.5 °C higher than the national average [36]. A detailed plan to implement climate change adaptation measures in Gimhae (2015) revealed that Gimhae is highly vulnerable to both health and infrastructure risks caused by heatwaves, ranking in the top 1% among 230 local governments nationwide [37]. The demographics and physical environment of Jangyumugye are also associated with high heatwave susceptibility; in 2015, a large percentage of the population was aged > 65 years, and approximately 67% of buildings were >20 years old.

Examples of heatwave damage in Gimhae include the death of an older woman because of heatstroke while working outdoors in summer 2018, which raised health concerns for vulnerable groups [37]; the deaths of approximately 7291 chickens and 1445 ha of fruit trees [38]; and green algae outbreaks around the Nakdong River, which is the main water source for Gimhae, after a heatwave-induced drought. Farm damage and water purification costs associated with algal contamination represent indirect costs incurred in relation to heatwave occurrence.

Thus, we targeted the Jangyumugye district of Gimhae, Republic of Korea, which is highly susceptible to heatwave damage, to examine the effects of heatwave mitigation measures.

3. Data and Methods

A vulnerability improvement study aimed at adaptation to climate change is currently underway in Jangyumugye, which involves the installation of cool roofs and cool pavement in downtown regions. Cool roofs were installed on existing houses and other buildings in residential areas, and cool pavements were installed in a parking lot near a traditional market and in alleys in a residential area. We measured surface temperatures for comparison between control and experimental groups to assess the effects of these heatwave mitigation measures. The experimental setup for this field study is described in

Table 1. Description of experimental setup for surface temperature measurements at heat mitigation facilities in 2019.

	Cool Roof		Cool Pavement
Experimental date	26 August	19 September	6 October	26 October
Measurement times	1 h intervals (9:00–15:00)		2 h intervals (07:00–21:00)
Location	Residential area *		Traditional market	Residential area
Type of material	Slab	Panel	Parking lot	Alley
Control group	R-1	R-2	P-1	A-1, 2
Experimental group	R’-1	R’-2	P’-1~6	A’-1~4
Monitoring equipment	Thermal imaging camera, temperature data logger		Thermal imaging camera
Material	Heat-reflective paint Adgreencoat (NCK Global Paint, Singapore)
Solar reflectance	RGB 96%, Infrared 89.4%

* Indoor and outdoor.

, including specifications of heat mitigation facilities.

3.1. Cool Roofs

Cool roofs are painted with bright colors using thermal barrier paint that reflects and radiates both sunlight and solar heat; these effects result in less heat accumulation by the roof and less heat transfer into the building. Cool roofs were installed at 134 locations (23,264 m²) within the study area, excluding roof materials such as slate and asphalt shingles, to which it is physically impossible to apply thermal barrier paint. Our experiment involved four households that were able to cooperate for the duration of the study period. Experimental sites were matched with control sites that had the same roofing material (slab or panel; see Table 1); matched sites were selected within nearby locations. Surface and indoor temperatures were measured for both groups after cool roof installation. The locations of the measurement sites are provided in Figure 3 and Figure 4, and

Table 2. Cool roof monitoring sites on one-story houses.

Site	Address	Roof Type	Group
R-1	32, Jangyu-ro 288 Beon-gil, Gimhae, Gyeongsangnam Province	Slab	Control
R’-1	21, Jangyu-ro 316 Beon-gil, Gimhae, Gyeongsangnam Province	Slab	Experiment
R-2	30-2, Jangyu-ro 324 Beon-gil, Gimhae, Gyeongsangnam Province	Panel	Control
R’-2	18-9, Jangyu-ro 316 Beon-gil, Gimhae, Gyeongsangnam Province	Panel	Experiment

.

The monitoring of slab and panel roofs was conducted on 26 August and 19 September 2019, respectively. A thermal imaging camera was used to measure roof surface temperature; a wireless temperature data logger was used to measure indoor air temperature (

Table 3. Equipment used to monitor cool roofs and cool pavements.

Item	Purpose	Model	Image
Thermal imaging camera	Surface temperature	FLIR E6
Wireless temperature data logger	Indoor air temperature Roof surface temperature	TR-71WF

). From 9:00 to 15:00, the roof surface temperature and indoor air temperature were measured at 1 h intervals.

3.2. Cool Pavements

Cool pavement installation involves thermal barrier packaging, which can lower surface temperatures or reduce heat absorption. Cool pavements are constructed to mitigate urban heat island effects, relieve heat for pedestrians, and improve transportation and the traffic environment, thereby reducing vulnerability during heatwaves.

Thermal barrier paint can be applied to the road surface and/or to sidewalk blocks to reduce radiant heat adjacent to roads. In this study, thermal barrier paint was applied to parking lot and alley surfaces. We established six experimental sites at the corners and central points of a parking lot at the Jangyu Traditional Market. The asphalt parking lot and nearby roads were coated with thermal barrier paint, covering approximately 2916 m². The same thermal imaging camera was used for both cool roof and cool pavement monitoring.

Construction was completed on 4 October 2019, and monitoring began on 6 October 2019. Thermal imaging was performed at 2 h intervals for 14 h (07:00 to 21:00) by using a thermal imaging camera (Table 3). There were six measurement points inside and outside of the parking lot (P’-1–6), one of which (P-1) was assigned to a control site (Figure 5).

In a nearby area, we established four alley and two road sites (Figure 6). The insulative coating was applied to alleys covering approximately 1113 m² at Jangyu-ro 322 Beon-gil, Gimhae, Gyeongsangnam Province. Cool pavement construction was completed on 25 October 2019. Thermal imaging was performed at 2 h intervals for 14 h (7:00 to 21:00) on 26 October 2019, for the four alley experimental sites (PA-1–4) and two road (control) sites (PA’-1, 2).

3.3. User Satisfaction Surveys

A qualitative heatwave mitigation effect survey was conducted to evaluate the perspectives of residents who used the heatwave mitigation technologies. The survey was conducted from 10 October 2019 to 25 October 2019. We interviewed 64 residents living in zones with cool roofs. The survey included questions about residents’ opinions regarding heatwaves, damage caused by heatwaves, thermal comfort, and air conditioner use during heatwaves. In addition to basic personal information, we obtained information regarding indoor temperature, air conditioner usage, sleep disruption, and satisfaction with temperature reduction after cool roof installation.

A similar survey was conducted among users of the cool pavements installed on the market parking lot. The survey included questions about participants’ opinions regarding heatwaves (including heatwave damage), thermal comfort, satisfaction with temperature reduction after cool pavement installation, and the number of uses after construction, as well as basic personal information. The detailed survey topics are listed in

Table 4. User satisfaction survey conducted after the installation of heatwave mitigation technology.

Category	Survey Questions
	Cool Roof	Cool Pavement
1. General information	- Residence time/range - House/roof type	- Hours of use - Timing of use
2. Opinions	- Need for climate change mitigation - Awareness of facility - Interest in cool roof installation	- Need for climate change mitigation - Awareness of facility - Interest in cool pavement installation
3. Perception of effects	- Perceived room temperature change - Perceived reduction in air conditioner use - Perception of sleep disorder alleviation - Perceived reduction in heatwave damage	- Perceived road temperature change - Perceived reduction in heatstroke - Perception of reduced inconvenience - Perceived reduction in heatwave damage
4. Satisfaction	- Satisfaction with temperature reduction - Overall satisfaction	- Satisfaction with temperature reduction - Overall satisfaction

.

4. Results and Discussion

We conducted on-site measurements and personal surveys to analyze the effects of cool roof and cool pavement installation compared with control sites.

4.1. Cool Roofs

Surface temperatures measured by a thermal imaging camera were compared between experimental and control sites with slab (R’-1 and R-1, respectively) and panel (R’-2 and R-2) roofs.

The slab roof results are shown in Figure 7. The maximum temperature difference between the experimental and control sites was 22.9 °C; the average temperature of the cool roof sites was 15.5 °C lower than the average temperature of the control sites. Temperature differences between experimental and control sites were lower in the morning, when air temperatures were generally lower; however, there was a statistically significant difference in daytime temperatures. The maximum indoor air temperature difference between experimental and control sites was 3.4 °C; the average temperature of the cool roof sites was 2.7 °C lower than the average temperature of the control sites. There were non-significant differences in atmospheric temperature between experimental and control sites, although indoor sites had non-uniform atmospheric temperatures because those sites were located in residential homes.

The panel roof results are shown in Figure 8. The maximum temperature difference between the experimental and control sites was 16.9 °C; the average temperature of the cool roof sites was 11.6 °C lower than the average temperature of the control sites. The maximum indoor air temperature difference between the experimental and control sites was 0.3 °C; the average temperature of the cool roof sites was 0.9 °C lower than the average temperature of the control sites.

Thus, surface temperatures decreased after cool roofs were installed; however, indoor air temperatures did not significantly change. As a result of environmental factors such as air temperature and ambient longwave radiation, quantitative infrared thermography may be prone to inaccuracies. Thus, more reliable results could be expected by measuring continuous data cumulatively at more points in future research.

4.2. Cool Pavements

Measured surface and atmospheric temperatures were compared between experimental and control sites on parking lot (P’-1–6 and P-1, respectively; Figure 9) and alley (A’-1–4 and A-1, 2; Figure 10) surfaces.

In the parking lot, the maximum surface temperatures of the cool pavement sites were 31.3–38.6 °C (Figure 11). The maximum surface temperature at site P-1 was 42.1 °C; thus, the maximum temperature of the cool pavement sites was approximately 3.5–10.8 °C lower than the maximum temperature of the control site. The average surface temperatures of the experimental sites were 23.5–27.5 °C, whereas the average surface temperature of the control site (P-1) was 29.3 °C; thus, the cool pavement reduced the surface temperature by approximately 1.8–5.8 °C.

In the alleys, the maximum surface temperature of the cooling pavement sites was 26.8–32.9 °C, whereas the maximum surface temperatures of the control sites were 34.4–35.4 °C (Figure 12). The maximum temperature was 4.8 °C lower at cooling pavement site A’-1 than at the corresponding control site (A-1); the maximum temperature at site A’-2 was 1.5 °C lower than the maximum temperature at the corresponding control site (A-2). The average surface temperatures of the experimental sites were 19.0–22.7 °C, whereas the average surface temperatures of the control sites were 25.2–25.5 °C. The average temperature was 2.5 °C lower at cooling pavement site A’-1 than at the corresponding control site (A-1); the average temperature at experimental site A’-2 was 4.3 °C lower than the average temperature at the corresponding control site (A-2).

4.3. Survey

4.3.1. Cool Roofs

We interviewed 64 residents living in the cool roof installation zone in Gimhae. According to the survey in

Table 5. User satisfaction survey results after installation of cool roofs.

Category	Cool Roof	Percentage of Respondents (%)
		Very Unlikely (1)	Unlikely (2)	Neutral (3)	Likely (4)	Very Likely (5)
Opinions	- Need for climate change mitigation	0.0	1.5	6.3 (do not know)	60.9	31.3
	- Awareness of facility	-	34.4	-	50.0	15.6
	- Interest in cool roof installation	0.0	7.7	64.1	18.8	9.4
Perception of effects	- Perceived room temperature change	0.0	12.5	48.4	34.4	4.7
	- Perceived reduction in air conditioner use	0.0	20.3	46.9	29.7	3.1
	- Perception of sleep disorder alleviation	3.1	17.2	54.7	21.9	3.1
	- Perceived reduction in heatwave damage	0.0	18.8	57.8	20.3	3.1
Satisfaction	- Satisfaction with temperature reduction	1.6	15.6	51.6	28.1	3.1
	- Overall satisfaction	0.0	10.9	57.8	29.7	1.6

, most residents spent an average of ≤6–12 h (50.0%) in cool roof buildings, followed by 12–18 h (34.4%), 18–24 h (7.8%), and <6 h (7.8%). The results showed that 28.5% of residents spent the hours from 18:00 to 24:00 or from 6:00 to 12:00 in cool roof buildings, followed by 23.5% from 15:00 to 18:00 and 19.5% from 12:00 to 15:00. Detached houses constituted 70.3% of cool roof buildings, followed by apartments (9.4%), row houses (4.7%), multi-family houses (7.8%), and others (5%). Most residential roofs were composed of slabs (81.3%); only 10.9% of roofs were composed of panels.

Climate change mitigation activities were perceived as “very important” by 31.3% of respondents, followed by “important” (60.9%), “not important” (1.5%), and “do not know” (6.3%), suggesting that most respondents recognize the need to mitigate climate change. When respondents were asked if they were aware of cool roof technology, 15.6% indicated that they were well aware of it, 50.0% indicated that they had heard of it, and 34.4% indicated that they were not aware of it. We also found that 9.4% of respondents were very interested in cool roof technology, whereas 18.8% were interested, and 64.1% were neutral.

After cool roof installation, 4.7% of residents perceived a “very likely” decrease in indoor temperature, followed by 34.4% “likely”, 48.4% “neutral”, 12.5% “unlikely”, and 0% “very unlikely”; thus, most respondents felt a reduction in temperature as a result of cool roof installation. When asked whether air conditioner use would decrease after cool roof installation, 3.1% said it was “very likely”, followed by 29.7% “likely”, 46.9% “neutral”, 20.3% “unlikely”, and 0.0% “very unlikely”. When asked whether cool roofs would reduce the occurrence of sleep disturbance related to heatwave, 3.1% of respondents said it was “very likely”, followed by 21.9% “likely”, 54.7% “neutral”, 17.2% “unlikely”, and 3.1% “very unlikely”. After cool roof installation, 3.1% of respondents reported that a decrease in health issues such as stress or loss of stamina related to heatwaves was “very likely”, followed by 20.3% “likely”, 57.8% “neutral”, 18.8% “not likely”, and 0.0% “very unlikely”. The large proportion of “neutral” responses suggests that many respondents did not experience a significant reduction in stress.

Residents reported their satisfaction with indoor temperature reduction in relation to cool roof installation as “very satisfied” (3.1%), “satisfied” (28.1%), “neutral” (51.6%), “dissatisfied” (15.6%), or “very dissatisfied” (1.6%). These results indicated a generally satisfactory response. When asked to express their overall satisfaction levels, 1.6% of respondents gave the new technology 5 points, 29.7% gave it 4 points, 57.8% gave it 3 points, 10.9% gave it 2 points, and no respondents gave it 1 point; thus, the average score was 3.2 points, indicating a moderate level of satisfaction. Other comments revealed that some respondents found the new surfaces to be slippery, easily soiled (i.e., caused management difficulty), and unsightly because of glare or dirt. Thus, residents were generally satisfied with the temperature reduction effects of cool roofs, and they had high levels of interest in the technology; however, installation methods should be improved to resolve the reported issues during the future expansion and maintenance of cool roof systems.

4.3.2. Cool Pavements

We interviewed 13 users of the Jangyu Traditional Market parking lot about the installed cool pavement as

Table 6. User satisfaction survey results after installation of cool pavements.

Category	Cool Pavement	Percentage of Respondents (%)
		Very Unlikely (1)	Unlikely (2)	Neutral (3)	Likely (4)	Very Likely (5)
Opinions	- Need for climate change mitigation	0.0	0.0	30.8 (do not know)	30.8	38.5
	- Awareness of facility	-	15.4	-	61.5	23.1
	- Interest in cool pavement installation	0.0	0.0	25.0	0.0	75.0
Perception of effects	- Perceived road temperature change	0.0	15.4	53.8	23.1	7.7
	- Perceived reduction in heatstroke	0.0	30.8	38.5	15.4	15.4
	- Perception of reduced inconvenience	0.0	23.1	30.8	30.8	15.4
	- Perceived reduction in heatwave damage	0.0	46.2	23.1	15.4	15.4
Satisfaction	- Satisfaction with temperature reduction	0.0	7.7	23.1	53.8	15.4
	- Overall satisfaction	0.0	0.0	30.8	30.5	38.5

. In total, 53.8% of the respondents parked for 1–2 h, whereas 46.2% parked for less than 30 min. The main time zones during which people used the parking lot were from 12:00 to 15:00 (46.2%), 6:00 to 12:00 (38.5%), and 15:00 to 18:00 (15.4%).

We found that 38.5% of respondents rated climate change mitigation activities as “very important”, followed by 30.8% “important”, 0.0% “not important”, and 30.8% “do not know”. Only 23.1% of respondents were aware of cool pavement technology before the survey, 61.5% had heard of it, and 15.4% were not aware of it. Notably, 75.0% of respondents who described themselves as “very interested” and 25.0% of respondents who were “neutral” displayed high levels of interest in the technology.

Parking lot users scored their own perceptions of temperature reduction in relation to cool pavement as “very likely” (7.7%), “likely” (23.1%), “neutral” (53.8%), “unlikely” (15.4%), or “very unlikely” (0.0%). Perceptions of alleviated heat illnesses such as heatstroke or heat exhaustion after cool pavement installation were rated as “very likely” (15.4%), “likely” (15.4%), “neutral” (38.5%), or “unlikely” (30.8%), or “very unlikely” (0.0%). Reduced discomfort because of geothermal heat experienced during the transition to cool pavement was reported as “very likely” (15.4%), “likely” (30.8%), “neutral” (30.8%), “unlikely” (23.1%), or “very unlikely” (0.0%). Some users reported that heatwaves caused less health damage (e.g., stress or reduced physical strength) after cool pavement installation, with responses of “very likely” (15.4%), “likely” (15.4%), “neutral” (23.1%), “unlikely” (46.2%), and “very unlikely” (0.0%).

Users were generally satisfied with the temperature reduction effect of cool pavement, such that 15.4% responded “very satisfied”, followed by 53.8% “satisfied”, 23.1% “neutral”, 7.7% “unsatisfied”, and 0.0% “very unsatisfied”. Overall satisfaction scores were 5 points (38.5%), 4 points (30.8%), 3 points (30.8%), 2 points (0.0%), or 1 point (0.0%); the average score was 3.1 points. Among the reasons provided for user dissatisfaction, the most common comments were “water spattering in rain”, “dirty”, “publicity stunt”, “messy”, “expansion of construction area”, and “requires repair and management”. Local residents showed high interest in the technology after construction was completed.

5. Conclusions

In this study, we quantitatively evaluated the impact of urban heatwave mitigation measures in the Jangyumugye district of Gimhae, Republic of Korea. Cool roofs and cool pavement were installed in urban areas; surface temperatures were measured and compared between experimental (cooling technology applied) and control sites. Cool roofs were applied to residential houses, and cool pavement was applied to a parking lot and alleys. Cool roofs and cool pavement both substantially reduced surface temperatures, with cool roofs providing average temperature reductions of 15.5 °C and 11.6 °C for slab and panel roofs, respectively. Both parking lot and alley sites showed temperature reductions of approximately 4–5 °C after cool pavement installation.

As a result of evaluating the benefits of microclimate in previous study, the results indicate that asphalt pavement is the worst option, while cool pavement integrated with vegetation is much more beneficial to human health [39]. Also, it was discovered that green roofs and cool roofs have the effect of reducing surface temperatures by changing the albedo of the roof surface based on a microclimate analysis of the urban scale [25]. The results of this study also showed similar results to previous studies; urban heatwave mitigation strategies such as cool roofs and cool pavements are effective in reducing temperature and also have an effect on humans.

Our study had some limitations, including the small numbers of users interviewed in our surveys and the limited conditions evaluated. Thus, the temperature reduction effect of heatwave mitigation technology may differ depending on the structures of houses where cool roofs are installed. Cool roofs composed of either slabs or panels showed the greatest impact on surface temperature between 12:00 and 15:00; however, because fewer than 20% of residents are present during this period, the effect on temperature may have been underestimated. Thus, it is likely that cool roof installation will have a greater impact on older adults, who remain at home for extended periods of time during the day.

Conversely, approximately half of all parking lot visitors use cool pavement between 12:00 and 15:00. These daytime visitors are expected to experience the heat reduction effects of cool pavement. Despite the short monitoring period, more than 30% of respondents using either heatwave mitigation measure stated that they perceived a temperature decrease.

Urban heatwaves are influenced by diverse factors such as building materials, housing type, and topography and vegetation surrounding the city. Therefore, the effects of urban heatwave mitigation technology should be determined through empirical and experimental studies that incorporate the practical evaluations of the effects on citizens.

The long-term effectiveness of cool roofs should be assessed in conjunction with indoor temperature monitoring and building energy consumption. To alleviate urban heat islands and reduce building energy consumption, heatwave mitigation technologies should be implemented in appropriate combinations, such as cool roofs over industrial complexes and greening on buildings, which would comprehensively address potential consequences such as glare and management complications.

In this study, temperature measurements were conducted to estimate daytime temperatures during a heatwave; thus, there was limited representation of diurnal temperature cycles related to solar radiation during the day. The impact of each heat reduction technology should be examined during tropical nights, which are common heatwave periods in urban centers. Extreme weather events including heatwaves have recently exceeded recorded extremes and threatened human lives; because these events are increasing in frequency and intensity, a response initiative is needed to increase the short- and long-term public perception of heatwave risks. The results of this study provide an opportunity to improve temperature reduction strategies for urban heat islands and develop management policies for heatwave and heat island mitigation.