The Issue
In tropical regions, those inside buildings often encounter Radiant Asymmetry, where excessive and uneven heat radiation penetrates the building envelope, leading to thermal discomfort. Usually, thermal discomfort is caused by a hot ceiling resulting from a dark-colored and inadequately insulated roof absorbing the intense tropical sun.
Case in point, the air-conditioning system of a hypermarket located in tropical Java, was struggling to maintain the temperature set point of 21°C on hot days. Largely because the building was fitted with a dark metal roof. The hypermarket owner told of significant thermal discomfort for their customers and an unnecessarily high monthly energy bill.
Our Approach
The hypermarket owner decided to engage a cool roof supplier, SkyCool, that applied their high-performance roof coating with a high solar reflectance of 87%, refer to photo during the application process:
The previous dark metal roof had a solar reflectance of about 20%, which accounts for the significant brightness difference between the uncoated and coated roof in the photo above.
Needless to say, by reflecting 87% of solar radiation back into the atmosphere, the SkyCool roof paint considerably lowers the heat absorbed by the roof, alleviating the issue radiant heat inside the hypermarket.
Figure 2: Illustration of heat flow into the building before and after SkyCool roof paint application.
After applying the paint, the client wanted to assess its effectiveness in mitigating the issue and explore further optimizations to enhance thermal comfort and energy efficiency under these improved conditions.
Simulated performance
IEN Consultants undertook a series of detailed building simulations to quantify the effect of apply the SkyCool roof paint, both in terms of energy consumption and thermal comfort for the building occupants. The scope of the two simulation were:
Building Performance Simulation Assess annual electricity consumption to evaluate energy efficiency improvements after applying SkyCool roof paint
Occupant Well-being
Maintain a comfortable indoor environment by regulating the HVAC Temperature Setpoint.
Our approach leveraged advanced building performance simulations, as well as tools like the Mean Radiant Temperature (MRT) Calculator and the CBE Thermal Comfort Tool to ensure accuracy and reliability in our assessments.
Building Simulation - IESVE
Figure 3: Construction Materials and Air-Con zones of the Building
To accurately calculate Mean Radiant Temperature (MRT) — a critical factor in assessing thermal comfort — we needed temperature data for key surfaces, including surface temperatures of the ceiling, floor, and walls. Thermographic images provided us with the ceiling temperatures before and after the application of SkyCool roof paint. Then, we employed IESVE building simulation software to estimate the temperatures of the floor and walls based on the construction materials of the building as defined in above diagram.
In our simulations, we modeled six different scenarios to investigate the optimal thermal comfort conditions with SkyCool roof paint applied. These scenarios allowed us to conduct a detailed comparison of how varying the indoor air temperature impacted both thermal comfort and energy efficiency.
Table 1: Simulation Scenarios. T he solar reflectance of the dark metal roof and the SkyCool roof is 20% and 87%, respectively.
Each scenario as shown in Table 1 offered insights into how much the indoor air temperature could be increased, post-application of the SkyCool roof paint, while still maintaining thermal comfort for the occupants.
MRT Calculator & Thermal Comfort Tool
Once we obtained the building simulation results, we used the MRT Calculator to determine the MRT for each scenario. MRT represents the average temperature of surrounding surfaces, weighted by their respective view factors.
After calculating the MRT, we applied the results to the CBE Thermal Comfort Tool to evaluate occupant comfort using Predicted Mean Vote (refer to PMV scale inserted below) and Percentage of People Dissatisfied (PPD) standards. These metrics provided a detailed understanding of how the different scenarios affected occupant comfort levels. PMV measures perceived thermal sensation, while PPD quantifies the percentage of occupants likely to be dissatisfied with the thermal environment.
By combining these tools with our simulation data, we were able to comprehensively assess the impact of the SkyCool roof paint on both energy consumption and occupant well-being, hence, identifying the most energy efficient and thermally comfortable building operation strategies.
Figure 4: PMV scale for thermal sensation.
Our Findings
Our analysis focused on two key areas: Thermal comfort and Energy performance. We evaluated each simulated scenario using metrics such as Predicted Percentage of Dissatisfied due to Radiant Asymmetry (PPD RA), Predicted Percentage of Dissatisfied due to thermal condition (PPD), and Predicted Mean Vote (PMV) to assess occupant comfort. In addition, we conducted energy simulations to assess the impact on annual electricity consumption, cooling load, and peak cooling load.
Surface Temperature
Figure 5: Hourly Surface Temperature for Case A, B and C.
The simulation results shown above demonstrate a substantial reduction in surface temperatures after applying SkyCool roof paint. Most notably, SkyCool roof paint is applied, the average peak ceiling temperature drops from 41°C (Case A) down to 23°C (Case B). This temperature reduction of 18°C significantly decreases the amount of heat transmitted into the building. The above graphs show two different ceiling temperatures, namely "Ceiling, actual", which denotes the actual ceiling temperature of the ceiling if you were to touch it with your hand, and "Ceiling, weighted", which denotes the radiant surface temperature of the ceiling as experienced by people in the hypermarket. In much lower values for "Ceiling, weighted" stems from the fact that most of the ceiling is covered by a radiant barrier (aluminum foil), which is a low emissivity material (e = 0.1), hence, significantly lowering the perceived surface temperature of the ceiling. For reference, the emissivity of most materials, such as a painted ceiling, is about nine times higher (e = 0.9).
The lower indoor surface temperatures enable a higher indoor temperature setpoint, reducing the HVAC system's workload while maintaining comfortable indoor conditions. This adjustment not only helps alleviate thermal discomfort but also offers an opportunity to lower energy consumption and extend the lifespan of the HVAC system by reducing operational strain. Case in point, the hypermarket was able to increase their temperature setpoint by 3°C after applying the SkyCool roof paint
Thermal Comfort
Occupant Well-being
Maintain occupant thermal comfort while increasing the setpoint temperature by up to 3°C, enhancing both energy efficiency and comfort.
Table 2: Simulated results for each scenario with low-e ceiling.
The key finding is that the SkyCool roof paint allows for a significant reduction in surface temperatures without any additional energy input. This reduction in heat absorption opens up the possibility of raising the HVAC setpoint temperature, leading to reduced cooling demand and lower energy consumption, all while maintaining or even improving occupant thermal comfort.
As demonstrated in Case C in Table 2, raising the setpoint temperature by 3°C did not compromise thermal comfort. Instead, it achieved a Neutral thermal sensation, which is ideal for the majority of occupants. This improvement in comfort, coupled with the reduction in energy use, showcases the efficiency of SkyCool roof paint as a sustainable building solution.
By reflecting heat away from the building, SkyCool roof paint reduces the need for cooling and enables higher temperature setpoints, contributing to both economic savings and environmental benefits. The result is a win-win scenario: a more comfortable indoor environment and significantly lower operating costs, as shown below.
Annual Energy Consumption and Cooling Load
The Building Performance Simulation indicates substantial improvements in energy efficiency:
Annual Cooling Load: Reduced from 145 kWh/m²/year to 13 kWh/m²/year
Annual Cooling Electricity Consumption: Decreased from 79 kWh/m²/year to 7 kWh/m²/year
Figure 6: Comparison of the annual electricity consumption, cooling load and peak cooling load between Case A, B and C.
By reflecting more solar radiation back into the environment, SkyCool roof paint significantly reduces the cooling demand on the HVAC system as indicated in Figure 6. This reduction is further enhanced when the setpoint temperature is raised by 3°C, leading to a sharp decrease in the cooling load required to maintain comfortable indoor conditions.
Additionally, the Annual Peak Cooling Load also drops by 73%, as a higher setpoint results in less frequent and intense cooling cycles. This reduction in peak demand lessens the strain on the HVAC system during the hottest periods of the year, further enhancing the building's overall energy performance.
The combined impact of SkyCool roof paint and the increased setpoint temperature leads to a 91% reduction in cooling electricity consumption. This reduction translates into significant economic savings and a lower environmental footprint, as the hypermarket uses substantially less energy to maintain optimal comfort conditions year-round.
BONUS: How does low-e ceiling help in saving energy?
Figure 7: Construction Materials of the Building Structure without radiant barrier.
To evaluate the benefits of a low-e ceiling, a simulation was conducted by removing the radiant barrier aluminum foil, effectively converting the low-e ceiling to a high-e ceiling. This adjustment allowed for a direct comparison of performance under identical conditions.
Table 3: Simulated results for each scenario with high-e ceiling.
Table 4: PPD RA comparison between high- and low-e ceilings.
The absence of the radiant barrier resulted in significant 54% thermal dissatisfaction rate from heat radiation coming off the ceiling surface. Once the SkyCool roof paint is applied, the thermal dissatisfaction from radiant asymmetry drops to just 4%. This significant different is the PPD RA value (Predicted Percentage of Dissatisfied due to Radiant Asymmetry) occurs due to a high-emissivity ceiling surface that effectively re-radiates the roof heat into the occupied hypermarket space below. Consequently, this leads to greater thermal discomfort and higher cooling energy demand to maintain acceptable indoor conditions.
In other words, both the solar reflectance of the roof surface as well as the ceiling emissivity play important roles for the energy consumption and the thermal comfort of the occupants. Our building simulations of the hypermarket in Java with a slightly insulated roof (just 25 mm roof insulation) shows that a cool roof paint kills two birds with one stone, as it both significantly reduces energy consumption and improved thermal comfort. Whereas, a low-emissivity ceiling only effectively tackles the thermal comfort issue, but only reduces energy consumption to a limited degree. An energy comparison is shown below for hypermarkets with different combinations of solar reflectance roof values (20% versus 87%) and ceiling emissivity values (0.1 versus 0.9). The graph below shows the annual monetary savings for hypermarket in Java, which had it's 10,000m2 dark metal roof painted with SkyCool:
Figure 8: Annual Energy Savings from reducing energy transmission through the roof
Given the floor size of the hypermarket in Java, which spans 10,000 m², the potential energy and cost savings from adopting a low-e ceiling are substantial. The graph shows that majority of the energy savings comes from applying cool roof paint. Using a low-e ceiling also helps to save energy, but to a lesser degree, especially once the cool roof paint has already been applied.
For the hypermarket in Java, the application of SkyCool roof paint allowed them to save about 60,000 USD per year. If the hypermarket did not have a radiant barrier ceiling, the energy savings from applying SkyCool roof paint would have been even higher, namely about 68,000 USD per year.
It is also notable, that the cool roof significantly reduces the peak cooling load across the year, namely a reduction about 60 kW of cooling capacity, or 17 RT (refrigerant ton). This means, once the cooling system is up for replacement, money can be saved by installing a smaller cooling system with a 60 kW (17 RT) lower cooling capacity.
Conclusion
In conclusion, our study shows that cool roof paints are effective in saving energy and resolve thermal comfort issues stemming from hot ceiling radiant asymmetry. In this case study of applying SkyCool roof paint to the 10,000m2 roof of an air-conditioned hypermart in Java, the energy savings were 60,000 USD per year. The cooler roof also allows the cooling system to be downsized by 60 kW (17 RT), which is another saving, both in the purchase of cooling equipment and in terms of the maximum demand charge on the electric bill. This case study also demonstrates that most of the energy savings result from using cool roof paint. While implementing a low-e ceiling also contributes to energy savings, its impact is less significant, particularly after the cool roof paint is already in place.
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