Can Roof Art Cool Our Cities?
by Juhi Kundu
Our urban centers are facing a classic boiling frog problem. Every year, as more people migrate to major cities, more buildings go up to house them. Building by building, our urban climate grows ever warmer, and just like frogs that won’t escape from the gradually increasing temperature of boiling water, we continue to "cook" ourselves in this escalating heat.
It's a known phenomenon that urban centers are several degrees warmer than rural areas due to a lack of vegetation and, instead, the use of man-made materials with dark surfaces such as asphalt roofing, roads, sidewalks, and other infrastructure that retain heat. This phenomenon is known as the “urban heat island effect” and is only becoming more prominent with climate change. This heat absorption causes ambient city temperatures to rise so much so that urban centers are as much as 7 degrees warmer in the day and up to 5 degrees warmer at night compared to rural areas [1]. According to Yale Environment 360, the heat waves resulting from the urban heat island effect “can be a killer. Counter-intuitively, the biggest effects are often at night. Vulnerable people such as the old who are stressed by heat during the day badly need the chance to cool down at night. Without that chance, they can succumb to heat stroke and dehydration…This appears to be what happened during the great European heat wave of 2003, during which some 70,000 people died, mostly in homes without air conditioning” [2].
Of course, those of us with air conditioning have the option to crank it up to counter the heat outside. The problem is that air conditioning is costly and destructive to the environment. Much of existing cooling equipment uses materials that release potent greenhouse gases and use lots of energy [3]. However, air conditioning’s cost and climate impact could be mitigated by combining its use with more climate-friendly approaches to reduce summer heat.
One such approach is the use of “cool roofs” which are, most commonly, roofs painted white so they reflect more sunlight, thus decreasing the inside temperature of a building. However, in the winter, the increased reflectivity of cool roofs results in increased heating costs, also known as a “heating penalty,” which is why, according to Oleson et al., “any benefits gained from a reduction in the summertime heat island need to be considered in the context of increased heating costs in winter” [4]. Art Rosenfeld, California Energy Commissioner, summed up the issue well when he pointed out, “The ideal roof would be 100 percent reflective in the summer and 100 percent absorbent in the winter” [5].
Very little research has been done on switchable roofing materials that change with temperature. However, one study conducted by researchers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory found a material called vanadium oxide which undergoes a temperature-based phase change where infrared rays can pass through at room temperature but are blocked when the material is heated to a certain temperature [6]. This material is promising as a temperature-dependent roof coating but requires further testing on a larger scale to determine “its performance as a practical roof coating” [7].
Therefore, instead of utilizing a material science approach, my study explored a mechanical solution that might be more quickly adoptable with the goals of optimal durability, scalability, and accessibility. To reduce summer temperatures while also ensuring no heating penalty in the winter, this study designed and tested the utility of a “chameleon” roof that mechanically flips between a black absorptive roof in the winter and a white reflective roof in the summer.
Two flippable roofing materials were tested: Black asphalt shingles painted white on one side, and aluminum tiles painted white on one side and black on the other. The tiles were configured in an aluminum frame with hinges so that they could flip between black and white. The frame system was integral in making the tiles easily implementable as it ensured the existing roof required no alteration. Therefore, the tiles and shingle only needed to sit atop the roof, which is how they were placed while conducting experimentation during the study. Each type of flippable roofing was placed on top of a foam core box, acting as a model house, with an existing grey shingled roof (the control). The house was exposed to an infrared heat lamp and tested under both winter and summer conditions, which were modeled using specific sun angles of 32° and 70°, respectively. The ambient temperature inside the house and the surface temperature of the roof were recorded in each condition and compared to the control.
In the winter, black asphalt shingles proved the most effective by heating the inside 8.7% more than the control roof. In the summer, white metal and white asphalt shingles were most effective since they heated the inside 36.9% and 34.5% less than the control roof, respectively. Therefore, regarding the inside temperature, the flippable shingle material was most productive since it cooled significantly in the summer and had zero heating penalty in the winter.
However, as mentioned before, one thing that contributes to heat waves, and thus an increased risk of severe heat-related illnesses, is when an area does not cool down overnight. Although an object’s color can affect how much heat it absorbs, the material it is made out of affects how long it can retain that heat. Metals, like aluminum, for instance, are conductors, which means they are good at getting really hot really fast, but this also means that they lose that heat very quickly. This makes them ideal materials to ensure cooling overnight, which can reduce the effects of heat waves. Asphalt, however, is thicker and retains more heat than aluminum, meaning it cannot cool down as fast. Therefore, as the white metal roofing proved efficient in cooling the inside temperature during the day and is also effective at cooling itself overnight, it is the most beneficial for roofing out of the two materials tested, but only in the summer. This led to the possibility of not only utilizing different colors for the different climates but also utilizing different materials. Is there a way metal roofing could only be present in the summer? Could it possibly be retractable instead of flippable?
A future area of exploration for this study is the use of retractable roofing designs using kinetic architecture. Suparna Kadam, a solar panel industry veteran who has worked at Pivot Energy, EDF Renewables, and SunEdison, pointed out that a major reason why people do not implement solar panels atop their homes is aesthetics [8]. These observations suggest that when designing a flippable/retractable cool roof, an artistic and aesthetically pleasing design could enhance adoption. Future designs inspired by kinetic architecture might opt for using a visually interesting pattern to achieve the retractability of white metal roofing. This would ensure optimal cooling during the summertime, and leave the existing roof exposed for the wintertime, thus eliminating any heating penalty.
Although such retractable designs as shown above are visually interesting, they require more intricate joints, whose durability would need to be tested in certain weather conditions. Therefore, more research is required to test such designs and compare their efficiency to the flippable roofing design presented in this study.
In terms of the flippable design, using only one material, the asphalt shingle material proved the most effective considering its effects on the inside temperature for both summer and winter. It achieved the dual goal of increasing winter temperatures and decreasing summer ones. Such a solution that prioritizes easy implementation could influence more homeowners to make roofing choices that help reduce the urban heat island effect. In the future, if such designs included more artistic appeal, adoption might be further enhanced.
References:
US EPA, O. (2014, February 28). Heat island effect [Collections and Lists].
Urban heat: Can white roofs help cool world’s warming cities? (n.d.). Yale E360.
Air conditioners fuel the climate crisis. Can nature help? (2023, June 30). UNEP.
Oleson, K. W., Bonan, G. B., & Feddema, J. (2010). Effects of white roofs on urban temperature in a global climate model. Geophysical Research Letters, 37(3), 2009GL042194.
Cool roofs in cold climates. (n.d.). Sika Roofing.
Tang, K., Dong, K., Li, J., Gordon, M. P., Reichertz, F. G., Kim, H., Rho, Y., Wang, Q., Lin, C.-Y., Grigoropoulos, C. P., Javey, A., Urban, J. J., Yao, J., Levinson, R., & Wu, J. (2021). Temperature-adaptive radiative coating for all-season household thermal regulation. Science, 374(6574), 1504–1509.
Duque, T., LBL. (2021). New Smart-Roof Coating Enables Year-Round Energy Savings. UC Berkeley.
Phone communication with Suparna Kadam. (January 28, 2024).
