A new study conducted by UCL researchers found that cool roofs, painted white or covered with a reflective coating, could be more effective at reducing temperatures and cooling down cities during hot spells than solar panels and green roofs. Meanwhile, air conditioning (AC) raises the outside temperature by 1 degree Celsius.
This study comes as cities around the world are coping with the menace of rising temperatures. Urban areas tend to be warmer than their neighboring rural zones, exacerbating the impact of heat waves and subsequently increasing health risks in these locations.
An examination of the difference in land surface temperatures between urban areas and their rural surroundings from 2003 to 2020 by the Joint Research Centre of the European Commission revealed that city surface temperatures were sometimes as much as 10-15°C higher than in rural areas.
Take London, for example. Residents here are facing a new reality with temperatures consistently above 30°C. According to research by the International Institute for Environment and Development (IIED), over the past three decades, London has experienced 116 days with temperatures exceeding 30°C, over half of which occurred in the last decade alone.
Consecutive days above 30°C are now more common, and even extreme temperatures have increased in frequency, with five days registering above 35°C in the last five years in London. This extreme heat is raising the risk of heat exhaustion and heatstroke and exacerbating existing health conditions.
As the climate changes, those living in cities are becoming increasingly vulnerable to warming temperatures because cities tend to trap heat, creating the Urban Heat Island effect, which leads to significant health issues for residents during hot spells.
With a large global population living in cities, it is critical to find sustainable solutions to mitigate these effects. City planners, designers, councils, and governments are all working to reduce these temperatures.
Some of the passive cooling methods being actively explored include green roofs, expanded urban vegetation, and cool roofs, alongside air conditioning, an active method for cooling building interiors.
Green Roofs & Solar Panels Not That Effective
Studies comparing the impact of building and street-level interventions on temperature at metropolitan scales are lacking despite the increasing urban heat-related mortality and morbidity. The researchers of UCL model the impact of nine interventions on air temperatures at 2 m during two hot days in the summer of 2018, the 26th and the 27th of July.
Using detailed urban climate modeling and recent data on the actual coverage of passive and active interventions, the research studied the effect of both of these heat adaptation strategies on the outdoor air temperature and surface energy balance (SEB) in the Greater London area.
The study discovered that deploying both solar panels and green roofs was ineffective in significantly reducing temperatures on a large scale. However, a feasible deployment of solar panels could offset its associated energy consumption, according to the study.
Solar panels and widespread street-level vegetation provided only a modest net cooling effect, averaging around 0.3 degrees Celsius. While these systems are not highly effective, they still offer environmental benefits.
Meanwhile, solar photovoltaic (PV) panels, which are considered primarily a source of electrical power, can also be used as a passive-active strategy to reduce temperatures by capturing sensible heat flux. They do this by increasing the roofs’ albedo, which is the fraction of light that a surface reflects.
Roofs that incorporate vegetation also have some great benefits, such as wildlife habitats and water drainage. But while green roofs do provide a daytime cooling effect, their net cooling effect, on average, is negligible. The researchers found that the effect of green roofs on temperature was significantly varied throughout the day.
These green roofs were modeled with sedum vegetation because it is the most common plant in London, but other designs may lead to other effects.
During the day, when it is warmest, the wide adoption of green roofs can bring down urban temperatures by an average of 0.5 degrees C. However, the green roofs end up increasing night-time temperatures by approximately the same amount due to the fact that the thermal mass from the roofs retains daytime heat and releases it when the sun goes down.
Now, changing the vegetation type from grasslands or croplands to deciduous trees can decrease outside temperatures, but only at night. During the day, the researchers found that it would have mixed results.
The study noted that this could be because of the rise in the surface albedo, which results in increased sensible heat emission. The increased latent heat here would be equivalent to that from green roofs when deployed at full scale.
While reducing air temperature, this kind of cooling is likely to increase the amount of water vapor in the atmosphere, which would then increase the humidity in the air. So, while this could be beneficial to avoid the water stress of vegetation, it affects the heat stress of the city’s residents.
Both solar panels and a mixture of croplands, grasslands, and deciduous trees had stronger cooling effects in the east of London.
What Actually Works for Cities, Then?
Published in Geophysical Research Letters, the study used a 3D urban climate model of Greater London to test the thermal effects of various active and passive urban heat management systems. These included:
- Green roofs
- Ground-level tree vegetation
- Rooftop solar panels
- Painted “cool roofs”
- AC during the two hottest days of the 2018 summer
To gauge each system’s full potential effect, the team modeled each one as though it had already been widely adopted and theoretically feasible in housing, commercial, and industrial buildings throughout Greater London.
Out of all the solutions, cool roofs were found to have the greatest reduction in 2 m air temperature. Cool roofs involve implementing highly reflective roofs made of various materials such as metal, concrete, or single-ply membranes.
If deployed at full capacity, all cool roof interventions contribute to a reduction, with major impacts in the south and east of GLA.
On average, cool roofs reduce outdoor temperatures by 1.2 degrees C if they are adopted widely throughout the city. Some locations in London could see as much as a 2 degrees C reduction in temperature.
“We comprehensively tested multiple methods that cities like London could use to adapt to and mitigate warming temperatures and found that cool roofs were the best way to keep temperatures down during extremely hot summer days. Other methods had various important side benefits, but none were able to reduce outdoor urban heat to nearly the same level.”
– Lead Author Dr. Oscar Brousse, UCL Bartlett School Environment, Energy & Resources
Cool roofs actually have dual benefits:
- Cool the outside urban environment by reflecting heat instead of absorbing it
- Cool the insides of buildings
When looking into the colling’s horizontal advection, the study noted it is all dependent on the position of the cool roofs. So, if cool roofs are deployed in central and mid-rise locations of the city, the low-rise and open residential areas will seem to enjoy the cooling effect.
Interestingly, establishing cool roofs only on low-rise residential buildings provides a substantial outdoor reduction by up to ∼2°C in the southeast but is unable to cool London’s central parts. Implementing cool roofs only on large low-rises produces a marginal heat reduction over London up to ∼1°C reduction in large low-rise areas of east London.
For cool roofs, which are found to be the “most effective way” of reducing outdoor temperature, the study stated:
“At the maximum, this temperature reduction is between 3.2°C and 2.8°C at times when daily spatial average temperatures reached 33°C and 37°C, respectively.”
The study, however, hasn’t been without its constraints. The researchers acknowledged several limitations. These included the use of only one urban canopy model (UCM), focusing on just two hot summer days due to computational constraints, and assuming a single type of application per intervention. Additionally, the absence of completely clear-sky conditions and concentrating solely on outdoor temperatures were noted as significant constraints. The study also did not account for the impacts of these solutions on other aspects, such as wind circulation, biodiversity, and indoor temperatures.
As such, researchers call for future research to estimate the impact of each of these strategies on longer time periods, the costs related to the deployment of each of these interventions to perform cost-benefit analyses, and foster the deployment of SEB (surface energy balance) monitoring systems of urban scale to validate model simulations.
ACs Having a Negative Impact
As temperatures climb, the adoption of air conditioners (ACs) for heat protection has significantly increased. According to the International Energy Agency (IEA), global AC sales tripled between 1990 and 2016, and they continued to grow in the following year. Consequently, the worldwide market size of air conditioners is projected to exceed $200 billion in 2024 and is expected to rise to $259.5 billion over the next five years.
This growth comes as nations around the world turn to ACs, with nearly 90% of US homes having air conditioning. However, not only do these cooling devices account for about 10% of global electricity consumption, but they also contribute to rising temperatures.
An AC works by absorbing heat from inside the building and releasing it outside while the cooled air is circulated throughout the home. This way, the air conditioning ends up warming the outdoor urban environment even more.
According to the latest study, using air conditioning to maintain the indoor temperature of all of London’s building stock to 21°C increases the temperature by about 0.15 degrees C across the city. This increase can be as much as 1 degree C in dense central London.
The researchers noted that ACs still protect at-risk populations from scorching temperatures within buildings. Moreover, solar photovoltaic (PV) panels can be used as a passive-active strategy by transforming incoming solar radiation into electrical power, which can then be used to run the AC system.
If solar panels are implemented on rooftops in a practicable manner, the energy production can cover the energy consumption of AC systems as it never goes above 8.78 MW during the hottest hours. Electric power production, if unconstrained by buildings, could go to 8.79 MW on average, stated the study. These local impacts, however, may be dependent on many factors, such as building height and increase in rooftop albedo.
Having said that, given the negative effect of AC on our environment, researchers have been actively looking into ways to bring down heat and cooling costs as well as reduce the fossil fuel requirement to run in-home HVAC systems.
As we noted in our previous articles, scientists at the University of California have developed adaptive tiles with a radiative switch to toggle between heating and cooling states. These devices reduce cooling and heating needs by over 2.5x compared with static devices. They can further attain varying temperature targets by simply changing the phase change material.
In another instance, scientists at Berkeley Lab created a smart-roof coating that maintains optimal home temperature regardless of the season. For this, they developed a unique material called TARC (temperature-adaptive radiative coating), which switches off the radiative cooling during cold seasons automatically to ensure there isn’t any excess cooling and waste of energy.
But this is not all; innovation is also happening in refrigerant products. Recent research by the Department of Energy’s Oak Ridge National Laboratory gained a better and deeper scientific understanding of materials and devices that help achieve solid-state cooling.
Now, let’s look at some of the companies that could benefit from the adoption of cool roofs:
#1. Owens Corning (NYSE: OC)
As a leading manufacturer of roofing materials, Owens Corning could integrate cool roof technology into its products to meet the growing demand for sustainable and energy-efficient building materials, particularly in urban areas affected by the heat island effect.
Financially, Owens Corning reported net sales of $2.3 billion and net earnings of $299 million in the first quarter of 2024. They expect a consistent demand in North American building and construction markets and have set a financial target of achieving annual revenue of $10 billion by 2024 with mid-teen operating margins.
#2. 3M (NYSE: MMM)
Potential Benefit from Cool Roof Technologies: 3M could leverage its expertise in material sciences to develop reflective coatings for roofs. These products could be marketed as solutions for reducing cooling costs and mitigating urban heat, aligning with global trends toward more sustainable urban planning.
Q12024 proved to be a mixed bag for 3M, financially speaking. The company saw a slight decrease in total sales, reporting $8.0 billion, a 0.3% year-on-year decline, with adjusted earnings per share rising 21% to $2.39. Despite the challenges in sales, 3M demonstrated strong operational execution and financial discipline, which contributed to significant growth in adjusted earnings and an improvement in adjusted operating income margins to 21.9%.
#3. Sherwin-Williams (NYSE: SHW)
As a top producer of paints and coatings, Sherwin-Williams could develop and market specialized reflective paints that could be used for cool roofs. This not only aligns with urban sustainability goals but also opens up a new product line that could be highly relevant in cities looking to combat the urban heat island effect.
In 2023, Sherwin-Williams reported a decrease in net sales and segment profit. Net sales declined due to a mid-single-digit percentage drop in sales volume, impacted by demand softness in North America and the divestiture of the Chinese architectural business. Adjusted segment profit also decreased, primarily because of lower net sales volume and higher foreign currency transaction losses, notably due to the Argentine Devaluation. Nonetheless, the company generated $3.52 billion in net operating cash during the year.
Final Thoughts
Rising temperatures have been a cause of concern for nations, governments, and people all over the world. With cities bearing the brunt of this, solving the problem of urban heat has become a priority. As this study noted, given cool roofs’ efficiency at reducing urban temperatures and, in turn, adverse health impacts, they should be explored for widespread adoption.
However, just one solution can’t effectively address all the problems. A mixture of measures, such as greener alternatives, advanced technology, and better coating materials, must be adopted across cities to effectively reduce outdoor temperatures and increase indoor temperatures while mitigating any adverse impacts.
Click here for a list of biotech companies working on global warming solutions.