During the past decades, several studies have been conducted concerning roofs as a means to keep buildings cooler, thus becoming more energy efficient. One might think this subject is as simple as having a light-coloured roof that reflects more solar radiation. However, other factors such as thermal performance, air infiltration or vapour drive need to be accounted for.
These are known as cool roofs, which can be white when painted in white, or green roofs, if they are covered by plants. Ideally, they should have materials with higher reflectance, so that more energy is reflected from the surface, and higher emittance, so that less energy is released to the environment, thus not tending to trap heat (figure 1). If the buildings do not have cool roofs, then they tend to consume more energy for people’s thermal comfort through air conditioning, with that increasing the energy demand and overloading the power grid, ultimately increasing energy bills. Despite building more power plants, overhauling ageing power grids, and purchasing energy through interstate markets, these efforts are not meeting the electricity demands of rapidly growing cities.
Nonetheless, a problem is usually not alone. Thus, besides demand issues, there are also environmental impacts in the so called “heat islands” – urban areas where temperatures reach as high as 12 ºC warmer than surrounding rural areas due to extensive use of man-made materials like asphalt and concrete, which results in the reduction of evapotranspiration and greater heat storage. Some evidence suggests that reducing heat island effects allow cities to diminish mortality rates associated with extreme heat events. In addition, the problems increase when taking smog into account – it was observed that the ozone concentration began to exceed the National Ambient Air Quality Standard of 120 parts per billion by volume (ppbv) when the daily maximum temperature in Los Angeles hit about 22 ºC, and reached 240 ppbv around 32 ºC, going from acceptable to terrible in just 10 to 15 ºC.
With all the associated problems, some cities are fighting ferociously to improve the quality of life while at the same time trying to reduce energy demands. New York introduced rules on white roofs into its building codes in 2012 and volunteers painted nearly 9.2 million ft2 (854 thousand m2) of tar roofs white, though that is only about 1% of the total roof area. In 2019, New York required green roofs and/or solar panels on newly constructed buildings (an initiative supported by Washington State in the US). Los Angeles began a similar program in 2018 but painted asphalt road surfaces with grey paint since asphalt reflects only 4% of sunlight. In comparison, natural areas such as grasslands reflect as much as 25%, and up to 90% of incoming energy is reflected for a white surface such as fresh snow. In Europe, some buildings were built having this in mind.
But these benefits are dependent on climate and building characteristics. For cities where hot days are numerous, white roofs might be more appropriate. The Lawrence Berkeley National Laboratory found that white roofs are the most cost-effective over a 50-year time span, when compared to black and green. Over the course of a typical hot August in Scottsdale Arizona (average temperatures from 27 to 39 ºC), a white roof is 4.62% cooler than a non-white roof. During the hottest point of the day (5:50pm and around 39 ºC) the roof is 8.49% cooler, and during the maximum internal temperature hours (7:00am to 5:00pm) it is 6.97% cooler. A practical example can be seen applied to the Nationwide® Scottsdale Insurance Company headquarters which reduced its consumption in 7.79% from 719,000 kWh of electricity in 2008 to 663,000 in 2009 using the PolyKool roof system (a type of white roof).
However, the studies have no consensus in terms of warding off climate change, since the temperature decrease in one building does not necessarily mean an overall temperature decrease or that other buildings that do not have cooler roofs will have lower energy demands. A study done at Stanford University shows that roof membranes direct the heat upward into the atmosphere, where it then reacts with black and brown carbon in soot particles to form compounds that contribute to global warming. Thus, the Stanford study further found that even though the conversion of rooftops worldwide to white roofs accounted to cool population-weighted temperatures by 0.02 ºC, an increase of 0.07 ºC was observed in the overall Earth temperatures.
Even for energy demands, determining climate change effects is not so simple. At high latitudes in winter white roofs are counterintuitive due to low incoming solar radiation and the existence of snow on the roofs – they need to invest in better insulations as they have increased energy demands to compensate for reduced solar heating. In colder climates, white roofs are more prone to develop condensation, which leads to lower wind resistance, lower insulation values and mold formation. This is where green roofs become more beneficial (figure 2) – even though white roofs yield lower temperatures, green roofs deliver cooling in summer due to evapotranspiration, shadowing effect and have higher insulation potential since they are not in direct contact with the building surface, which reduces the energy needs in winter (it is observed an average of 25% savings in both kinds of weather).
One way to reduce emissions while simultaneously reducing air conditioning demand is to install photovoltaic panels on roofs (figure 3). They reduce the amount of sunlight absorbed by the building as they transform part of it into electricity. Since they do not reflect as much sunlight back to the atmosphere as white rooftops, the energy is not available to be absorbed again by pollutants in the air, thus storing heat. Solar panels can be guaranteed for at least 25 years and once the initial investment is made, a solar installation will continue to produce clean energy.
In conclusion, it is obvious that different roofs affect building efficiency and its surroundings in different ways. Before choosing an option, one needs to carefully consider their location and their allowed budget, taking into consideration maintenance costs. If you are interested in improving your rooftop’s energy efficiency performance and you happen to live in the USA, Arizona State University has created a simulation tool to tackle your needs. However, the final decision might be made by a combination of strategies, such as a mix between green roofs and solar panels used in many countries (like Switzerland, Germany and Austria).