Cool Roofs a Wise Choice for Warming
The greenhouse effect of climate change involves primarily the absorption of infrared radiation by greenhouse gases (GHGs) in the atmosphere. Other components besides GHGs also enhance or decrease the distribution of incoming solar radiation by either absorbing or reflecting the sunlight. Land and ocean surfaces have a variety of different albedo values for reflecting or absorbing solar radiation.
Albedo describes a surface’s ability to reflect light; transparent surfaces reflect and radiate heat much better than dark colored surfaces, which absorb radiation and have a very low albedo. Because snow and clouds are light in color, they’re beneficial to reflecting incoming solar radiation and thus mitigate the greenhouse effect. Fresh snow has an albedo value of around 0.80 or 0.90, which means 80-90% of the solar radiation is reflected back out to space rather than being trapped in the atmosphere.
What’s puzzling is the fact that dark asphalt is a very prevalent material used in buildings and construction, even though it has an incredibly low albedo value (0.05 to 0.15). Switching from black roofs to white roofs, as well as changing dark asphalt surfaces to light concrete surfaces, could greatly increase the amount of radiation reflected back out into space. Communities, as well as individual homeowners, should look into this shift to curb climate change, reduce the heat island effect, and cut down on energy use.
Lessened Energy Use
When a building has a dark roof, it suffers from low solar reflectivity because heat is absorbed rather than emitted, causing the building below to heat up. During the summer, this is especially problematic because hotter buildings mean amplified air conditioning, and thus increased energy use.
If dark roofs were swapped out for roofs painted with solar-reflective white coating, or “cool” roofs, then the amount of heat conducted to the building below would decrease significantly. Benefits of cool roofs include reduced utility bills caused by a decreased usage of air conditioning, enhanced comfort to residents due to cooler inside temperatures, prolonged air conditioning system life due to less frequent use, and shrunken roof maintenance costs.
Diminished Urban Heat Island Effects
It’s a widely known phenomenon that cities tend to be hotter than surrounding rural areas due to the absorption of solar radiation by built structures (sidewalks, roads, buildings, and pavement). The urban heat island effect is harmful not only because of the abnormally warm temperatures involved, but also because it causes increased energy use (via air conditioning) and augments urban pollution, triggering many health hazards such as asthma. Urban albedo is usually quite low because darker materials constitute most of the surfaces (asphalt, roofs, and pavement).
If reflective coatings were administered to both roofs and pavements in cities, the reflectivity of these surfaces could rise 15 to 25%, which could increase overall urban albedo values by 0.10. That may seem like a somewhat small number, but if applied to urban areas worldwide, it could cause roofs and pavements to induce a negative radiative forcing effect on the Earth. Cool white roofs could potentially decrease the dichotomy between hot cities and cool surrounding landscapes.
Curbing Climate Change
Because pollution and heat are connected in a positive feedback loop, the utilization of cool roofs could result in lower urban temperatures and subsequently reduce the formation of smog. Cool roofs also reduce GHG emissions since less electricity is used to power air conditioning, thus decreasing CO2 emissions from power plants.
Astoundingly, one study found that if reflective roofing was used worldwide, a global cooling effect could take place that would be equivalent to offsetting 44 gigatons of CO2 over the roofs’ lifetimes. This equates to roughly $1.1 trillion! Cool roofs can increase the albedo of cities by a value of 0.10, yielding remarkable energy savings, and reducing the harmful effects of urban heat islands. They ought to be a priority, to say the least, when designing or renovating green buildings.Featured Image: Rooftops of Cusco. Image via Kenneth Moore.