A cool roof is a roof that uses a highly reflective building material, regardless of the color. Cool roofs can help cool buildings, cities, and the planet by reducing the percentage of sunlight converted to heat by the building surface. White roofs are one type of cool roof. Cool colored roofs are typically treated with a coating that makes them reflective. Because white roofs have a higher reflectivity of any color, they are generally a better choice than cool colored materials. One caveat is that a any colored roof made out of metal or that has a metallic coating, will still absorb a lot of heat (because it will have a high thermal emittance) and is therefore not considered “cool.”
Green roofs are living vegetative systems located on rooftops. Their benefits include cooling the building through shading and insulation, reducing peak storm runoff, and potentially growing food. Green roofs do not, however, provide enhanced reflectivity and thus would have a negligible effect on global temperature if they were widely implemented.
Solar photovoltaic (PV) installations provide neither the reflectivity benefits of a cool roof nor the storm water management benefits of a green roof. However, PV installations can generate clean energy, an important benefit in our global effort to transition to a low-carbon energy economy. Because PV installations typically do not take up the entire surface of the roof, and sometimes require highly reflective roofing to ensure sufficient system efficiency, cool roofs and PV are complementary technologies.
Cool roofs, green roofs and PV are all excellent options for improving the environmental performance of a building. Which system or combination of systems is most appropriate for an individual roof will need to be evaluated on a case by case basis. We strongly believe that each of these options have an important role to play in reducing the environmental impact of our cities and see plenty of room for each solution to thrive.
We do want to point out, though, that cool roofs are by far the cheapest option of the three. They are at least an order of magnitude less expensive on a dollar per square foot basis than both green roofs and PV. When resources are available, we certainly encourage PV and/or vegetative roofs. However, we believe that cool roofs are a solution that can be more readily adopted by a larger percentage of cities globally.
Many people believe that the converse effect is also true; that white roofs increase the heating requirements for buildings in northern climates, the so-called “winter penalty”. There is sound evidence that the winter penalty is non-existent or minimal, since the sun is at a low angle in the winter months and heating loads are more pronounced in the evenings, especially in residential buildings. White roofs will still provide a higher albedo surface in these areas, creating a climatic benefit, even if the cost savings to the building inhabitants are minimal.
Visit the US Department of Energy’s Roof Savings Calculator to estimate what your energy and cost savings would be if you installed a cool roof.
On steep-sloped roofs (typically found on residential buildings), changing the reflectivity of the roof is usually not as simple. Because most people do not like the aesthetics of a white roof (which would be visible thanks to the sloped nature of the roof), increased reflectivity is best gained through highly reflective shingles that can come in any color. Currently, these roofing materials do cost more than standard materials, but as the market grows, the price is likely to come down.
There are also important non-cost factors to consider:
• Comfort: In unconditioned buildings, cool roofs can maintain cool indoor temperatures.
• Durability: Cool roofs may degrade slower and last longer than similar non-cool roofs, but more research is needed to prove this benefit.
Additionally, Lawrence Berkeley National Laboratory has a directory of suppliers of cool roofing materials.