White Roofs, Cool Savings

Did you know that painting a roof white can save money on cooling costs? We’ll explain how this process works and which paints are most effective. And as legislators and architects embrace
the idea that building design has a huge impact on energy consumption and sustainability, white roof coatings will become more and more mainstream. However, building owners sometimes do not appreciate the immediate benefits of white roof coatings and convincing them of their value is part of what we will be covering. One of the best approaches is to combine two arguments: energy savings and the extension of roof life cycle. In the latter case, white coatings protect membranes with a chemical barrier and reflect sunlight, both of which contribute to a longer roof life cycle. The primary benefit, of course, is that the reflection of solar radiation significantly lowers air-conditioning costs, especially during hot summers in temperate climates and year-round in warmer climates.

In roof design, a cool roof is a roofing system that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the building) and high thermal emittance (the ability to radiate absorbed, or non-reflected solar energy). Most cool roofs are white or other light colors, which naturally reflect light. Silver-colored zinc-galvanized sheeting does not reflect heat as well as the color of white because metallic surfaces fail to emit infrared back to the sky. The opposite is also true. Dark colors like black naturally absorb the most heat, which is why black roofs and asphalt drives get so hot in the summer. Cool white roofs enhance roof durability and reduce both building cooling loads and the urban heat island effect.

Cool white roofs are not only great for urban environments in America, but are an effective alternative to bulk attic insulation under roofs in humid tropical and subtropical climates. Bulk insulation, in these situations, can be entirely replaced by roofing systems that both reflect solar radiation and provide emission to the sky. This dual function is crucial, and relies on the performance of cool roof materials in both the visible spectrum (which needs to be reflected) and far infra-red which needs to be emitted.

Cool roof can also be used as a geoengineering technique to tackle global warming based on the principle of solar radiation management, provided that the materials used not only reflect solar energy, but also emit infra-red radiation to cool the planet. This technique can give between 0.01-0.19 W/m2 of globally averaged negative forcing, depending on whether cities or all settlements are so treated. This is generally small when compared to the 3.7 W/m2 of positive forcing from a doubling of CO2. However, in many cases it can be achieved at little or no cost by simply selecting different materials. Further, it can reduce the need for air conditioning, which causes CO2 emissions which studies have shown worsen global warming. For this reason alone it is still demonstrably worth pursuing as a geoengineering technique.

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