We learn a valuable lesson from Mediterranean cities; roofs are white or at least light colored. They reflect light and keep heat from the interior of houses and buildings. Some jurisdictions in the U.S. (states and cities) have suggested contractors use solar panels or required them to put a ‘green’ roof with planted vegetation or paint a roof white, on the top of office buildings and multi-tenant dwellings. The laws and regulations in most locations exempt single-family homeowners. This frames our opportunity.
Scientists have studies that show that dark surfaces, especially tar roofs, create ‘heat islands’ and will significantly warm the surrounding cities. Unfortunately, living in a large city with lots of row houses I see roof after roof that have black tar coverings that absorb as much light and heat as the sun will put out. These houses put out their retained heat all day.
For this reason cities are hotter than their surrounding rural areas. The temperature change continues into the night. The retained heat is emitted at night raising the city’s nighttime temperature as well. This is something each of us can change.
There are two measures of a surface. One is the absorption where light and warmth is allowed in and retained under the surface that raises the house’s temperature. For example, if it is a sunny day and you walk barefoot on asphalt, because it is black and absorbs heat, you will burn your feet.
Another measure is the amount of light that is reflected back into the sky from a white or lighter surface, its albedo. No light with its accompanying heat is absorbed. Instead the light is sent back into the atmosphere. This keeps everything, including the roof and the underlying structure, cooler to the touch. This article is about reflecting the light (and its heat) back into the atmosphere.
A California state government database includes two charts that show how high the temperature gets on a roof in direct sunlight in a variety of conditions. These charts show temperatures roofs can reach with a variety of covers. For instance, it shows a measured temperature of a roof that is single ply covered by EPDM (ethylene propylene diene monomer (M) class rubber) at 173°F. A BUR (built up roof) topped with aggregate rocks measured 159°F. A BUR topped with a capsheet was measured at 158°F in full sunlight. However, a roof with a “cool” rated paint on a single ply roof came in significantly cooler at 121°F and a “cool” coating over a BUR roof was measured at an even cooler 108°F.
All roofing paint manufacturers set up a rating scale for their roof coverings to comply with, among others, the California Title 24 rating for emissivity and reflectivity in roofing materials. This gives a rating to their roof paints and coverings that states how they will reflect light and reduce heat.
Both scales are expressed as a percentage between 0 and 1. The higher the percentage number in reflectivity the more light and heat it reflects and keeps from penetrating the material. So paint with a perfect rating of 1 (or 100%) reflects all of the light that hits it. The same is true for thermal emittance. A material with a high heat emittance rating will give up its absorbed heat more readily. A perfect rating of 1 gives up all its retained heat very quickly.
This is a simplistic description of actions and benefits that does not represent all of the contrasting opinions that painting one’s roof represents. There are contrary positions that cite a variety of limitations. For instance, in snow areas white roofs defeat keeping a home heated, many would argue that snow on the roof is just as damaging. Recognize there is controversy. I would suggest you talk to your roofing painting company or Google the appropriate sources for their advice. I would argue that a white roof is beneficial in almost all surroundings.
We should each paint our roofs with the highest reflectivity and emittance ratings we can find. This will reduce our collective contribution to climate change and global warming. At the very least it will keep our homes cooler in the hotter temperatures and not create controversy if we live in the hotter regions of the country.
This is finally something, a small thing, most of us can do. We need to move ahead and take a personal stand against climate change. We need to, for example, conserve water, drive a hybrid car, turn off lights, conserve energy, and paint our roofs white to reflect the sun’s rays back into space. Of course there are many other things we can champion to our elected representatives. You may not agree with one or two of the steps we need to take but we each need to act. Only our collective action will save our planet and keep a healthy world to pass on to our children.
Use the attached links to obtain more information or look at the source documents used for this article.