Urban Heat Islands and Cool Roofs
When I left the PolycreteUSA office in Richmond Virginia’s Manchester neighborhood one day recently, the thermometer in my truck said it was 101 degrees outside. Back at my farm, 20 miles and a half hour later, it was 8 degrees cooler.
That’s a perfect example of what’s called the Urban Heat Island Effect. Cities generally have few trees and lots of dark-colored paved areas, roofs, and hard masonry or concrete surfaces that absorb and hold heat.
These so-called “heat islands” become and remain much hotter than the surrounding areas that have less built-density, more green spaces and more extensive tree canopy.
Summers are getting gradually hotter each year, and each year the heat island effect has a greater impact on HVAC cost, electric grid capacity, and community health. The heat island effect also appears more pronounced in economically depressed areas where buildings may not have air conditioning.
In large city buildings without air conditioning, it's often hotter indoors than out -- particularly at night. Senior citizens, folks with chronic health problems, and anyone who has difficulty getting out to find a place to cool down are at greater risk of dying from heat-related causes.
The design and construction community can help reduce urban heat island effect. We can help save lives, money, and stress on the electric grid. We can do this by including high insulation values, thermal mass, and cool roofs on all new construction.
Designing with insulated concrete forms provides R21+ insulation values and creates thermal mass in the concrete. The thermal mass in the concrete, encased in a sandwich of continuous insulation, keeps indoor temperatures very stable. The is because the insulation prevents the concrete walls from changing temperature. Once the inside of the building reaches an optimum temperature, rising or falling air temperatures outside have little effect on the temperature inside.
The weak points in this energy efficient ICF building are the doors, windows, and roof. Door and window technology has come a long way and there are many highly energy efficient solutions to choose from. The roof is the final piece in the puzzle.
It’s been standard practice for many years to highly insulate a building’s roof in order to manage indoor temperatures most economically. The next standard to create is the cool roof. It’s not even a new concept, but it’s getting more attention lately because our summers are getting so much hotter. The City of New York’s Department of Buildings says a cool roof can save 10% to 30% in air conditioning costs on hot days.
US Department of Energy says, “Cool roofs have surfaces that reflect sunlight and emit heat more efficiently than hot or dark roofs, keeping them cooler in the sun. In contrast, hot roofs absorb much more solar energy than cool roofs, making them hotter.
Solar reflectance and thermal emittance are the two key material surface properties that determine a roof’s temperature, and they each range on a scale from 0 to 1. The larger these two values are, the cooler the roof will remain in the sun.”
Reflectance is just that – the amount of sun’s radiation the roof reflects. Emittance is the ability to emit absorbed heat, or the rate at which a roof cools itself back down.
According to C40 Cities (C40.org), New York City DOB requires that 75% of the roof area or setback surface on all new or substantially renovated low-slope roofs have minimum initial solar reflectance of 0.7 and minimum thermal emittance of 0.75. This means that all new roofs in NYC must reflect a minimum of 70% of the sun’s radiation.
You can’t create a properly reflexive and highly emmitive roof surface with paint alone. Specially designed elastomeric coatings, sheet products, and/or shingles are needed. That’s where the Cool Roof Rating Council comes in.
The Cool Roof Rating Council (CRRC) is a non-profit organization established to “develop accurate and credible methods for evaluating and labeling the solar reflectance and thermal emittance (radiative properties) of roofing products and to disseminate the information to all interested parties.”
The CRRC Rated Products Directory is a free online resource that lists the roofing products that have undergone testing, weathering, and rating in accordance with the CRRC’s protocols.
Finally, your choice of cool roof design is impacted by your geography. In some regions or locales, a cool roof may not be appropriate. The Department of Energy and CRRC can help you determine the appropriateness and economics of a cool roof in your area.
The Urban Heat Island Effect is real phenomenon, and its adverse impacts will only become more of an issue as our cities continue to warm. A recent study found that Richmond, Virginia is warming at twice the rate of rural Farmville, VA - 60 miles away.
Planting trees and using light colored pavements can help with Urban Heat Island mitigation, but in the end, it’s the responsibility of the design and construction community to build resiliency into our cities. Cool roofs are one more tool in your toolbox.
PolycreteUSA believes sustainable construction must also be fast, strong and economical. That's why we're dedicated to making it EASY to design, build and own energy efficient buildings that can stand up to Mother Nature.