Multifamily housing developers – including those who build affordable housing – can reduce peak temperatures by replacing hot, dark surfaces – interior roads, rooftops, playgrounds, and parking lots – with cooler alternatives.
With each summer seemingly getting hotter than the one before, apartment owners who can develop and market cooler properties will have an advantage over those who do not.
While there has been some movement toward “cool roof” and “cool pavement” programs, truly meaningful results will only come from plans that involve all heat-trapping surfaces. Any surface that is dark and impervious should be replaced with surfaces that are green, porous, and reflective.
The Smart Surfaces Coalition has partnered with the City of Baltimore as a demonstration of what can be done to cool the urban environment. The idea is to cover the city with reflective roofs and highways, solar panels, trees, porous pavements, and “urban meadows” – areas of the median where mown grass is replaced with unmanicured native grasses. These same ideas can be applied in a micro way at individual properties.
“Cool roofs” are an immediate solution for properties undergoing renovation if roof replacement is part of the renovation plan. These building materials help mitigate the urban heat island effect, which can have important implications regarding the comfort of residents.
In the past ten years, there has been a significant increase in the cool roof and urban heat island policies in U. S. cities. As of the start of the pandemic (March 2020), Austin, Chicago, Chula Vista (CA), Dallas, Denver, Houston, Los Angeles, Miami Beach, New York, Philadelphia, and Washington, DC had all implemented cool roof mandates.
Many cities encourage cool roofs through voluntary green building programs, income-qualified programs, and financial incentives. Anaheim, CA, Austin, Chicago, Los Angeles, Louisville, Pasadena, and Orlando offer a cool roof rebate, while Baltimore, New York City, Philadelphia, and San Antonio use programs that install cool roofs on low-income homes. Affordable housing developers in these cities should investigate how they may use these local benefits.
The adoption of cool roofs is also present at the state level. California has had a prescriptive cool roof requirement in its building code (Building Energy Efficiency Standards, Title 24, Part 6) since 2005. Alabama, Florida, Georgia, Hawaii, Nebraska, and Texas also have building codes with cool roof requirements. It is likely that as more states update their building codes, cool roof and other energy-efficient measures will be included.
How Do Cool Coatings Work?
During the day, a cool roof reflects solar radiation away from the building, and, at night, releases any heat that was absorbed by the roof. In addition to resisting urban warming, a cool roof also lowers the demand for air conditioning, decreases peak electrical demand, and increases resident comfort. When used collectively, cool roofs also improve outdoor air quality and assist with stabilizing electrical grids. Cool roof materials also complement green and solar roofs.
Cool roofs are available in a variety of product types, including field-applied coatings and factory-coated metal. Historically, flat or low-sloped roofs have been transformed into cool roofs by coating them white. However, there are now “cool color” products on the market that use darker colored pigments that are highly reflective in the near-infrared (non-visible) portion of the solar spectrum.
The ”coolness” of a roof coating is determined by two basic properties: solar reflectance (sometimes called albedo) and thermal emittance. Solar reflectance is the fraction of solar radiation that is reflected away from the roof, while thermal emittance is the efficiency by which the roof can radiate any heat that was absorbed into the building. The values of both properties range from 0 – 1. In addition to these two metrics, the “coolness” of a roof can also be represented by its solar reflective index (SRI) value, a calculated metric that combines solar reflectance and thermal emittance into one value. SRI values are usually between 0 and 100, with very cool materials exceeding 100. It is this metric that is most easily understood by builders and developers.
With numerous products available for installation on commercial and residential buildings, identifying roofing materials that meet the needs of a project can be difficult. The Cool Roof Rating Council (CRRC) has published a Rated Products Directory, which is an excellent resource for developers looking to receive LEED credits, comply with building codes, or qualify for rebates or loans.
The CRRC also is looking at rating exterior wall products. Recent research by Lawrence Berkeley National Laboratory found that cool walls can save as much energy as a cool roof, and when combined, savings are multiplied.
What About Cool Pavement?
While not as thoroughly studied as cool roofs, cool pavement research is well underway. The City of Phoenix Street Transportation Department and Office of Sustainability have announced the results of the first year of its Cool Pavement Pilot Program. The program and analysis of the cool pavement process is being conducted in partnership with Arizona State University (ASU).
Year one of the study done by scientists at ASU’s Global Institute of Sustainability and Innovation, Healthy Urban Environments, and the Urban Climate Research Center shows that reflective pavement surface temperatures are considerably lower than traditional roadway pavement.
Cool pavement coating reflects a higher portion of the sunlight that hits it, hence absorbing less heat. Because of this higher reflectivity, the coating has the potential to offset rising nighttime temperatures in the hottest regions of the country.
Findings from the first year of the study include:
- Cool pavement revealed lower surface temperatures at all times of the day versus traditional asphalt.
- Cool pavement had an average surface temperature of 10.5 to 12 degrees Fahrenheit lower than traditional asphalt at noon and during the afternoon hours. Surface temperatures at sunrise averaged 2.4 degrees Fahrenheit lower.
- Sub-surface temperatures averaged 4.8 degrees lower in areas treated with cool pavement, which indicates a longer lifespan for the surface.
- Nighttime air temperature at six feet of height was on average 0.5 degrees lower over cool pavement than on non-treated surfaces.
While the expense of the cool pavement technology may not be practical for any but the largest housing developments, cool roof technology offers immediate benefit for virtually any multifamily housing development. Builders and owners who are in a position to do so should strongly consider the use of cool roofs for their next new or renovated development.