Green Roofs and the Urban Heat Island Effect

DensDeck Roof Board has always been your strongest choice for weather and moisture resistance. Now it’s the choice that adds even more value when it comes to meeting the stringent requirements of greener buildings and provides a more energy-efficient future.

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Increased air pollution. Volatile organic compounds mixed with nitrous oxides, heat, and sunlight produce ground-level ozone. In many cities, summer peak demand load for energy results in the burning of more fossil fuels and more air pollution.

Negative health impact. An increase in air pollution can trigger respiratory problems, like asthma and cardiac irritability, which translates into higher public and private expenditures on healthcare. Hotter cities also mean increased numbers of heat stress and other heat-related illnesses. In the United States, an average of 1,000 people die each year due to extreme heat.³

Increased stress on ecosystem health. Air pollutants and heat can damage plants and trees by affecting their ability to photosynthesize and produce seed or fruit. Some plants and animals are more sensitive to increasing temperatures and may not survive for long in the urban landscape.

Increased water consumption. This is the result of the need to support stressed vegetation and generate the energy required to meet the higher summertime energy demand load. Different sources of energy require varying amounts of water resources, which are becoming increasingly scarce.

Negative economic impacts. In addition to healthcare, energy supply, and transportation-related economic impacts, hotter cities also have negative impacts on tourism and related local economic activity because many residents leave urban areas to escape the excessive heat.

In 1998, five cities were studied under the Urban Heat Island Pilot Project (UHIPP). Data was collected using flyovers to measure surface temperatures, identify hotspots, and quantify UHI reduction strategies by analyzing meteorological, air-quality, and energy-demand data. They found the hottest spots coming from rooftops, with upper temperatures measuring 160 degrees F. The coolest areas were those covered with vegetation or water bodies; these surfaces ranged from 75 to 95 degrees F. Rooftops account for 5 to 35 percent of the total land area of cities, so there’s significant potential to help address the UHI with mitigation strategies focused on roofs.

How Do Roofs Contribute to UHI?
To understand how roofs and other surfaces contribute to or mitigate UHI, it’s helpful to understand a basic roof energy balance model (see Figure 1).

Incoming long- and short-wave solar radiation strike a roof surface and are either reflected or absorbed by the building envelope. Most conventional roof surfaces are dark and reflect very little solar radiation. These surfaces have what is referred to as low albedo or reflectance, which is a measure of the ability of surface to reflect solar radiation. Albedo is a unitless measure with possible values ranging from 0 to 1. Objects that reflect lots of light have a high albedo approaching 1, while a perfect black body would have an albedo of 0.

Solar reflective index is a measure of a constructed surface’s ability to reflect solar radiation by combining reflectance and emittance (a measure of a material’s ability to radiate absorbed energy) into one number. It is defined so that a standard black (reflectance 0.05 and emittance 0.90) is 0. A standard white (reflectance 0.08 and emittance 0.90) is 100. Cool roofs are able to reduce heat gain in buildings due to the high solar reflective index.

Green Roof Basics

Green roofs, also known as vegetated roofs, have been in existence for thousands of years, and have been used for winter insulation and summer cooling. Modern lightweight green roofs emerged from research and development work centered in Germany in the 1960s and ‘70s.

Almost all green roofs are comprised of layers that work together to achieve three primary goals: 1) ensure the structural integrity of the building, 2) maintain the waterproofing of the building, and 3) ensure the long-term health of the plants. Green roof layers are typically comprised of the following elements, although there are many variations on the market:

• High-quality waterproofing.
• Waterproof membrane protection layer.
• Root repellant system.
• Drainage layer.
• Filter cloth or fabric.
• Engineered growing media.
• Plants.

Green roofs can be loose laid (built up), in which each of the layers is installed and the plant seeds are sown in the engineered growing media. There are also modular systems that combine several of the layers, including the plants; these units are brought to the roof and laid out.

There are three types of green roofs:

1. Extensive roofs have 6 inches or less of growing medium, low maintenance, low capital costs, and low loading capacity. The plants on these roofs are typically sedums.
2. Semi-intensive roofs have 25 percent of the roof either above or below 6 inches of growing medium.
3. Intensive roofs (also known as rooftop gardens) have more than 6 inches of growing media, greater cost and maintenance, and are usually irrigated.

The effective design and installation of green roofs requires multidisciplinary expertise, and the very best green roof projects are part of a holistic green building design process.

How Can Green Roofs Reduce UHI?
Green roofs are more effective than conventional and white roofs for cooling buildings because they utilize a heat-transfer mechanism known as evaporative cooling, which is unavailable to most conventional roofs.

When water changes from a liquid to a gas, energy is needed to overcome the molecular force of attraction between the particles. This energy is known as latent heat. Because green roofs store water in the growing media and plants, latent heat loss is accomplished via transpiration from plants and evaporation of moisture from the growing medium – collectively referred to as evapo-transpiration. Vegetation utilizes incoming solar radiation for evaporating moisture from the plants and the growing media around it, as well as for transpiration during the process of photosynthesis. Conventional roof surfaces are designed to shed water, and thus are unable to accomplish much evaporative cooling.

A green roof is able to reduce UHI through the following means:

• The plants and growing media provide the basis for evapo-transpiration, reducing ambient air temperatures, and generating a net cooling effect for the surrounding buildings, almost like an “outdoor” air-conditioning system.
• Evapo-transpiration – combined with the effects of shading, reflection, thermal mass transfer, and insulation – significantly reduces heat gain within buildings, reducing the need for air-conditioning. As a side effect, less overheated air is discharged into the surrounding environment.
• Lower ambient temperatures above a green roof can be used to supply intake air to roof-mounted HVAC systems, thereby increasing efficiency. Air-conditioning systems begin to lose efficiency at about 95 degrees F. Drawing cooler air into the system can help to reduce energy costs. Green roofs tend to maintain an ambient temperature of 90 degrees F., creating optimal conditions for air-conditioning.⁴

Developing a UHI Strategy
Addressing UHI requires investment in a variety of measures, including white or reflective roofs, green roofs, urban forests, and reflective paving. Several studies have looked at the effect of wide-scale green roof and urban forest implementation on UHIs. In 2006, a report prepared for the New York State Energy Research and Development Authority by the Columbia Center for Climate Systems Research explored opportunities to reduce New York City’s UHI. The study utilized a regional climate model in combination with observed meteorological satellite and GIS data to determine the impact of urban forestry, green roof, and light-colored surfaces on UHI. During the summer months, the daily minimum surface and near-surface air temperature in the city was 7 degrees F. warmer than that in the surrounding rural and suburban areas.⁵ Nine mitigation scenarios were evaluated with six case study areas.

The results indicated that vegetation rather than surface albedo alone or other features of the urban physical geography, such as road density, was crucial in determining the urban heat potential. Research by David Sailor at Portland State University also concluded that low latent heat flux due to lack of vegetation in urban areas is the most significant contributing factor to the UHI phenomenon.⁶ The report concluded that a combined strategy of implementing green roofs and maximizing the amount of vegetation in New York by planting trees along streets and in open areas offers more potential cooling than any one strategy. New York has recently approved a tax abatement of up to $100,000 per project to support green roof installation in New York City and is investing significantly in urban reforestation. Toronto has recently made green roofs a mandatory requirement for almost all new buildings to help reduce the UHI effect and address climate change.

Maximizing Green Roofs

Green roofs are an important component in an overall strategy to offset the negative impacts of UHIs and enhance the value of green building practices for occupants and surrounding citizens. They also have the ability to improve air quality, reduce combined sewer overflows, provide usable spaces for building occupants, support biodiversity, reduce noise, and extend waterproofing longevity. When green roofs are combined with other measures, such as urban forests and reflective roofs and paving, scientific research has clearly demonstrated significant economic, social (health related), and environmental benefits associated with investing in UHI reduction.

Here are some ways to maximize the benefits of green roofs to reduce UHIs and save building owners additional funds:

• Increase the area of coverage of the green roof and use extensive and semi-intensive roofs wherever possible. Increase the depth of growing media on load-bearing columns and walls for additional plant diversity and greater leaf area.
• Increase the field capacity of the growing media (i.e. the amount of water available for plant growth).
• Store rainwater in the drainage layer and moisture retention mat for supplemental irrigation.
• Ensure full plant coverage by clearly specifying performance criteria in a minimum 5-year maintenance contract.
• Provide supplemental irrigation during periods of drought for extensive green roofs. Intensive green roofs almost always require permanent irrigation. Utilize non-potable water sources wherever possible.
• Strive to integrate mechanical-system design with the green roof, taking advantage of the summer reductions in heat gain to “right size” cooling systems.
• Take advantage of the cooler ambient air temperatures to direct cooler air into the building (see California Academy of Sciences) or directly into the air intakes.
• On semi-intensive and intensive green roofs, try to position trees so that they are able to provide shade to roof-mounted air-conditioning systems – this can improve their efficiency by as much as 10 percent.
• Combine green roofs with reflective (cool) roofs, green walls, and trees planted on the south side of the building to further reduce the UHI effect.

Steven W. Peck is the founder and president of Green Roofs for Healthy Cities, a non-profit, membership-based industry association whose mission is to increase the awareness of the economic, social and environmental benefits of green roofs, green walls, and other forms of living architecture. Jordan Richie is a research associate with Green Roofs for Healthy Cities who is working directly on the Energy Calculator project.

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For more information and resources, including a lifecycle cost benefit calculator that compares up to three roofing systems and a searchable online database of more than 200 research projects, visit www.greenroofs.org.