By Jessica Clark, LEED AP BD+C and Sarah Schneider
From the August 2013 issue of Today’s Facility Manager
Cool roofs are a simple and effective approach to lowering the cooling energy loads of buildings and for increasing occupant comfort. By reflecting solar radiation, cool roofs reduce the amount of sunlight that is absorbed as heat in a building. As a result, the need to control indoor air temperatures with air conditioning systems (A/C) decreases, which in turn lowers energy bills.
In addition to energy cost savings, cool roofs contribute to grid stability by lowering the need for A/C use during the hottest times of the year. Cool roofs also reduce outside ambient air temperatures, which helps mitigate the urban heat island effect, a phenomenon where cities can become several degrees hotter than surrounding less developed areas.
Cool Roofs: The Basics
A “cool roof” is any roof surface comprised of highly reflective materials that keep the sun from heating up a building. Today, cool roofing options are available in almost any roofing material, and innovations in product formulation have enabled manufacturers to create products in a variety of colors as well.
The performance of these materials is rated based on two radiative properties: solar reflectance (SR) and thermal emittance (TE) (illustrated in the figure above). The reflectivity of a roof’s surface is important because on hot, sunny days, standard dark roofs can be up to 90°F hotter than the ambient air temperature.1 Both SR and TE values range from 0 to 1, with higher performing products having higher ratings.
Also considered when rating cool roofing materials is Solar Reflectance Index (SRI). The SRI is a calculation that combines both radiative properties into one value between 0 and 100; higher performing products sometimes exceed 100.
Building codes and voluntary programs may require an “aged” or “weathered” SR, TE, and SRI value for products, so manufacturers often have their products weathered at test farms in various climate zones. These tests are completed over a three year period to determine how a product will perform over time after being installed. Aged ratings give a better estimate of how reflective a product is after it has been subject to the natural elements, microbiological growth, and the sun’s ultraviolet rays. Interestingly, a study on the performance of various cool roofing materials found that washing the roof to the manufacturer’s specifications may restore some products to their original reflective performance.2
Cool Roofing Considerations
As with any building envelope strategy, there are several factors to take into account when considering the installation of a cool roof. While cool roofs are ideal for many buildings, their success depends on several factors, including the attributes of a building and the scope of the project. In general, cool roofs have the most benefits for facilities in very hot climates that rely significantly on the use of A/C to control indoor ambient air temperatures and on facilities with little insulation, such as some warehouses.
Project scope. Cool roofing options vary greatly based on whether the project is a new construction, complete reroof, or renovation of an existing roof. Examples of renovation options include coating an existing flat roof with a cool or light colored coating or installing a metal roofing system over an existing shingle roof.
Budget. Though most cool roofing options and the cost of installation are similar to standard roofing products, some types of cool roofing products have a price premium depending on the material type used. However, local utility rebates and incentives can help offset those higher costs.
Moreover, the cooling energy savings gained during the hot, summer months also help offset the higher capital costs. The U.S. Department of Energy has a thorough guide for installing cool roofs that includes a price comparison chart.
Facility specifics. Roof slope, location, and size greatly impact the reflective properties of a cool roof. Because low slope and flat roofs cannot be seen from street level, highly reflective, smooth, and bright white materials can be used without visually impacting the surroundings. One thing to keep in mind in urban areas, however, is that glare coming off these bright surfaces may be an issue for the occupants on higher floors in adjacent buildings.
In addition, other building factors are important to consider, such as the number of floors and the amount and type of insulation. Cool roofs have the most impact on the upper floors of a building, with cooling energy savings between 15% and 25% for the top floor and savings of 5% to 10% for the next floor down.3 Unfortunately, there is usually less of an impact on overall cooling loads for high-rise buildings. Cool roofs also have less of an impact on buildings with substantial amounts of existing insulation, which mitigates temperature fluctuations.
Duke Medicine Going Green With Roofing
A world-class academic and health care system, Duke Medicine in Durham, NC integrates the Duke University Health System, the Duke University School of Medicine, and the Duke University School of Nursing. Duke Medicine endeavors to “green up” its operations, in everything from large-scale construction projects (designed according to LEED standards) to housekeeping services. Green (vegetative) roofs support this commitment to sustainability.
Tim Pennigar, project manager for structural systems at Duke Medicine for 25 years, provides in-house consulting and project management for building enclosure construction and exterior rehabilitation. His experience spans ongoing expansion on the campus as well as restorations of complex and historical building facades and management of roofing assets. In this interview with Today’s Facility Manager (TFM), Pennigar discussed green roof initiatives.
TFM: How many green roofs has Duke Medicine installed?
Pennigar: We have a 6,000 square foot green roof over the entrance and in a courtyard of the Duke University Hospital. That was our first site, installed in July 2008. A 6,000 square foot installation on the Duke Cancer Center was completed in October 2011.
TFM: Why did Duke decide on green roofs? What criteria did you consider for selection?
Pennigar: Duke seeks to be a leader in stewardship of the environment. Green roofs are a natural complement to campus initiatives—particularly in the area of stormwater management and water quality. Our two roofs retain a total of more than 250,000 gallons of stormwater annually.
Operationally, green roofs are very much in line with our long-standing principles of roofing design. We protect waterproof membranes on roofs to extend their service life. As a protected membrane assembly, a well constructed green roof provides greatly enhanced service life.
Early on, we created a roof test site with multiple plots to evaluate a variety of options, including build in place with plug plants, modular trays, and pre-vegetated mats. Principally, we wanted a system that offered minimal live load, rapid vegetation coverage, ease of installation and maintenance, and an established track record.
TFM: What system did you choose?
Pennigar: We chose an extensive system with pre-vegetated mats based on a design engineered in Germany. It has been proven and refined over more than 40 years. The system is from Xero Flor America. The option we selected is 2.5″ in depth and fully saturated weighs only 10-11 pounds per square foot. The mats, pre-vegetated with Sedum, arrived in sod-like rolls. Installation yielded essentially a mature green roof with complete plant coverage. Immediate coverage reduces one of the biggest risks (that immature plants will not grow and fill in).
The roof over the Duke Hospital entry developed a leak around a defective roof drain, a condition we isolated with electronic leak detection. Being lightweight and designed with a permanent carrier layer, the mats were easy to roll back for repairs. This contrasts with other options we studied. The saturated weight of modular trays can approach 30 pounds per square foot. That makes them tough to lift. And, with build in place or mat systems with bottom layers that decompose, we would have had to dig out and discard the section over the leak.
TFM: What are the maintenance requirements?
Pennigar: With vegetative roofs, the goal is to keep maintenance to a minimum. You do this by paying attention to small routine chores before they become large, difficult tasks. Both our roofs get an application of an organic, granular fertilizer once a year in the spring. We walk the roofs every spring with four college interns to remove invasive weeds. In 2013, we spent about 30 minutes on the Duke University Hospital roof and an hour or so on the Cancer Center roof.
I should caution that it’s difficult to generalize. Every green roof develops its own ecosystem. It depends on what type of system is in place and how long it has been established. The deeper the system is, the higher the maintenance.
Supplemental irrigation is important, particularly during the initial growing season. Thereafter, irrigation should be available to protect the plants during periods of summer drought and extreme heat. I think the hospital green roof had eight irrigation events last summer. Be careful not to overdo it. Sedum are drought tolerant. Overwatering can threaten plants even more than drought.
TFM: Are there plans for additional green roofs?
Pennigar: We have six new installations, approximately 12,000 square feet in total, in final stages of construction on our new Duke Medical Pavilion and a dietary storage facility.
TFM: What lessons learned would you share?
Pennigar: A roof, of course, must function as a reliable, waterproof barrier. Particularly on retrofit projects, conduct a thorough engineering study to identify existing and potential leakage pathways before installing any overburden materials.
Become familiar with the roles of various team members such as architect, roofer, horticulturalist, irrigation contractor, and code official. Green roof success largely reflects how well these perspectives are balanced. And, two blind spots that pop up frequently are coordination of irrigation needs and building or insurance code compliance.
Monoculture planting of rows of single species may look good in a landscape design, but it is not a good idea. A green roof that blends different plants with 12 to 15 species promotes biodiversity and makes for a dynamic plant community that better adapts to the environment of each rooftop. That requires less maintenance.
Also, pay attention to where the plants are grown. Regional varieties locally grown and adapted to the particular area climate where a green roof is going up helps ensure success.
Finally, while the green roof industry is long established in Europe, it is still fairly new in North America. Do your homework. Get good advice from knowledgeable, experienced experts.
Climate. Cool roofs have the greatest impact on overall energy loads in regions that rely heavily on A/C systems to regulate indoor temperatures. In climates where heating is needed in winter, cool roofs may result in a heating penalty (requiring more heating than a building with a traditional roof material would). This penalty is small in most climates because there is less intense sunlight in the winter and the angle of the sun is also lower, meaning less exposure to direct solar rays. Furthermore, the presence of snow on roofs also reflects solar radiation, thus making the color of the roof irrelevant. Moreover, the reduced cooling-energy usage in summer months often offsets the slight increase in energy usage in colder months.4 More research is currently being conducted for cool climates. An online calculator is available for those interested in estimating the energy savings of a cool roof in their climate.
Other roofing material factors. Facility managers (fms) are also encouraged to consider standard roofing factors when choosing products, such as compatibility with existing roof systems, ease of maintenance, and warranty. If the project includes additional green building goals, fms may also wish to consider the raw material extraction and fabrication methods used by manufacturers as well as the product’s ecological footprint and recyclability.
Cool roofs are an optimal approach to reducing the cooling-energy load of buildings, particularly during the hot summer months when A/C demand is high. The benefits of reduced energy usage from cool roofing materials includes lower energy bills, energy grid stability, and cooler cities overall. Though the use of cool roofing products is an ideal way to lower a building’s energy usage, fms are encouraged to take into account all the factors that affect the performance of a cool roof in addition to the intended outcomes of the project overall.
Clark is the marketing liaison for the Cool Roof Rating Council, a non-profit organization that maintains a third-party rating program for measuring and labeling the radiative properties of roofing products. She received a B.A. in Architecture from the University of California, Berkeley. Schneider is the technical liaison for the Council, and she holds a Master of Public Policy degree from Cal Poly San Luis Obispo and a B.S. degree in Environmental Science from Humboldt State University.
1Garrison, Noah, Cara Horowitz, and Chris Ann Lunghino. Looking Up: How Green Roofs and Cool Roofs Can Reduce Energy Use, Address Climate Change, and Protect Water Resources in Southern California (New York: Natural Resources Defense Council, 2012).
2Snyder and Roodvoets. (n.d.). Economic Feasibility of Cleaning Roofs to Maintain their “Solar Reflectance” Ratings.
3Garrison, N. Looking Up.
4Global Cool Cities Alliance and R20 Regions of Climate Action (2012). A Practical Guide to Cool Roofs and Cool Pavements.