By Helene Hardy Pierce, FRCI
Published in the August 2005 issue of Today’s Facility Manager
As an emerging technology, cool roofing has been defined indifferent ways by various groups in the past. Today, however, roofsthat provide a high level of both reflectivity and emissivity areconsidered to be the class of roofs that make up cool roofing.
Reflectivity is the amount of light that the roof reflects ratherthan absorbs. Emissivity indicates how much heat is emitted back out tothe environment from the roof surface when compared to the amount thatwould be emitted back from a black body. A reflectivity of either 65%or 70%, combined with an emissivity requirement of 0.9 (meaning 90% ofheat is emitted back from the surface), is the generally accepted levelof performance from a cool roof.
The reflectivity and emissivity factors are important, because thesework in reducing the temperature loading of the roof and, in turn, thebuilding as a whole. This lessens the heat gain that the building mustovercome during warmer months through the use of air conditioning orventilation.
Simply stated, cool roofs typically require that less energy be usedto cool the interior conditioned space they house than “non-cool”roofs—when other factors, like the amount and type of insulation,remain the same. And in southern climates or those with longer periodsof warmer weather, the reduction in energy usage is greater for abuilding with a cool roof that has maintained its reflectivity.
Facility managers can discover the savings that a cool roof canprovide their facilities by using an energy calculator. Oak RidgeNational Laboratory (ORNL) in Oak Ridge, TN offers a calculator on itsWeb site. Using this calculator, the facility manager can estimate thepotential annual savings based on factors specific to his or herbuilding.
It is important to understand the premise for the savings is basedon a change from a truly black roof (reflectivity of less than 5%) to aroof with a reflectivity greater than 5%. Because the presence of atruly black roof is often not the case, instructions are provided toestimate the energy savings for a change from a roof with areflectivity greater than a black membrane to one with a higherreflectivity. These instructions should be followed if a facilitymanager is starting with a membrane that is not black and wants to knowthe annual savings that would be derived with a highly reflectivemembrane.
Another reason cool roofing is being touted as important is thecontribution that roofing can provide to the Urban Heat Island Effect.Research has concluded that roof surface temperatures can influenceoutside air temperatures, and concentrations of buildings in an urbanarea with non-reflective roof surfaces can cause the outside airtemperature to rise approximately 1°F to 2°F on a hot summer day.1 Thisrise in air temperature can increase smog, harm air quality, and add toenergy usage in urban areas.
Typical Reflectivity And Performance
If a cool roof is one with a reflectivity greater than 65% or 70%,how can facility managers determine the reflectivity of their existingroofing materials? Typically, white surfaces such as white single plymembranes or white coatings have an initial reflectivity value ofgreater than 70% and often as high as 80%. A white granule surfacedmembrane such as a modified bitumen membrane or white cap sheet has aninitial reflectivity of 25% to 30%.
The initial reflectivity of gravel surfaced and ballasted membranes is clearly dependent on the type of gravel or ballast, but typical reflectivity values are in the 15% to 30% range. A black surfaced single ply membrane has a typical reflectivity in the 6% to 10% range.
Therehas been a great deal of discussion within the roofing industry aboutthe reflectivity of a roofing membrane after the system has been inplace for a period of time. What happens to the reflectivity after theroof accumulates dust, dirt, or ponding water? If the reflectivitychanges, what happens to the estimated savings from a highly reflectivesurface?
Roofs do accumulate dirt and dust. Through measurement, it has beendetermined that this accumulation reduces the reflectivity of a whiteor reflective membrane. On the other hand, dirt and dust increases thereflectivity of a dark or less reflective membrane, because the coloris “lightened” by the debris.
Data presented at the Cool Roofing Symposium in Atlanta, GA in May of this year2 shows that the reflectivity of a black, EPDM membrane with areas of ponding water is almost three times higher (29.5% versus 10.3%) than the same membrane without the areas of ponding water and its associated dirt. Likewise, a white single ply membrane that has not been kept clean can go from an initial reflectivity of 75% to a reflectivity of 50% or less due to the accumulation of dirt on its surface.
Some Important Considerations
For facility managers thinking about cool roofing for theirbuildings, there many things to consider. Listed below are severalfactors that can affect whether or not cool roofing will provide abenefit for the facility in question.
Consideration: Does the existing roof pond water?
Why It’s Important: As mentioned above, ponding watertypically accumulates dirt and debris and reduces the reflectivity of ahighly reflective membrane.
If the standing water is not removed, the actual reflectivity of theroof can end up being between 30% to 50%, regardless of the type ofmembrane.
Consideration: What is the geographic location of the facility?
Why It’s Important: The benefits of cool roofing obviouslyincrease in southern climates, where there is a longer cooling periodversus a shorter heating period for the facility’s conditioned space.In fact, a non-reflective roof may be the best roof color in an areawith heavy snow loadings, low cooling degree days, and low solarradiation.
Energy savings in the summer can be offset by heating costs in thewinter with a reflective roof. Consider that a reflective roof does notbenefit from sunshine heat gain in the colder months of the year. Useof an energy calculator takes this geographic concern intoconsideration in its estimation of potential energy savings.
Curbing energy costs may be the top reason many facility managers are interested in green roofing, but there are other environmentally friendly benefits to consider as well. When taking a life cycle approach, facility managers can examine all stages of a roofing system’s existence from manufacture to disposal. Durability and maintenance are also factors here since, reducing the physical and financial resources needed for upkeep will also impact the facility.
In looking at roofing materials, metal is one choice that can satisfy sustainability criteria as well as day to day performance and economic demands. The most commonly used metals in roofing are steel and aluminum; however, copper, zinc, and titanium are also found in some applications.
The inherent durability of metal offers facility managers a roofing option that can withstand extremes of weather. Various coatings available also contribute to the durability of the surface. Steel, for instance, typically requires a coating to protect it from the elements to prevent the formation of rust. This galvanized roofing will feature a zinc coating or a combination of zinc and aluminum.
There are also different types of painting systems for metal roofs. If facility managers want a cool performance roof, they can specify what is referred to as superpigments. These pigments provide not only a high degree of reflectivity but also a high degree of emissivity.
Even without the superpigment, a metal roof will reflect sunlight and reduce the amount of heat load on a building envelope. If there is a regular roof with the galvanized coating or regular paint, the roof is going to have an inherent reflectivity aspect.
“Metal roofing in and of itself is reflective, almost regardless of what kind of metal it is,” says Greg Crawford, executive director of the Cool Metal Roofing Coalition. “It has a reflective quality to it—allowing a good portion of the solar energy to be reflected away.”
Maintenance needs can be reduced with a metal roof. Whether the roof is simply galvanized or painted with superpigments, the smooth, hard surface of a metal roof enables it to shed dirt, fungus, and other organic threats to its condition.
Its surface qualities also enable rain and wind to play a part in removing those threats and other debris from the roof. Crawford notes that maintenance needs may vary depending on the facility location. For example, a roof in an urban environment with many contaminants in the air will need more attention.
The fact that metal can be easily recycled is another sustainable aspect of the material. According to The Metal Initiative, a coalition of manufacturers, individuals, and associations in the metal industry, the steel, aluminum, copper, and zinc used in metal roof panels are 100% recyclable. Of the metals used for roofing and wall panels, steel is the most recycled; the annual rate is currently more than 70%, and the recovery rate is near 90%.
In many cases, a metal roof can be installed over an existing roof, thus preventing the old roof from having to be torn off and perhaps ending up in a landfill. Crawford advises facility managers to check with local building codes to make sure it’s permitted. “Typically it will be,” he says. “Metal roofing is lightweight, and if a municipality allows a first or second roofing to take place, metal roofing will qualify.”
In addition to code compliance, facility managers should also ascertain that the existing roof is in good condition to allow proper roofing. A contractor should be able to help in determining that.
When shopping for a new roof, facility managers have a variety of materials to consider. The sustainable aspects of metal can fit the bill for many buildings.
Consideration: Is there a roof cleaning program in place?
Why It’s Important: Reflective roofs need to be kept clean, or else its reflectivity properties will be greatly reduced.
In many geographic areas, as well as many industrial locales, roofsaccumulate dirt and dust. If roofs are not washed on a routine basis,the benefit of a reflective roof can be significantly reduced. Thefacility manager should be sure to include the cost of cleaning incalculating any benefit of a cool roof. If there is no cleaning programin place, then the facility manager should be sure to use a lower value(30% to 50% less) for reflectivity when calculating the benefits of areflective roof.
Consideration: How much insulation does the roofing system have?
Why It’s Important: If the existing roof is inadequatelyinsulated, adding insulation may have the same effect on energy costsas a cool roof would.
In areas that may require cleaning of the membrane, addinginsulation may be more cost effective than using a cool roof. Theenergy calculator from ORNL includes information on how much insulationcould be added to achieve the same energy savings.
Consideration: Is the building located in California or a “cool” community?
Why It’s Important: There are some areas where local code requirements may, or will, require cool roofing.
Effective, October 1, 2005, the state of California’s Energy CodeTitle 24 will require cool roofing for low slope installations on newcommercial buildings as well as on most major commercial re-roofingprojects for conditioned buildings (e.g., warehouses without airconditioning or heat would not be included). These installations musthave a reflectivity factor of at least 70% and an emissivity factor ofat least 75%. Otherwise, a whole building energy usage study must beperformed.
Facility managers will be required to provide reflectance andemittance data as rated by the Cool Roof Rating Council (CRRC).Products that do not have a CRRC rating will be assumed to have areflectance default value of 0.10 regardless of actual performance.(See Title 24 for more details.)
Compliance with local codes should be considered mandatory by thefacility manager. Understanding the requirements is crucial for theroofing contractor to obtain a building permit and can prevent seriouscomplications to this process.
Consideration: Who are the facility’s neighbors? Where are they located relative to the building’s roof?
Why It’s Important: Replacing a non-reflective roof with areflective roof may seem like a great idea for a building, but facilitymanagers need to consider the effect this may have on neighboringbuildings. There have been many instances where a light coloredreflective roof was installed that caused serious glare problems forother buildings and their occupants.3
These cool roofs have to reflect sunshine somewhere, and if aneighboring building or other component structures are in the path ofthis reflected light, then unintended consequences (such as damage toadjacent materials or occupant discomfort) can arise.
Weighing The Options
Cool roofing has perhaps been oversimplified, with many peoplethinking it’s a must with clearly applicable benefits. This is notnecessarily true. Understanding the roofing needs of the facility andtaking into consideration all of the factors that go into the selectionof a roofing system is still how facility managers should proceed. Thebody of research on this topic is expanding as the industry attempts toeducate itself on different considerations of cool roofing such asthose mentioned here.
Certainly, a cool roof can provide benefits for many buildings, butone size doesn’t fit all. The prudent facility manager understands notjust the benefits that a cool roof may provide, but the otherrequirements necessary to realize these benefits.
- Oak Ridge National Laboratory (www.ornl.gov/sci/roofs+walls/facts/CoolCalcEnergy.htm)
- Cool Roof Rating Council (www.coolroofs.org)
Pierce is director of contractor services at GAF Materials Corp. in Wayne, NJ. She can be reached at [email protected].
- Akbari, H., A.H. Rosenfeld and H. Taha. 1990. “Summer HeatIslands, Urban Trees, and White Surfaces.” ASHRAE Transactions 96, No.1.
- Cool Roofing…Cutting Through the Glare, Proceedings, Roof Consultants Institute Foundation, May 12-13, 2005.
- Murphy, C., 2005. “Cool Roof Design and Case History.” Cool Roofing….Cutting Through the Glare Proceedings, RCIF.