Architectural Record BE - Building Enclosure

Cool Roofing for Cool Climates

TPO and PVC membrane reflective roofs continue to prove their effectiveness in all climate zones, debunking myths that cool roofing is only suitable in warm environments
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Northern Cities Embrace Cool Roofing Programs

In the bigger picture, cool roofs can play a significant role in reducing peak demand across all climate zones, which is why a number of northern cities—including Chicago, New York, and Toronto—have cool roofing programs, and why California’s Title 24 energy code, ASHRAE 90.1, the U.S. Green Building Council’s LEED program, and the Green Building Initiative’s Green Globes program include cool roofs in their requirements and credits.

“Most energy-related codes and green building programs such as LEED place heavy emphasis on installation of reflective roofs,” reports Mark Kim, director of architecture, MVE + Partners, Irvine, California.

For example, NYC CoolRoofs is a part of the Big Apple’s goal to reduce carbon emissions 80 percent by 2050. To help move things along, the city offers cool roof installations at no cost or low cost to select buildings in its quest to whiten 6 million square feet of New York rooftops.

Driven by a similar greenhouse gas emissions reduction goal of 80 percent by 2050, Toronto’s Eco-Roof Incentive program offers residential, industrial, commercial, and institutional buildings the opportunity to receive $2 to $5 per square meter, up to a maximum of $50,000, for installing a reflective roof.

Meanwhile, in the Windy City, the tragic 1995 heat wave led Chicago to eventually require cool roofs on all new roofs. To help support this, Chicago’s Cool Roof Grants program gives a $0.55 rebate per square-foot on reflective roofing for a commercial facility and $0.60 for an industrial building. To qualify, both building types must have an initial reflectance of greater than or equal to 0.65 for low-slope roofing and greater than or equal to 0.25 for a medium-slope roof. Reflectivity values must either be Cool Roofs Rating Council or ENERGY STAR rated.

Backing up its confidence in the effectiveness of cool roofs, a 2012 study from Yale University titled “Remotely sensing the cooling effects of city scale efforts to reduce urban heat island” used satellite images to determine that new reflective surfaces installed in Chicago between 1995 and 2008 raised the city’s overall reflectivity by 0.016.10

More recently, a Notre Dame study, “Green and cool roofs to mitigate urban heat island effects in the Chicago metropolitan area: evaluation with a regional climate model,” reported that the use of roofs with vegetation or reflective surfaces on top of Chicago’s current infrastructure could reduce the UHI effect by lowering roof temperatures between 5.4 and 7.2 degrees Fahrenheit.11 In collaboration with the City of Chicago, the researchers examined the efficacy of green or cool roofs using a regional climate model to simulate various real-world urban rooftop conditions.

In light of all these programs, Andre Desjarlais, program manager, U.S. Department of Energy’s Oak Ridge National Laboratory Building Envelope research program, states in a Building Operating Management (BOM) article titled “Do Cool Roofs Fit in Cool Climates?”, “The peak demand benefits of cool roofing are the same no matter how far north you march.”12

Similarly, Scott Kriner, technical director, Metal Construction Association and chairman of the Cool Metal Roofing Coalition, asks why utility companies would offer incentives in cold climates if there wasn’t a benefit.

Further stating the case for cool roofing in cool climates, Hashem Akbari, staff scientist and group leader for Lawrence Berkeley National Laboratory’s Heat Island Group, Berkeley, California, states in the BOM article, “the only building that won’t benefit from a cool roof energy wise is one that’s not air-conditioned.”

In addition to incentive programs and demand charges, it’s also important to understand if and how a climate’s number of heating days and cooling days fits in. Addressing a misnomer here, Taylor explains that heating days vs. cooling days is not the right metric to look at, but rather the air-conditioning costs vs. heating costs. If heating costs are larger, then it can make sense to evaluate nonreflective roof membranes.

One question often asked about cool roofs is the extent to which they retain their reflectivity.

Most reflective materials, particularly TPOs, will retain their solar reflectance quite well as they are naturally cleaned by the rain.

Furthermore, the U.S. Environmental Protection Agency’s ENERGY STAR cool roofing program requires an initial solar reflectance of greater than or equal to 0.65 and a three-year age value of greater than or equal to 0.50. Incidentally, many of today’s reflective membranes exceed the standard.

In fact, in 2013, the Cool Roof Rating Council’s rated product directory reported that more than 90 percent of the 448 cool roof products in its directory had an aged reflectance value of 0.60 or better, and only 3 percent were below 0.55.

By cleaning the roof, those values can be restored to almost 100 percent, according to the LBNL and National Research Council Canada.

But even at the point where the membrane is not looking its cleanest, the heat energy coming from the sun makes up 50 percent of the sun’s rays, and with a wavelength longer than visible light, it goes through thin layers of dust and dirt and is still reflected by the membrane.

Debunking Condensation Myths

In convincing architects and building owners about the viability and benefits of cool roofing in northern climates, it’s important to address the myth that light-colored roofing materials are more prone to moisture build up in colder climates.

Some have questioned whether cool roofs enable more condensation to build up in the winter months, as opposed to darker membranes, and being reflective, they will not heat up sufficiently in the summer months to dry out completely. As moisture continues to accumulate, they say, eventually the roofing system’s performance is compromised.

Delving into this issue, SPRI, the association representing the single-ply roofing industry, reported on a study investigating whether cool roofs were susceptible to condensation issues. Based on WUFI modeling of 10 selected roofs with both white and black membranes, the modeling showed that any moisture would completely dry out in the summer for both the light and dark-colored roofs. Incidentally, WUFI analyzes the movement/accumulation of moisture and heat through an assembly over a period of time.

In another more well-known study, Target investigated white PVC roofs, between 10 and 14 years old on 26 stores located in the northern states of Connecticut, Illinois, Massachusetts, Michigan, Minnesota, New York, Washington, and Wisconsin. After investigating test cuts on each roof, the researchers discovered that apart from some minor leaks, there was no evidence of any moisture buildup or damage.

In yet another telling study, SPRI teamed up with Oak Ridge National Laboratory studying 10 reflective roofs located in climate zone 5. In seven of the cases, no damage was discovered on the polyisocyanurate foam insulation or facer. On three of the roofs, condensation was found on the backside of the highly reflective membrane, but minimal damage had been done to the polyiso foam insulation, and there was no rust on the roof deck.

“The conclusion from the study found that although there were signs of moisture condensation in three of the 10 roofs observed, minimal effect had occurred to the roofing assembly that would affect its integrity, insulating value, or performance,” Taylor reports.

Further commenting on the issue of condensation, the DOE’s Guidelines for Selecting Cool Roofs states, “While this issue has been observed in both cool and dark roofs in cold climates, the authors are not aware of any data that clearly demonstrates a higher occurrence in cool roofs.”

Steering the condensation conversation away from cool roofs, Phil Dregger, RRC, FRCI, P.E., Technical Roof Services, Concord, California, explains that the causes of moisture buildup in roof systems are many and varied.

“Buildings with conventional non-cool roofs can develop condensation problems, and changes other than those to roof reflectance (e.g., building use, HVAC operation) can negatively tip previously maintained balances between the wetting and drying of roof systems,” he states in a Western Roofing article titled “Cool Roofs Cause Condensation: Fact or Fiction?”13

Furthermore, Dr. Jim Hoff, president, TEGNOS Research, and vice president of research, Center for Environmental Innovation in Roofing, Washington, D.C., reports that moisture condensation in roofs is a relatively rare phenomenon that only occurs in unusual circumstances, such as extremely cold external temperatures, extremely high internal temperatures and humidity, unusually low amounts of above-deck roof insulation, or unusually high levels of air movement within the roofing system.

It is the case that in humid conditions, the warm air can sometimes migrate through a roof system up to the underside of the roof membrane, causing condensation to occur during the winter months in northern climates. However, this is not related to the roof material type or color and is related to roof system design.

Granted, “self-drying” dark-colored roofs will heat up and dry out in the summer should condensation occur in the winter, whereas reflective roofs tend not to dry out in the summertime, as the membrane doesn’t get hot enough.

That said, in order to ensure that condensation is not an issue with cool roofing, Taylor recommends specifying roof insulation to meet current code requirements, as this helps to move the dew point down into the insulation layers where airflow is close to zero. He also advises building teams to install two layers of nonporous insulation, such as polyiso, as opposed to mineral wool, in a staggered fashion with non-overlapping joints.

“With proper insulation, condensation is not an issue on the bottom side of the membrane,” confirms Polhill.

While this is usually sufficient, a recommended best practice is fully adhering the roof membrane as this prevents the membrane billowing during high-wind events and pulling air up into the system.

 

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Originally published in Architectural Record

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