Virginia Tech Study Measures the Impact of Membranes on the Surrounding Environment

Equipment tripods are set up to hold air temperature and EMT temperature sensors.

Equipment tripods are set up to hold air temperature and EMT temperature sensors.

For much of the past decade, the debate over when and where to install reflective roofing has been guided by two basic assumptions: first, since white roofs reflect heat and reduce air conditioning costs, they should be used in hot climates. Second, since black membranes absorb heat, they should be used in cool-to-colder climates to reduce heating costs. This reasoning has been broadly accepted and even adopted in one of the most influential industry standards, ASHRAE 90.1, which requires reflective roofing on commercial projects in the warm-weather portions of the United States, Climate Zones 1–3.

But as reflective membranes have become more widely used, there has been a growing awareness that the choice of roof color is not simply a matter of black or white. Questions continue to be debated not only about the performance and durability of the different types of membranes, but on the impact of other key components of the roof system, including insulation and proper ventilation. The issue of possible condensation in cooler or even cold climates is garnering more attention. Given these emerging concerns, the roofing community is beginning to ask for more detailed, science-based information about the impact of reflective roofing.

One recent area of inquiry is centering on the impact of “the thermal effects of roof color on the neighboring built environment.” In other words, when heat is reflected off of a roofing surface, how does it affect the equipment and any other structures on that roof, and how might the reflected heat be impacting the walls and windows of neighboring buildings? Put another way, where does the reflected heat go?

THE STUDY

To help answer those questions, the Center for High Performance Environments at Virginia Tech, supported by the RCI Foundation and with building materials donated by Carlisle Construction Materials, designed and implemented a study to compare temperatures on the surface and in the air above black EPDM and white TPO membranes. In addition, the study compared temperatures on opaque and glazed wall surfaces adjacent to the black EPDM and white TPO, and at electrical metallic tubing (EMT) above them.

Specifically, the Virginia Tech study was designed to answer the following questions:

  • What is the effect of roof membrane reflectivity on air temperatures at various heights above the roof surface?
  • What is the effect of roof membrane reflectivity on temperatures of EMT at various heights above the roof surface?
  • What is the effect of roof membrane reflectivity on temperatures of opaque wall surfaces adjacent and perpendicular to them?
  • What is the effect of roof membrane reflectivity on temperatures of glazed wall surfaces adjacent and perpendicular to the roof surface?

To initiate the study, the Virginia Tech team needed to find an existing roof structure with the appropriate neighboring surfaces. They found a perfect location for the research right in their own backyard. The roof of the Virginia-Maryland College of Veterinary Medicine at Virginia Tech was selected as the site of the experiment because it had both opaque and glazed wall areas adjacent to a low-slope roof. In addition, it featured safe roof access.

In order to carry out the study, 1.5 mm of reinforced white TPO and 1.5 mm of non-reinforced black EPDM from the same manufacturer were positioned on the roof site. A 12-by-6-meter overlay of each membrane was installed adjacent to the opaque wall and a 6-by-6-meter overlay of each was installed next to the glazed wall. At each “location of interest”—on the EPDM, on the TPO, and next to the opaque and glazed walls—the researchers installed temperature sensors. These sensors were placed at four heights (8, 14, 23, and 86 centimeters), and additional sensors were embedded on the roof surface itself in the TPO and EPDM. Using these sensors, temperatures were recorded on bright, sunny days with little or no wind. The researchers controlled for as many variables as possible, taking temperature readings from the sensors on and above the EPDM and TPO on the same days, at the same time, and under the same atmospheric conditions.

The roof of the Virginia-Maryland College of Veterinary Medicine at Virginia Tech is the site of the experiment because it has opaque and glazed wall areas adjacent to a low-slope roof.

The roof of the Virginia-Maryland College of Veterinary Medicine at Virginia Tech is the site of the experiment because it has opaque and glazed wall areas adjacent to a low-slope roof.

THE RESULTS

The output from the sensors showed that at the surface of the roof, the black membrane was significantly hotter than the white membrane, and remained hotter at the measuring points of 8 cm and 14 cm (just over 3 inches and 5.5 inches, respectively). However, the air temperature differences at the sensors 23 centimeters (about 9 inches) and 86 centimeters (just under three feet) above the surface of the roof were not statistically significant. In other words, at the site the air temperature just above the white roof was cooler, but beginning at about 9 inches above the roof surface, there was no difference in the temperature above the white and black membranes.

On the precast concrete panel adjacent to the TPO and EPDM, temperatures were warmer next to the TPO than adjacent to the EPDM, leading the study authors to hypothesize that the TPO reflected more heat energy onto the wall than did the EPDM. Exterior glazing surface temperatures were found to be approximately 2 degrees Celsius hotter adjacent to the TPO overlay as compared to the EPDM overlay.

Elizabeth Grant led the team that designed and implemented the study. She says her findings show that you need to take the entire environment into account when designing a roof system. “You need to think about what’s happening on top of the roof,” she says. “Is it adjacent to a wall? Is it adjacent to windows? Is it going to reflect heat into those spaces?”

Samir Ibrahim, director of design services at Carlisle SynTec, believes the study results will help frame additional research. “These findings are an important reminder that the full impact of reflective roofing on a building and on surrounding buildings is not fully understood,” he says. “Additional research and joint studies, covering different climatic conditions, are certainly warranted to broaden the knowledge and understanding of the true impact on the built-environment.”

Energy-efficient Cool-roof Legislation: Creating Jobs and Reducing Energy Costs

Building on two roofing trends—higher thermal performance and cooler roofs in hotter climates—that have policymakers and architects seeing eye to eye, energy-efficient cool-roof legislation offers a significant opportunity to increase building energy efficiency and create jobs. Known in the last Congress in the Senate as S. 1575, the Energy-Efficient Cool Roof Jobs Act, and in the House of Representatives as H.R. 2962, the Roofing Efficiency Jobs Act, the legislation is scheduled to be reintroduced this spring.

The intent of the legislation is to encourage improvement in the thermal performance of existing roofs and, where appropriate in the designer’s judgment, encourage the use of a white or reflective roof surface in hotter climates. This is a clear win-win for the environment and building owners in terms of reduced energy costs and reduced pollution associated with energy consumption.

energy efficiency

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SIGNIFICANT SAVINGS lie within the commercial roofing sector, where more than 50 billion square feet of flat roofs are currently available for retrofit, 4 billion of which are typically retrofitted each year. The legislation would provide a 20-year depreciation period (instead of the current 39 years) for commercial roofs that meet minimum R-values that are significantly higher (requiring more insulation) than those required under state and local building codes and that have a white or other highly reflective surface. This change would correct an inequity in the current depreciation system (the average life span of a low-slope roof is only 17 years). By providing this incentive, the federal government would allow building owners and architects to decide whether the combination of thermal insulation and reflective roofs are appropriate for a given climate.

The required R-values under the proposed legislation are identical to the prescriptive requirements found under ASHRAE 189.1-2011, “Standard for the Design of High-Performance, Green Buildings Except Low-Rise Residential Buildings”. This legislation would be limited to retrofits of existing low-slope roofs and would not be available to new buildings. The cool roof requirement would only apply to buildings in ASHRAE Climate Zones 1 through 5, which covers approximately the area of the country from Chicago and Boston south. Roofs may qualify for the depreciation in zones 6, 7 and 8 but would not need a cool surface. View a map of the ASHRAE Climate Zones.

According to the U.S. Department of Energy’s Annual Energy Review, 2011, buildings account for 19 percent of the nation’s total energy usage and 34 percent of its electricity usage. Policies directed at commercial buildings are important to improving the economy, reducing pollution and strengthening energy efficiency. Although the country has over time maintained a steady pace in improving energy efficiency, a huge potential still exists, especially for commercial buildings. A wide range of credible estimates are available that point to this potential for cost-effective energy-efficiency improvements (see the graph).

THIS PROPOSED legislation complements the approaches taken in more comprehensive energy-efficiency proposals by focusing on the roof, which is the only building-envelope component that is regularly replaced but rarely upgraded to address energy and other environmental impacts.

Most buildings were constructed before building energy codes were first developed in the mid-1970s, or buildings were constructed under relatively weak codes, so these older, under-insulated roofs offer an important opportunity for increased energy savings. During the next 17 to 20 years, most of the weatherproof membranes on all commercial roofs will be replaced or recovered, which is the most cost-effective time to add needed insulation.

By accelerating demand for energy-efficient commercial roofs, the proposed legislation would:

    ▪▪ Create nearly 40,000 new jobs among roofing contractors and manufacturers.
    ▪▪ Add $1 billion in taxable annual revenue to the construction sector.
    ▪▪ Save $86 million in energy costs in the first year.
    ▪▪ Eliminate and offset carbon emissions by 1.2 million metric tons (equal to emissions of 229,000 cars).

THE LEGISLATION has the support of the Polyisocyanurate Insulation Manufacturers Association; National Roofing Contractors Association; Alliance to Save Energy; American Council for an Energy-Efficient Economy; Associated Buildings & Contractors Inc.; Building Owners and Managers Association International; United Union of Roofers, Waterproofers and Allied Workers; and several more construction industry associations.

When Sens. Cardin and Crapo reintroduce the Energy-Efficient Cool Roof Jobs Act, they hope it will influence the future debate about tax and energy policy. Although consideration of tax reform has stalled for the moment, when Congress returns to this issue it will be a golden opportunity to consider ideas for reforming cost-recovery periods and removing the disincentives that overly long depreciation schedules currently place on building energy-efficiency improvements.

Green-building Innovation Is Important, But So Is Refinement

In March 2006, I swore allegiance to the wildly popular green-building movement. I even put the kibosh on my favorite joke about recycling in the landfill—you know, so not to deprive future generations of fossil fuels and diamonds. Nice.

I’ve worked in facility management at Duke University Health System for 26 years. In this profession, being overly pragmatic is an occupational hazard. So, why did an “old-school” guy (no pun intended) show up at a green love fest alongside folk with funny-colored hair and way too many bumper stickers? Quite simply, I came to the party to plea for intellectual honesty.

Unfortunately, early on, the sustainability movement offered myriad earth-friendly materials often with little thought to their durability or life cycle. Similarly, early building rating programs focused largely on the merits of individual products without factoring their proper integration into functional systems or assemblies. Consider, for example, the many thousands of squares of reflective “cool roofing” membranes applied over non-durable assemblies. A LEED-applicable roofing membrane that fails prematurely because of inferior quality or misapplication does not look very sustainable buried in a landfill.

It’s no longer 2006, and the greenie you’re partying with may be a blue-haired, retired architect. It’s encouraging so many in the building industry, and particularly the roofing industry, have embraced the concept of durability as the essence of green and sustainable building design. Moving beyond mere branding “strategery,” sustainability can be good for the bottom line. On the Duke campus, a 2007 roof replacement used forward-thinking design to divert 718 tons of solid waste. Salvaged materials from this effort included 296,000 board feet of XPS insulation, which was repurposed in new roofing construction on three Duke buildings. It’s our story. And it’s simply good business.

It has been said “architecture is storytelling.” The story of our 2007 roof replacement project settled forever how Duke University Health System will conduct itself in regard to sustainable roofing design and environmental stewardship. We distilled our story into the following “Guiding Principles of Sustainable Roofing”:

  • 1. Favor insulations or insulating assemblies that are highly resistant to water and physical damage.
  • 2. Favor roof assemblies that position the roof membrane directly over a permanent or semi-permanent substrate.
  • 3. Favor roof designs that prohibit or highly discourage the entrapment of water within the roof assembly.
  • 4. Favor membrane and insulation designs capable of in-place reuse or recycle in future roof iterations.

Through the years, these guiding principles have produced a dramatic improvement in roofing performance on our campus. In particular, our emphasis on adaptive reuse of materials will minimize our impact on the environment, as well as reduce future demand on hospital resources–resources best used in support of outstanding patient care or cancer research, not funding a premature roof replacement. Interestingly, the U.S. General Services Administration, Washington, D.C., has recently incorporated our guiding principles in facilities standards for future public building construction. Now our story has legs.

In April 2013, I attended the Energy Efficient Roofing Conference in Charlotte. I was invited to participate in the program, offering a building owner’s perspective about emerging roofing technologies. The focus, primarily, was energy-efficient roofing as a value proposition: how to achieve it and how to sell it. The format leaned heavily on panel discussions, which produced large amounts of banter and at times outright tension regarding the subjects at hand. It was as if someone handed a microphone to the elephant in the room. Has the proposition become a “solution” in search of a “problem”?

Don’t misunderstand; everyone can see the benefits in much (but not all) of the new energy-efficient roofing innovations and building codes. But should we be excited about reflective or solar membranes on massively thick R-30 minimum insulation while still far too many roof installations will fail prematurely because of shortsighted design and construction? If quality and durability are of utmost value, do you—the roofing contractor— know how to achieve it and how to sell it? Should you care?

Back in 2006, I believed everyone was trying to “out green” each other; durability be damned. Today, I wonder if the problem is that everyone wants to “out innovate” each other. As we’ve witnessed with green, the danger when innovation means everything is that it can soon mean nothing.

Innovation is exciting and necessary, but so is refinement. Refinement may be the most powerful strategy of all, yet it remains under emphasized. The most effective way to celebrate refinement is by creating new stories–new institutional memories. Roofing contractor, you are the biographer. Run with that.