Cool Roof

Cool Roof

By: Daniel Overbey It’s one of those “easy” LEED credits. Add a “cool roof” to your building and you’ll earn a credit for mitigating the urban heat island effect. However, recently the cool roof strategy has been challenged. Recently, my Indianapolis-based office has received multiple memos from roofing giant Carlisle Construction Materials contending that dark-colored roofing membranes are a more economic option than white membranes in colder climates, such as Indianapolis, New York City, and Boston, to name a few. Carlisle memos say cool roofs are bad in colder climates. Indianapolis is among the “top 25 U.S. cities in colder climates” presented by Carlisle in an undated comparison of energy performance between black and white roof colors using the Roof Savings Calculator (RSC) developed by Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory (ORNL/LBL). According to the RSC results, a black roof would apparently outperform a white roof in the Circle City (see Table 1). The memos go on to criticize EPA’s ENERGY STAR program for qualifying cool roof products because doing so “may cause individuals to select a white roofing membrane when a dark-colored membrane would be a better option.” Another memo from Carlisle (dated March 31, 2014) cites a 2011 study titled “Effects of Urban Surfaces and White Roofs on Global and Regional Climate,” published in the Journal of Climate and authored by Stanford University researchers Mark Z. Jacobson and John E. Ten Hoeve, that suggests cool roofs may potentially increase the earth’s temperatures and could increase space heating in some instances. The memo goes on to scrutinize USGBC’s LEED rating system for rewarding projects for using a cool roofs in any climate, regardless of energy cost savings. It’s worth pointing out that the LEED BD+C v4 Heat Island Reduction credit is designed to combat the urban heat island effect, while the Carlisle data specifically considers a building’s site energy cost savings. Body of research refutes Stanford study, suggests cool roofs are good. The Jacobson and Hoeve study was published in September of 2011. The LBNL Heat Island Group, which has conducted extensive research on cool materials and their effects on regional and global climate, promptly examined the Stanford study. Their findings were published in a November 2011 executive summary titled “Cool Roofs and Global Cooling,” authored by Surabi Menon, Ronnen Levinson, Marc Fischer, Dev Millstein, Nancy Brown, Francisco Salamanca, Igor Sednev and Art Rosenfeld. The LBNL peer review group determined the Stanford results to be “preliminary and uncertain” and suggested that the conclusions be “analyzed more carefully.” LBNL cited four primary concerns. In particular, the group determined that the numerical results of the Sanford study may be “statistically insignificant.” The Heat Island Group offered three studies that challenged the Jacobson and Hoeve findings. Questioning the objectivity of the Carlisle memos. I question the objectivity of the Carlisle memos, considering that they were authored by the Marketing Manager of EPDM and FleeceBACK Roofing Systems. On their EPDM Product Page, Carlisle SynTec is highly critical of cool roofs. (By the way, the company does offer the only ENERGY-STAR-qualified EPDM membrane – and, yes, it is white.) A Carlisle roofing product representative recently told me that there is a higher profit margin for black ethylene propylene diene monomer (EPDM) single-ply roofing membrane systems versus white thermoplastic polyolefin (TPO) single-ply roofing membrane systems. In all honesty, I consider myself a proponent of LEED and ENERGY STAR. However, as a practitioner, I just want to know the truth. With an extensive research background in passive solar heating, I understand how solar absorption atop a building can benefit heating loads under certain circumstances. I decided to investigate – starting with the Roof Savings Calculator. Are cool roofs appropriate for colder climates, like Indianapolis? My firm deals mostly with commercial projects, so I carefully reviewed the Carlisle application of the ORNL/LBNL Roof Savings Calculator (RSC) in Indianapolis. I wanted to see if I could replicate the $342 net annual savings for using black EPDM for 10,000-square-foot commercial structure. RSC Inputs The Carlisle memo states several assumptions regarding their input into the RSC:

  • 10,000 square foot building
  • 1 floor
  • 40% window-to-wall ratio
  • Post-1990 construction
  • Mid-efficiency heating and cooling equipment
  • Aged reflectance: black EPDM at 9%; white TPO at 70%
  • Aged emittance: black EPDM at 84%; white TPB at 86%
  • R-20 roof insulation
  • Electric cooling (at 9.14 cents/kWh)
  • Natural gas heating (at 7.29 $/1000 cu. ft.)

The RSC requires several additional inputs. After several trial-and-error attempts, I was able to most closely mimic the Carlisle data using the following assumptions:

  • Office building type
  • No above-sheathing ventilation
  • Low roof pitch (slope < 2:12)
  • No radiant barrier present
  • Duct location: Conditioned Space
  • Duct leakage: Inspected (4%)

Results of the direct RSC analysis. RSC Scenario 1 There are many unknowns about how the RSC is structured and whether or not it has been updated since the Carlisle study. Nevertheless, my attempts at replicating the result of the Carlisle study yielded results of the same magnitude. The cool roof produced energy savings in cooling mode, but the black EPDM was more advantageous in heating mode. Over the course of the entire year, the white TPO came energy cost penalty of $350 based on the inputs listed above. RSC Scenario 2 At the risk of belaboring this issue, suffice it to say that changing various parameters in the RSC will yield varying results. After an unscientific investigation, however, it became clear that changing the fuel source for the HVAC equipment made a decisive impact on the net annual impact of a white TPO versus a black EPDM membrane. Unlike Scenario 1, which utilized a “natural gas furnace” (RSC language), Scenario 2 used an “electric heat pump” (RSC language). Under Scenario 2, the white TPO had a positive net annual energy cost impact in Indianapolis. (See Table 2.) As a side note, in both RSC scenarios above, the cool roof required less MBtus of total annual energy, despite the varying energy costs associated with different energy source types (see Table 2). What does it all mean? Are cool roofs appropriate for commercial structures in colder climates? It really just depends. Occupancy type, number of floors, orientation of the room plane(s), HVAC equipment, insulation levels – all of these things, and my other factors, impact the economic advantage of one roof type over another. The research providing the foundation of Carlisle’s anti-cool roof stance in colder climates is preliminary and inconclusive at best. Even the RSC clearly states in red text that it “is currently undergoing validation due to discrepancies with previous studies.” Clearly, the notion of changing energy types (e.g., fossil fuel-derived electricity, natural gas, solar-derived electricity, etc.) will impact the economies of building energy costs. If we’re talking about reducing carbon emissions, we have to consider source energy in our analysis, which could yield entirely different results. Carlisle’s contention is proactive because it counters conventional best practices. However, I make a living questioning best practices through analysis. It is always best to leverage energy and design performance modeling tools to cut through the rhetoric, certification credits, ratings, and one-size-fits-all solutions to arrive at the right answer for a specific project, program, and location.   Table 1: Annual Cool Roof Energy Savings. Data derived from an internal study by Carlisle Construction Materials. Image 1: This Las Vegas Walmart utilizes a white TPO cool roof studded with skylights to reduce building energy consumption. (Photograph courtesy of Walmart/Flickr) Figure 1: A screen capture of the full range of HVAC equipment options in the Roof Savings Calculator (http://rsc.ornl.gov/). The options leave a lot to be desired, considering this parameter largely determines the economic advantage of a white TPO roofing membrane. Table 2: Table 2: Annual Cool Roof Energy Savings. Data derived from the author’s personal analysis using the Roof Savings Calculator (RSC).

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