Clarifying cool roofs
I am writing regarding "The un-cool consequences of cool roofing," July issue, page 36, by Samir Ibrahim, director of design services for Carlisle Construction Materials, Carlisle, Pa.
Although, as the author points out, recognition of the energy and environmental benefits of cool roofing goes back only 15 years or so, many highly reflective membranes have been in use for much longer than that.
My company has produced nothing but light-colored, highly reflective membranes for more than 50 years. Being a Boston-based company, many of our earliest reference projects, which continue to perform problem-free, are located in New England and exhibit none of the condensation or loss of adhesion issues attributed to cool roofs in the article.
The article is fundamentally a treatise on good roof system design. Billions of square feet of cool roofs have been installed in northern climates using the same "self-drying concept" with at least the same "high rate of success" attributed by the author to "dark-colored" membranes for roughly half a century. The points raised regarding eliminating thermal shorts, using multiple insulation layers, accommodating moisture from concrete, etc., apply to all low-slope roof systems regardless of membrane color, material or number of plies.
Lawrence Berkeley National Laboratories (LBNL) consistently has been at the forefront of research on cool roofs and other technologies to reduce the heat-island effect and has quantified the potential effects of heat islands. Independent studies by the National Center for Atmospheric Research in America and University of Perugia in Italy show that, if anything, the LBNL projections of the greenhouse gas reduction benefits that result from cool surface strategies are conservative and underestimated.
It is instructive to note many of the biggest retail companies in the U.S. use cool roof membranes exclusively in all areas of the country. For "big box" retail stores, roofs typically represent more than 75 percent of a building's envelope and, therefore, significantly affect heating and cooling costs. One retailer settled on thermoplastic PVC roof membranes more than 20 years ago for its 2,000 stores located throughout the U.S. in part because of the net energy savings achieved by doing so. Furthermore, the same company has a proactive roof system replacement program whereby a roof membrane is removed before failure and replaced, allowing the building to maintain its existing insulation. Barring any localized damages to the roof membrane, the insulation is always found to be dry and is left in place for decades.
Attempts to discredit the use of cool roofs in northern climates do not stand up to the overwhelming empirical evidence of thousands of such roofs performing without problems for many decades in cold climates.
Stanley P. Graveline
Ibrahim responds: Carlisle Construction Materials is a major producer of black and white membranes, and the article was not meant to discredit the use of reflective membranes but to bring awareness to possible consequences when a change in roof color is not properly understood.
It is a fact that dark-colored membranes are far more effective in a "self-drying" roof assembly than reflective materials regardless of manufacturer. This has been a topic of research and published papers in many industry publications.
Cool roofs, because of their colder surface temperatures, fall below the dew point frequently and for longer periods of time, increasing the level of moisture collected during winter. During summer, their drying cycles are significantly reduced because of their lower temperatures. One exception is when a reflective membrane is severely soiled and no longer functioning as a cool roof membrane, it will experience a higher surface temperature and the drying cycle may be increased.
Single-ply membranes have been used throughout North America for several decades, but we must recognize the market dynamics and evolution of construction practices. White membranes represented a small percentage of roofs in northern climates, and, in the early days, most installations were used in re-cover applications. Thus, membranes were not in direct contact with migrating warm air, and condensation was not an issue.
In those assemblies (new construction or tear-off with no vapor retarder), products such as wood fiber and perlite boards, common in earlier times, acted as sponges for residual condensation. Even though a building owner may not have experienced moisture drips, eventually these boards disintegrated. This was likely not covered by the single-ply manufacturer's warranty because it was a "product by others."
Currently, most or all manufacturer warranties contain exclusions for condensation to make sure owners are well-informed. Therefore, an owner should be alerted to possible consequences before selecting roof assemblies.
The article is about not limiting choices and is against offering one option as a universal fix for climate change and energy savings.
"No roof system should be shoved down people's throats," said Andre Desjarlais, group leader for building envelope research at Oak Ridge National Laboratory, Oak Ridge, Tenn. "I'd like to see the availability of all options and let people choose based on what is the most economical and efficient for their needs."
His article, "Energy Efficiency: More than a Simple Black and White Issue," published in American School and Hospital Facility's March/April 2009 issue features a comprehensive overview of roof color, energy impact and performance issues. Focusing on roofs to reduce the urban heat-island effect when they only represent about 27 percent of horizontal surfaces in a metropolitan area is outrageous!
To date, there is no study that contains empirical field data about the effects of cool roofs and their contribution to the reduction of heat islands despite the fact that Title 24 in California has been in place since 1989.
Current studies do not account for the increased level of carbon dioxide in cities and ignore the effect of deflected heat on air intake units and vertical surfaces (windows and walls) that absorb much of the deflected heat.
A good example is the Disney Concert Hall in Los Angeles; the deflected sunlight from this structure contributed to a verified increase of 15 degrees in the adjacent Promenade condominiums.
A single-component approach leads to increased probability of unintended consequences, greater risk to the designer and higher costs to the building owner and does not deliver the perceived heat-island reduction.
Concerning "The un-cool consequences of cool roofing," July issue, page 36: We represent leading manufacturers of white reflective vinyl roof membranes in North America. Our trade group is committed to making sound, scientifically backed information available about the environmental and functional performance of our products.
We were disappointed and perplexed by the article's stance that reflective roofing is a problem in cold climates, especially given the statement: "This is not to say reflective roofs can't or shouldn't be used in places with harsh winters; as long as certain design modifications are carried out, reflective roofs can perform quite well in cold climates." This comes after a lengthy discussion about the effects of condensation on roof decks, insulation and the underside of roof membranes because of white membrane temperatures the author says fall below the dew point and remain there longer than dark membranes.
Most of his commentary could have been avoided had he simply acknowledged sound roof system design is critical for roof systems of all colors. Obviously, these are complex assemblies; this is why reputable roof consultants and design engineers provide expertise to building owners who express an interest in a reflective roof—or any roof. Moisture can occur under all of them, not just white ones.
The Department of Energy (DOE) Building Technologies Program's handbook 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." If building owners are concerned, there are standard remedies that can be incorporated into a design as there are with any roof.
The article repeatedly notes reflective roofing has been touted as a "quick fix" for stemming the tide of climate change and achieving energy savings. We disagree—this terminology is not typically used to describe either of these two benefits. Instead, what we say is a variety of research institutions, national laboratories, universities and government agencies that have long studied the impact of cool roofing in service on buildings for decades typically find two things:
Research on white roofs' effects on global cooling still is in its infancy, but projections of greenhouse gas reductions that we are aware of are promising, and no study can be said to trump any other at this point. However, it must be noted Arthur Rosenfeld, who has studied this area of science longer than anyone, recently said in the International Energy Agency's spring journal that every 100 square meters of roof area that is white instead of black cancels the warming effect of 10 tonnes of carbon dioxide over a roof's lifetime (typically 20 years or more). Rosenfeld pioneered energy-efficiency breakthroughs at the Lawrence Berkeley National Laboratory's Heat Island Group and has served as a DOE senior advisor and commissioner of the California Energy Commission.
With roofs accounting for roughly 25 percent of urban surface area and cities occupying 1 to 2 percent of global land area, converting most low-slope roofs to white would cancel warming from more than 1 gigatonne of carbon dioxide per year for the average lifetime of the roofs. In terms of emissions, it's equivalent to taking half the world's cars off the roads for 20 years.
Finally, the notion that there can be no net savings in colder climates ignores a number of differences in the seasons. Although it's natural to assume a white roof will reflect solar energy that could have been used to warm a building, in reality the sun is lower to the horizon in winter and not hitting roofs as directly or intensely as it would in summer; the sun shines fewer hours; and there are more cloudy days. And regardless of roof color, snow cover always will be white—and reflective.
We hope the article will not deter building owners and specifiers from considering all the available evidence on the benefits of cool roofing. For a comprehensive compilation of the facts, please visit www.vinylroofs.org/cool and follow @reflectiveroofs on Twitter.
The Vinyl Roofing Division of Chemical
Fabrics and Film Association
Ibrahim responds: Carlisle Construction Materials is a major producer of reflective membranes though it is not a member of the Chemical Fabrics and Film Association. The article was not meant to discredit the use of white membranes but to bring awareness to possible consequences when a change in roof color is not properly understood.
The thought regarding good design by "reputable roof consultants and design engineers," oddly enough, does not include roofing contractors, who are the primary members of NRCA and readers of Professional Roofing.
Roofing contractors in many parts of the U.S. do not need a consultant or architect to replace a roof, which currently is 70 to 80 percent of roofing sales in the U.S., and are in need of information that may affect them or their clients. The points made in the article are not false or unfounded. In Guidelines for Selecting Cool Roofs (the same reference material mentioned in the response letter), it also states: "In colder climates, like Chicago or Alaska … there is less heat available to dry out the roof and more opportunities for condensation to occur." This runs contrary to the point about using cool roofs "in practically all areas of the U.S." because the energy savings is "worthwhile."