Heating penalty
While cool roofing reflects heat and lowers roof temperatures, it does this both during the summer months (i.e. when being cooler is a benefit) and in the winter (i.e. when it is not necessarily beneficial). Lower winter temperatures contribute to a greater transfer of heat from the building. This is due to a greater thermal differential and can be lowered by increasing the amount of insulation.
The projected energy savings for any building using a cool roof is the cooling savings minus the heating savings, or a total savings or negative cost for a given year. As a cool roof assembly used on one building is moved to colder areas of the country (i.e. from Miami to Charlotte to Philadelphia to Boston), the heating penalty increases and overall savings are diminished.
EnergySTAR
EnergyStar, a U.S. Environmental Protection Agency (EPA) voluntary program, helps businesses and individuals save money and protect the climate through energy efficiency. The agency assists consumers by listing appliances and building products on its website to aid in energy-efficient selection. Recently, EnergySTAR rated cool roofs—previously listed for their reflective values—as ‘incomplete systems.’ Relying on reflectivity alone is not a guarantee of energy savings and insulation and coverboards should be part of the system. The program states:
Although there are inherent benefits in the use of reflective roofing, before selecting a roofing product based on expected energy savings consumers should explore the expected calculated results that can be found on the Department of Energy’s Roof Savings Calculator. Please remember the energy savings that can be achieved with reflective roofing are highly dependent on facility design, insulation used, climatic conditions, building location, and building envelope efficiency.(For more information, visit www.energystar.gov/products/building_products/roof_products.)
Design guides
Energy efficiency design guides have been available for architects, designers, and specifiers for many years. These guides are designed for new construction as well as existing buildings.
A series of Advanced Energy Design Guides (AEDGs) were developed in cooperation with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), American Institute of Architects (AIA), the Illuminating Engineering Society of North America (IESNA), the U.S. Green Building Council (USGBC), and U.S. Department of Energy (DOE) in 2011. These guides were aimed at achieving 50 percent energy savings and working toward a net zero energy building. They encompassed small to medium office buildings, medium to big box retail buildings, large hospitals, and K–12 school buildings.
ASHRAE Climate Zones 4 and above recommend following ASHRAE 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings for roof reflectance, which currently does not require roofs to be reflective in these zones. In Climate Zones 4 and above, cool roofs are not recommended design strategies.(To see more from ASHRAE, visit www.ashrae.org/standards-research–technology/advanced-energy-design-guides.)
A series of Advanced Energy Retrofit Guides (AERGs) for “Practical Ways to Improve Energy Performance” were developed in cooperation with DOE and Pacific Northwest National Laboratory (PNNL) in 2011. These guides were aimed at improving existing retail and office buildings which could improve their energy efficiency. Cool roofs were not recommended for all locations. The AERGs state:
This measure is likely more cost-effective in the hot and humid climate zone, which has a long cooling season, than in the very cold climate zone, for example. For buildings located in warm climates, this measure is worth consideration.(To see this guide in its entirety, visit www.pnl.gov/publications/abstracts.asp?report=378139.)
Peak energy demand
One recent paper currently covers the idea of peak demand energy savings, in which a building owner may realize savings using a cool roof as a ‘hidden benefit.’ The premise of this is summer energy savings with the added cost savings of peak demand charges during times of high use.
This concept may have some affect on certain building owners with peak energy demand charges. However, a peak demand calculation is complex and is locally dependent on the energy generation or distribution provider. To further complicate the ‘hidden benefit’ of peak demand energy, not all electricity generators/providers charge for peak demand. Providers could include an additional charge, but could also increase the standard rate to compensate for the peak usage on their electricity grid.
Larger building owners may negotiate their rates due to high usage. They could also negotiate to experience peak demand charges only when they exceed a usage threshold for a given period of time. A peak demand charge will vary by state and possibly producer.
Conclusion
Establishing the benefits of cool roofing as a guarantee for energy savings throughout the country is a misconception and an overstatement of their benefits. When discussing cool roofing, a designer, specifier, and building owner must keep in mind results may vary based on geographic region, building use, building size, and orientation.
One should consider incorporating a vegetative roof or additional plants and trees throughout the project site to lower local ambient temperatures, especially in colder, northern climates. Increasing levels of building insulation—a product that works on sunny and cloudy days during the winter and summer months—can also help. One should review the locally adopted energy codes, and recognize design elements, such as lighting and HVAC equipment, that will save energy and lower the load on the local electricity grid.
Craig A. Tyler, CSI, AIA, CDT, LEED AP, is an architect and specification developer for Carlisle Construction Materials in Carlisle, Pennsylvania. He has a master’s degree in architecture from Savannah College of Art and Design (SCAD) and is a licensed architect in seven states. Tyler has more than 15 years of experience, and has worked on commercial projects ranging from offices to multi-family housing. He can be contacted at craig.tyler@syntec.carlisle.com.
