Calculating Cool Roof Energy Savings

Posted on September 20, 2016 by Candy McNamee

Whether it’s providing waterproofing, reducing thermal expansion and contraction, or supplying chemical and damage protection, cool metal roofing has much to offer. Of course, the most substantial benefit is the energy savings gleaned from reduced rooftop heat levels driving down air conditioning loads. In fact, the Lawrence Berkeley National Laboratory’s heat island group projects a whopping $1 billion reduction in cooling costs if cool roofs were to be implemented on a nationwide basis.

To assist architects in determining the kinds of energy savings that can be expected from cool metal roofing, the Oak Ridge National Laboratory (ORNL) has parlayed the data it gathered from a three-year evaluation of metal roofing products into a whole building energy savings calculator.

Cool metal roofs are offered in a variety of colors.
In addition to energy efficiency, cool metal roofs are known for extended durability and longevity.

Cool Roof Calculator

This calculator is called, simply enough, the Cool Roof Calculator. The easy-to-use tool is described as a quick way to compare overall energy costs and savings for a variety of roof and building conditions. Unlike some energy modeling calculators, which are limited to steep slope residential roofs with attics, ORNL’s tool models the typical low slope commercial roof with insulation placed directly over the deck and under the roofing membrane.

To calculate approximate energy savings offered by a cool metal roof, architects are instructed to input the building’s location, proposed roof R-value, roof reflectance and emittance, base energy costs, equipment efficiencies, electrical demand charges and duration.

While experts suggest that it may be difficult to accurately predict the base use and peak demand without detailed construction and cost information, tools such as the ORNL’s cool roof calculator can be a useful way to gather helpful performance estimations for a variety of building types and locations.

Attempting to do just that, the calculator outputs a number of values to offer an approximate estimate of potential energy savings, broken down into cooling energy savings—a calculation of air conditioning savings from base use and peak demand reductions—and cooling season demand savings, an estimate of the peak demand charge reduction enabled by enhanced roof reflectivity.

Accessible at http://rsc.ornl.gov, users can also compare the energy performance offered by a cool roof vs. a conventional black roof.

“It’s a nice tool to give people a feel for where a cool roof would actually help them and have the greatest impact in terms of energy use,” relates Robert A. Zabcik, PE, LEED AP BD+C, director, research and development, NCI Group Inc., Houston, in a Metal Construction News article.

Roof Reflectance Baseline

Roof reflectance and emittance, requirements and options, can be found in energy codes such as IECC, ASHRAE 90.1, California Title 24, and other local codes. Requirements may vary based on roof slope and climate zone, and may allow for either aged or initial solar reflectance, thermal emittance and/or SRI.

Fortunately, MBCI continues to stay current with individual testing and also maintains third-party tested and verified product listings through entities such as the Cool Roof Rating Council, and the U.S. EPA’s ENERGY STAR®.

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