Applying the Cool Roof Peak Calculator

Now that we’ve reviewed the basic workings of the Cool Roof Peak Calculator, we can examine in greater detail what the calculator may reveal about base use and peak demand savings throughout the U.S. and Canada. Although it is difficult to accurately estimate exact base use and peak demand without a detailed examination of the construction and cost conditions for a specific building, it may be possible to develop a useful model by applying conservative assumptions suitable to a wide array of locations and buildings across North America. In order to develop an informative portrait of peak demand and cool roofs throughout the U.S. and Canada, this paper provides a climatic analysis for a typical cool roof versus a black roof using the following parameters applied to the Cool Roof Peak Calculator:

• Climate Zones and Model Cities. Current energy codes divide the U.S. and Canada into eight primary climate zones, with Zone 1 the warmest and Zone 8 the coldest. Within each zone, demand for heating and air conditioning tends to fall within a relatively narrow range, allowing for a similar thermal analysis of buildings within the climate zone. A map of the eight climate zones in the U.S. and Canada is illustrated in Figure 7.

  Also included in Figure 7 is a listing of model cities used within the analysis. In the case of the most extreme zones, only one city has been selected since the zones are either small or sparsely populated. In the intermediate climate zones, however, two cities were selected and their climate data averaged to provide a more accurate representation for all cities within the zone.

• Representative Commercial Building. Within all climate zones, a representative building was selected. For the purposes of this analysis, the building was assumed to be a lowrise structure of one or two stories with a flat roof area of 20,000 square feet. In addition, it was assumed that the building was cooled with an electric air conditioning system with a Coefficient of Performance (COP) of 2.0 and heated with a natural gas-fired furnace¹² with an efficiency rating of 70 percent.
• Roof Insulation (R-Value) Level. Two insulation conditions were selected for the analysis to allow for a comparison of different roofing scenarios. The first condition (“new insulation”) assumes that the existing roof is completely removed and replaced with a new roofing system using R-value levels meeting the latest energy code requirements. The second condition (“old insulation”) assumes that the existing roof and insulation remains in place and is simply recovered with a new roofing membrane with no additional R-value. The new insulation condition is intended to model the installation of a completely new roof on an existing building or a newly constructed building, while the old insulation condition is intended to model the installation of a roof recovery over an existing roof that remains in place. Because the amount of roof insulation used in buildings varies according to climate zone, lower levels of insulation were assumed for the warmer climates and higher levels were assumed for colder climates. In addition, because code-mandated insulation levels have increased over the past decade, separate insulation levels were applied to the old and new insulation conditions. For the old insulation condition, R-value levels were based on an earlier (2006) version of the International Energy Conservation Code, and for the new insulation condition, R-value levels were based on the most recent (2012) edition of the code. These old and new insulation levels are summarized by Climate Zone in Table B.
• Roof Reflectance/Emittance. The long-term reflectance of most cool roofs tends to fall within a relatively narrow range, specifically from 0.55 to 0.63 for minimum aged reflectance as shown in Table A. Accordingly, the cool roof modeled in the analysis is based on a reflectance of 0.60, which falls approximately mid-range of the aged values in Table A. And because the Cool Roof Peak Calculator automatically compares this cool roof to a black roof with a reflectance of 0.05 and an emittance of 0.90, an emittance value of 0.90 also was selected for the cool roof.
• Base Use and Peak Demand Charges. Because the example electric bill from the state of Indiana shown in Figure 2 represents one of the lower rates available in North America, a comparison based on those rates obviously would provide a conservative estimate. As a result, the analysis assumes a base use rate of $0.033/kWh and a peak demand charge of $20.10/kW across all eight major climate zones. In addition, the analysis assumes a rate of $0.70/ therm for natural gas, which is very close to the average commercial rate across North America at this time.

Using these assumptions and values, estimated base use and peak demand savings for a typical 20,000-square-foot commercial building in all eight climate zones were calculated using the DOE Cool Roof Peak Calculator. For each climate zone, two different roof conditions were examined. The first set of calculations compared a cool roof against a black roof installed over new roof insulation meeting the most recent energy code R-value requirements. The second set of calculations compared the same cool and black roof installed over existing (old) roof insulation meeting an earlier version of the energy code. The comparison of the cool versus black roof over new roof insulation is shown in Figure 8, and the comparison of the cool versus black roof over old insulation is shown in Figure 9. In addition, the range of savings available for both old and new insulation conditions is graphically portrayed on a map of the eight North America climate zones in Figure 10.

The Bottom Line: Cool Roofs and Peak Energy Demand

Savings in all Climates and Conditions. As illustrated in Figures 8 through 10, the total value of base plus peak energy savings offered by the cool roof is sizeable, averaging more than $1,000 annually in most climate zones for a typical commercial building. In addition, these savings appear to be equally important for buildings with either “old” and “new” levels of insulation. As a consequence, cool roofs may offer a significant opportunity for net energy savings even at the highest levels of roof insulation mandated by the latest building codes. The savings value of cool roofs is further reinforced because modern cool roofing membranes frequently cost no more that darker non-cool roofs. As a result, all of the savings identified in the analysis tend to drop to the bottom line without any additional cost encumbrances.

Cool Roofs and Insulation Level. Differences in the level of new versus old insulation appear to have a significant effect on the amount of base use savings. In most cases, base use savings using the lower R-value levels of old insulation are reduced by half or more by the addition of the higher R-value levels of new insulation. However, this condition does not appear to hold for peak demand savings. In most cases, the savings available using either old or new insulation levels appears to be significant for all climate zones. As a consequence, it would appear that significant reductions in peak demand cost cannot be achieved simply by increasing insulation levels without also installing a cool roof covering.

Peak Demand Drives the Savings.

One of the most striking results from this analysis is that the estimated savings due to peak energy demand reduction provide a substantial majority of the net energy savings throughout all climate zones studied. In fact, peak demand savings account for over 50 percent of total savings in the warmest climate zones up to 100 percent in the coldest climate zones. In addition, while base use savings tend to vary widely by climate zone (even falling to negative values in the coldest climates), peak demand savings tend to more significant and consistent throughout all climate zones. As a consequence, the analysis clearly suggests that any estimate of cool roof savings that neglects to include peak demand reduction has little chance of providing an accurate estimate.

Effect of Fuel Selection on Net Energy Savings.

As stated previously, a natural gas forced air system was selected as the heating system for the building in these calculations. If electric resistance heat or an electric heat pump were selected in lieu of a natural gas system, the base use savings (shown in yellow in Figures 8 and 9) would decrease slightly due to an increased winter heating penalty applied to the electric heating system. In a similar manner, if an oil-fired furnace were selected in lieu of the natural gas system, the base use savings would increase slightly due to the higher cost of heating oil as compared to natural gas across the country. However, the peak demand savings (shown in red in Figures 8 and 9) would remain the same regardless of the heating system and fuel source selected.

Notes:

1. U.S. Utility Rate Database. Available http://en.openei.org/wiki/Utility_Rate_Database
2. Derived from Understanding Your Utility Bill: A Guide for Businesses in Indiana. Duke Energy, Plainfield, Indiana (2013). Available http://www.duke-energy.com/pdfs/ understand-bill-guide-in.pdf
3. 2013 Retail Commercial Electrical Rates by State (excluding Alaska and Hawaii) from U.S. Energy Information Administration Electricity Data Browser. For more information on average electric rates, please visit http://www.eia.gov/ electricity/data/browser/
4. Although stated as a percentage in this table, roof reflectivity is typically expressed as a ratio in reference standards. Initial values shown are based on measurements of roofing material as manufactured, while aged values shown are based on measurements after field exposure of test samples.
5. For more information on the Energy Star rated roofing products, please visit http://www.energystar.gov/productfinder/ product/certified-roof-products/
6. For more information on Cool Roof Rating Council rated roofing products, please visit http://coolroofs.org/products/results
7. Petrie, T. W., Wilkes, K. E., & Desjarlais, A. O. (2004). Effects of Solar Radiation Control on Electricity Demand Charges – An Addition to the DOE Cool Roof Calculator Proceedings of the Performance of the Exterior Envelope of Whole Buildings IX International Conference, December 5-10, 2004.
8. Hoff, J. L. (2014). Introducing the RoofPoint Energy and Carbon Calculator: A New Modeling Tool for Roofing Professionals.Proceedings of the Second International Roof Coatings Conference, Baltimore, MD, July 14-17, 2014.
9. U. S. Department of Energy (DOE) Cool Roof Peak Calculator. Available http://web.ornl.gov/sci/buildings/tools/cool-roof/
10. It should be noted that the Cool Roof Peak Calculator does not account for the potential for snow cover of the roof in the winter. The presence of accumulated snow on the roof surface may have two effects on overall energy savings. First, snow on either a cool or a dark roof surface will reduce the amount of solar energy absorbed into the building, which may increase heating costs. Conversely, a thick accumulation of snow may provide additional thermal insulation that may reduce heating costs.
11. Climate zones as defined by the International Energy Conservation Code and AS HRAE 90.1. Illustration courtesy of the Center for Environmental Innovation in Roofing and the Polyisocyanurate Insulation Manufacturers Association.
12. Natural gas was selected because it provides over 60 percent of all commercial building heating demand in the United States, according to the Commercial Building Energy Consumption Survey (CBECS) published by the U.S. Energy Information Administration (http://www.eia.gov/consumption/ commercial/) Figure 10: Estimated Range of Net Energy Savings for a Cool Roof by Climate Zone (Annual Dollars/20,000-Square-Foot Roof Area)

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