Dr. Tom Petrie of the U.S. Department of Energy’s Oak Ridge National Lab (ORNL) created a web-based energy savings calculator for low-slope commercial roofs, which shows the savings a building owner can expect when selecting a cool roof over a conventional black asphalt roof.  At the bottom of this page, you will find a link to the calculator on ORNL website.

The calculator requires users to input several different pieces of information, such as city and state, the type of energy (gas or electric) for heating and cooling, in-place insulation value, and gives some guidance to help the user select reasonable assumptions for the relative cost of energy in their area.  The calculator automatically fills in the correct HDD and other pertinent climate information from a database. 

Several examples of different roof reflectance and emittance are shown in Table 2 for various cities in the United States.  Each building in the example was assumed to be 10,000 square feet in size, have an average heating and cooling system, R-12 insulation in the roof, and initially a black built-up roof with a Solar Reflectance of 0.05 and Infrared Emittance of 0.90.  The savings shown in the example are for replacing the existing roof with a cool metal roof, with values shown for unpainted Galvalume™ and white PVDF painted Galvalume™.  

	Chicago, IL		International Falls, MN		Houston, TX		Miami, FL
	HDD65 = 6,536		HDD65 = 10,487		HDD65 = 1,371		HDD65 = 200
Roof Surface	Unpainted	Painted	Unpainted	Painted	Unpainted	Painted	Unpainted	Painted
R=Reflectance	R=0.69	R=0.65	R=0.69	R=0.65	R=0.69	R=0.65	R=0.69	R=0.65
E=Emittance	E=0.06	E=0.88	E=0.06	E=0.88	E=0.06	E=0.88	E=0.06	E=0.88
AC Savings/Yr	$260.00	$580.00	$   30.00	$    70.00	$820.00	$1,310.00	$890.00	$1,790.00
Heating Savings/Yr	$   60.00	$  (90.00)	$   80.00	$(1,10.00)	$   60.00	$    (30.00)	$   10.00	$  0
Combined Savings	$320.00	$490.00	$ 110.00	$   (40.00)	$880.00	$1,280.00	$900.00	$1,790.00

Table 2: Comparison of Savings from ORNL Cool Roof Savings Calculator

From the examples above, it can be seen that in low heating degree-day climates such as Miami, large savings can be expected.  In these areas, high-emissive ((0.80) white or light painted Galvalume™ roofs would be the best choice.  However, in areas where the heating degree-days dominate (in our example International Falls, Minnesota), the high-emissive roof actually increases overall energy costs!  Heat gained by the low-emissive (0.06) unpainted Galvalume™ roof can reduce the load on the building’s heating system, which saves energy. 

Because there are fewer heating degree-days in areas such as Houston and Miami, it can be seen that both a high reflectivity reflectance and emissivity emittance yields the greatest savings.  In these areas, white or light-painted Galvalume™ roof panels would perform best.  Although other products may have high initial reflectance, metal roofs have proven to retain their radiative properties, which allow the building owner to continue to see the savings year after year. 

To reach the ORNL calculator, click here.