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An Analysis of the Effect of Roofing Albedo on Ambient Temperature

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Introduction
It is becoming increasingly obvious that our climate is warming. I had seen news stories about global warming and Al Gore's film An Inconvenient Truth. I decided to do some research to try and better understand what is happening to our Earth. I had heard much discussion (and debate) about how people need to reduce their consumption of petroleum fuels and start driving electric cars. I had also heard a lot of disagreement about whether it is really practical to expect people to drastically change their daily behavior, and about whether people are ready or willing to abandon their gasoline-powered cars. I wanted to find out if there was anything that people might be able to do to reduce climate change without having to drastically change their lives.

I learned scientists have found that the Earth's increasing temperatures are due in part to the loss of snow and ice cover in the polar regions. One manner in which snow and ice cover serves to help keep the Earth cool is by reflecting sunlight. Loss of snow and ice cover reduces the albedo, or solar reflectivity, of the Earth. (Albedo is defined as incident radiation divided by reflected radiation.) When albedo is reduced, less sunlight is reflected away from the Earth. Sunlight that would otherwise be reflected is instead absorbed, further increasing global temperatures and in turn resulting in the melting of more ice and snow. This is called the feedback effect (Wang).

Obviously, it would be helpful if the Earth's albedo could be reduced. In my research, I learned about the role the albedo has in the "urban heat island effect." The urban heat island effect refers to increased temperatures in populated areas that result from reduced surface albedo (Konopacki). It has been estimated that "increasing albedo over downtown Los Angeles by 0.14 and over the entire basin by an average of 0.08 could decrease peak summertime temperatures by as much as 1.5°C" (Sailor). According to the EPA, "On hot summer days, urban air can be 2°-6°C hotter than the surrounding countryside." I also learned that high albedo roofs are thought to reduce buildings' interior temperatures, and therefore energy use and air-conditioning bills (Konpacki). In fact, in one side-by-side comparison, researchers found that the application of white reflective coating to a building reduced summertime air conditioning by 22% (Parker and Barkazski). Scientists have noted that, given the maintenance and replacement needs of most roofing systems, building owners could realize immediate savings in their energy costs by applying high albedo roofing materials in association with their regular roofing maintenance or replacement measures (U.S. Environmental Protection Agency).

I wondered whether high albedo roofing could affect the surrounding air temperatures—specifically, whether the air temperatures around buildings with higher albedo roofs would be lower than the temperatures around buildings with dark roofs. If so, perhaps increasing the solar reflectivity of roofs across a city would lower the temperature in that city, helping to counteract the urban heat island effect. Applied on a global scale, could the use of high albedo roofing materials potentially slow, or even counteract, some of the effects of global warming, essentially replacing some of the Earth's albedo that has been lost due to the loss of ice and snow? Could the feedback effect be reduced? At a minimum, lowered temperatures in urban areas would require less energy to cool the buildings in these areas, which would help in the effort to reduce global warming.

I decided to try an experiment to determine how much cooler the air would be around a building with a highly reflective roof surface, as compared to the air around a building with a dark roof surface. I constructed four small structures (doghouses), each with different kinds of roofing. My plan was to take the temperatures above, inside, and on the surface of the structures.

Hypothesis
I selected the following four roofing materials to be analyzed and compared for their effect on temperatures:

  • asphalt shingles painted with a highly reflective coating;
  • silver corrugated tin;
  • black asphalt shingles;
  • tin painted with a highly reflective coating.

The following measurements were to be taken:

  • ambient temperature;
  • air temperature six inches above the structures;
  • air temperature inside the structures; and
  • roof surface temperature.

I hypothesized that the structure with the white painted shingles would have the lowest temperatures in each category; that the structure with white painted tin would have the second lowest temperatures; that the black shingled structure would have the highest temperatures in each category; and that the structure roofed with silver tin would have the second highest temperatures in each category.

Ryan constructing a dog house

Ryan constructing a dog house


I hypothesized this because I believed that the white painted shingles would be the most reflective and would not retain much heat. I also thought that the tin might retain a bit more heat than the painted asphalt shingles. I expected that the black shingles would have a higher temperature than the silver tin because black has a lower albedo than silver.

Materials
The materials I used in my project were as follows:

  • four do-it-yourself pitched-roof unfinished doghouses (31"W x 30"L x 24"H), by Ware Manufacturing
  • one bundle of three-tab black asphalt shingles, from Home Depot
  • white Ultrashield topcoat shingle paint, from BestMaterials.com
  • non-contact infrared thermometer, from ScienceKit.com
  • red liquid-filled wall thermometer, from ScienceKit.com
  • rain gauge thermometer, from ScienceKit.com
  • silver tin roofing, from Home Depot

Procedure

The four test structures

The four test structures


I built four small structures and roofed one structure with silver corrugated tin roofing, one with black asphalt shingles, one with tin painted with the reflective white paint, and one with black asphalt shingles painted with the reflective white paint. I arranged the structures in an area where they would receive the same amount of sunlight and very little shade.

Ryan measuring surface temperatures

Ryan measuring surface temperatures


I measured the surface temperatures of the roofing materials using a non-contact infrared thermometer from three feet away. I also mounted a rain gauge thermometer on a pole six inches above each structure to determine what effect the roofing materials had on the temperature above each house. The interior temperatures of the structures were measured with thermometers mounted inside each doghouse. I took these measurements at irregular intervals over period of several months. In total I took 54 sets of measurements.

Data
For each of the types of measurements I took, I made a graph in order to compare the readings of the various roofing materials. Graph 1 is pictured below. This graph shows the relation between the ambient temperature and the surface temperatures of each structure. After I made graphs for each set of measurements, I then made graphs showing the average difference between each measurement type and ambient temperature. These graphs show how many degrees Celsius, on average, the temperatures six inches above the structures, the interior temperature, and the surface temperatures varied from the ambient temperature. The average variances for the four roofing materials, for these three measurements, are reflected in Figures 2, 3, and 4, respectively.

Graph 1: ambient surface temperature

Graph 1: ambient surface temperature


Graph 2 below shows the average difference between the ambient temperature and the temperature of the interior of the structures. (Note that the temperatures inside the structures were all lower than the ambient temperature, most likely because the structures were not insulated and the measurements were made in the fall and winter.) The interior temperature of the tin-roofed structure was, on average, 2.6°C cooler than the ambient temperature. The interior temperature of the white-shingled structure was, on average, 4.2°C cooler than the ambient temperature. The average interior temperature of the black-shingled structure was 1.3°C cooler than the ambient temperature. And the average interior temperature of the white tin-roofed structure was 4.5°C less than the ambient temperature.

Graph 2: structure interior vs. ambient temperature

Graph 2: structure interior vs. ambient temperature


Graph 3 shows the average difference between the ambient temperature and the air temperature six inches above the structures. The air temperature six inches above the tin roofed structure was, on average, 5°C warmer than the ambient temperature. The temperature six inches above the white-shingled structure was, on average, 3.7°C warmer than the ambient temperature. The temperature six inches above the black-shingled structure was, on average, 7°C warmer than the ambient temperature. The temperature six inches above the white-tin roof was, on average, 4°C warmer than the ambient temperature.
Graph 4, below, shows the average difference between the surface temperature of the structure's roof and the ambient temperature. The surface temperature of the tin-roofed structure was, on average, 0.71°C less than the ambient temperature. The surface temperature of the white-shingled structure was, on average, 1.8°C less than the ambient temperature. The surface temperature of the black-shingled structure was, on average, 20.8°C higher than the ambient temperature. The surface temperature of the white-tinned structure was, on average, 0.22°C less (so low that it almost does not show on the graph) than the ambient temperature.

I found that, on average, the structure roofed with black shingles had the warmest temperatures for all three measurements taken: on the shingles' surface, inside the structure, and six inches above the structure. The structure roofed with white painted shingles typically had the coolest temperatures in each of the three areas. The structure roofed with white tin was the second coolest, with slightly warmer temperatures than the structure roofed with white shingles. The structure roofed with black shingles was found to have the warmest temperatures for all three measurements. The tin-roofed structure was the second warmest, with slightly cooler temperatures than the structure roofed with black shingles.

Graph 3: 6 inches above structure vs. ambient temperature

Graph 3: 6 inches above structure vs. ambient temperature


Overall, the test results were just as I had hypothesized. The roofing material selected for the structures had a definite impact upon not only temperatures on the roof's surface and inside the structure—but also on the temperature of the area above the structures.
The interior temperature of each of the structures was found to be cooler than the ambient temperature. I believe this to be due to three factors: the structures were ventilated, with a doorway in the front and a small, screened opening in the back; the structures provided shade from the sun's heat; and each of the roofing materials provided at least some degree of reflection or absorption of the sun's heat.

Graph 4: Roof structure vs. ambient temperature

Graph 4: Roof structure vs. ambient temperature


Conclusion
A selective use of roofing materials can not only increase albedo and lower energy costs, but also lower ambient temperatures. From my observations, I have concluded that the use of high albedo roofing materials would be very helpful, both in terms of "albedo replacement" and overall energy savings. Also, I believe use of high albedo roofing could be one of the more cost-effective solutions to the problem of albedo loss.
One issue I had not considered in connection with my hypothesis was the potential heating benefit of low albedo roofing. Low albedo roofs absorb heat. With regard to helping to heat the interior of a home during the winter months, the heat-absorbing qualities of low albedo roofing could therefore be a good thing, reducing heating bills. Because of this, the heat-reflecting qualities of high albedo roofing can actually make it more difficult to heat a home in winter; in my research I learned that this cost of high albedo roofing is called a "heat penalty" (EPA). However, in the Texas Gulf Coast climate where my research was conducted, and in most of the United States, I would expect that the benefits of higher albedo roofing in providing more efficient cooling in hot weather would outweigh the heat-absorbing benefits of low albedo roofing. This conclusion is supported by other research that indicates that the cooling benefits of high albedo roofing are such that the heat penalty experienced in the winter months would be more than offset by lowered cooling costs during the rest of the year, except in very cold climates (EPA).

Future Research
To further research these issues, I will continue collecting and analyzing data during the spring and summer months. Also, some of my measurements were made in direct sunlight while others were not. In my future research, I would like to explore this variable further by taking all measurements using a shaded thermometer. I would also like to test other coolers and types of roofing materials. Finally, I would also like to test the air temperatures one foot above the structures to see if temperatures vary at that level.

Bibliography
Akbari, H., M. Pomerantz, and H. Taha. "Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas." Solar Energy 70 (2001): 295-310.
Gaffin, S., C. Rosenzweig, L. Parshall, D. Beattie, R. Berghage, G. O'Keefe, and D. Braman. "Energy balance modeling applied to a comparison of white and green roof cooling efficiency," in Proceedings of the Third North American Green Roof Conference: Greening Rooftops for Sustainable Communities, Washington, D.C. (2005): 583-597.
Konopacki, S., and H. Akbari. "Energy savings of heat-island reduction strategies in Chicago and Houston, including updates for Baton Rouge, Sacramento, and Salt Lake City." Lawrence Berkeley National Laboratory Report 49638 (2002): Berkeley, CA.
Konopacki, S., H. Akbari, L. Gartland, and L. Rainer. "Demonstration of energy savings of cool roofs . " Lawrence Berkeley National Laboratory Report LBNL-4067 (1998): Berkeley, CA.
Parker, D.S., J.R. McIlvaine, J.R. Barkaszi, D.T. Beal, and M.T. Anello. "Laboratory testing of the reflectance properties of roofing materials." Florida Solar Energy Center FSEC-CR670-00 (2000): Cocoa, Florida.
Sailor, D.J. "Simulated urban climate response to modification in surface albedo and vegetative cover." Journal of Applied Meteorology 34 (1995): 1694-1704.
Simpson, J.R., and E.G. McPherson. "The effects of roof albedo modification on cooling loads of scale model residences in Tucson, Arizona." Energy and Buildings 25 (1997): 127-137.
Streutker, D.R. "Satellite-measured growth of the urban heat island of Houston, Texas." Remote Sensing of Environment 85 (2003): 282-289.
Wang, Wei-Chyung, and Peter H. Stone. "Effect of Ice-Albedo Feedback on Global Sensitivity in a One-Dimensional Radiative-Convective Climate Model . " Journal of the Atmospheric Sciences 37.3 (1980): 545-552.
Heat Island Effect Glossary. U.S. Environmental Protection Agency. Retrieved from the World Wide Web on 19 March 2007. www.epa.gov/hiri/resources/glossary.html.

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