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TRENDS

Home Energy Magazine Online November/December 1998

TRENDS

Saving Energy with Reflective Roofs

A white cementatious coating is applied to a flat gravel roof in Cocoa, Florida (site 9).

A white coating is applied to the roof of a double-wide trailer in Cape Canaveral (site 8). The measured space cooling energy reduction was 22%.

In 11 Florida homes with roofs coated with a white material during midsummer, average air conditioning energy use decreased by 8 kWh/day, or 19%. This was the conclusion of a series of case studies that the Florida Solar Energy Center conducted in Florida over a period of four years. The case studies measured the effect of increasing the roof surface solar reflectance on air conditioning energy use.

The results of six of the case studies (sites 0-5) were previously reported in Home Energy ("Saving Energy With Reflective Roof Coatings," May/June '94, p. 15). We conducted these studies on buildings with asphalt shingle, modified bitumen, gravel, or tile roofs. The final five case studies (sites 6-10), discussed here, were conducted on houses with shingle, gravel, or metal roofs.

It is important to note that, while we did so for experimental purposes, we do not recommend painting or coating a conventional shingle roof white. The numerous seams in an asphalt shingle roof make it possible for water to accumulate under the shingle edges, particularly in humid climates. With a dark-colored shingle roof, water that has accumulated evaporates the next time the shingles heat up. With a white-coated roof, the shingles tend not to heat up enough to fuel water vaporization--leading to potential moisture damage.

Currently, the only proven residential reflective roofing systems (with a reflectance greater than 65%) are white tile or white metal roofs, although a white shingle roof is a marginally better choice than a dark one (reflectance is approximately 25% for white tile, as opposed to 10% for dark).

Paint, but Don't Insulate

One important finding was that, for homes with attic ducts (10 of the 11 houses), savings decreased with increasing ceiling insulation. The houses with R-19 or R-25 insulation had savings ranging from 1 to 4 kWh/day (a 2% to 13% reduction). The houses with R-11, R-7, or no insulation had even higher savings--8 to 15 kWh/day (22% to 43%).

Of the 11 homes, the 3 with the highest midsummer savings had either no attic insulation (15 kWh/day and 900W, 14 kWh/day and 900W) or R-7 insulation (12 kWh/day and 1,000W). Measurements and infrared thermography showed that much of the savings resulted from thermal interactions between the duct system and the attic space.

Following is a description of the new study sites.

Site 6

This home was located in Palm Bay and had a shingled roof, R-19 attic insulation, and ducts located in the attic. The reflectance of the dark roof was measured at 0.15. After a white coating was applied, the reflectance increased to 0.59, reducing absorbed solar energy by slightly more than half. Average midsummer daily air conditioning electricity use decreased from 34 to 31 kWh for a savings of 3 kWh, or 10%.

Site 7

This home was also located in Palm Bay and like site 6 had a shingled roof, R-19 attic insulation, and attic ducts. The reflectance of the dark roof was measured at 0.22. After a white coating was applied, it increased to 0.64, cutting absorbed solar energy from 78% to 36%. Average midsummer daily air conditioning electricity use decreased from 41 to 40 kWh for a savings of 1 kWh, or 2%. The reason for the small savings was that internal appliance use and occupancy increased in the post-retrofit period.

Site 8

This building was a double-wide manufactured home located in Cape Canaveral. It had a metal roof, R-11 attic insulation, and attic ducts. The reflectance of the dark roof was not obtained, but after a white coating was applied, it was measured at 0.64. Average midsummer daily air conditioning electricity use decreased from 35 to 27 kWh for a savings of 8 kWh, or 22%.

Site 9

This home was located in Cocoa and had a gravel roof, R-19 attic insulation, and attic ducts. The reflectance of the dark roof was measured at 0.21. After a white coating was applied, it increased to 0.63, reducing absorbed solar energy from 79% to 37%. Average midsummer daily air conditioning electricity use decreased from about 32 to 28 kWh for a savings of about 4 kWh, or 13%.

Site 10

This home was located in Cocoa Beach and had a flat gravel roof, no attic insulation, and attic ducts. The reflectance of the dark roof was measured at 0.25. After a white coating was applied, it increased to 0.64, reducing absorbed solar energy from 75% to 36%. Average midsummer daily air conditioning electricity use decreased from 53 to 39 kWh for a savings of 14 kWh, or 26%.

So What Does This Mean in Missouri?

Knowing that our experimental results were isolated to our experiences in Florida and related experiments in California, we needed a method of analyzing what reflective roofs could do elsewhere. Obviously, in a very cold climate, with little need for air conditioning, a reflective roof might not make sense.

To get at that question, we created a special version of the DOE-2.1E simulation program that solves a major deficiency of that model: It better calculates attic thermal performance and the interactions of duct systems that are often located there. This simulation model was then successfully compared with measured data from the eleven Florida locations, yielding very similar results to the monitored effects. After that point, we took the simulation model and created residential prototypes for both new and existing construction in 14 very different climates around the U.S. The model then was used to determine how reflective roofing will affect heating and cooling energy use in these locations.

Simulation Results

In all locations, reflective roofs reduced space cooling varying from 13% for new construction in St. Louis to 58% for existing residences in Los Angeles. Heating consumption was increased only slightly, from 3% in Miami to 6% in San Francisco. Still, the increase in heating was enough to increase combined heating and cooling costs in some colder places, such as Detroit and Seattle.

However, in the Sunbelt region (Los Angeles, Atlanta, Houston, Ft. Worth, Fresno, Miami and Phoenix) the annual energy savings from a new reflective roof in new construction was greater than adding another increment of R-11 ceiling insulation to current practice, and in all these places the combined heating and cooling costs were reduced. The highest savings was in the hottest and sunniest location: Phoenix, Arizona.

Conclusions

Except in northernmost locations and in cool, cloudy locations, the combined cost of heating and cooling is lower for houses with reflective roof surfaces than for houses with conventional roofs. While white roofing is effective at reducing cooling loads in all locations, it works best in hot sunny climates, in houses in which there is little or no attic insulation and where the ducts are located in the attic. Homeowners in any location who wish to cut down on their cooling bills can adopt reflective roofing; for those in hot sunny climates, white roofs are highly recommended.

--Steven Konopacki
and Danny Parker

Steven Konopacki is a principal research associate at Lawrence Berkeley National Laboratory. Danny Parker is a research scientist at the Florida Solar Energy Center.

The full report is detailed in the ASHRAE paper: D. Parker, Y.J. Huang, S.J. Konopacki, L.M. Gartland, J.R. Sherwin and L. Gu, 1998. "Measured and Simulated Performance of Reflective Roofing System in Residential Buildings," ASHRAE Transactions, Vol. 108, Pt. 1.

The paper can be obtained from the Florida Solar Energy Center, 1679 Clearlake Rd., Cocoa, FL 32922 Tel: (407) 638-1405; FAX (407) 638-1439. Web site: www.fsec.ucf.edu/~bdac.

Table 1. Results of Field Tests of Reflective Roofing Systems in five Florida Homes

Test Site Reflectance Before Reflectance After Cooling Energy Use Before (kWh/day) Cooling Energy Use After (kWh/day) Cooling Energy Use Savings (kWh/day) Reduction in Utility Coincident Peak Demand (W)

Site 6
white coating on shingles, R-19 attic insulation, attic ducts 0.15 0.59 34 31 3 (10%) 350 (16%)

Site 7
white coating on shingles, R-19 attic insulation, attic ducts 0.22 0.64 41 40 1 (2%) 300 (12%)

Site 8
white coating on metal roof, R-11 attic insulation, attic ducts N/A 0.64 35 27 8 (22%) 200 (12%)

Site 9
white coating on gravel roof, R-19 attic insulation, attic ducts 0.21 0.63 32 28 4 (13%) 220 (11%)

Site 10
white coating on gravel roof, no attic insulation, attic ducts 0.25 0.64 53 39 14 (26%) 920 (29%)

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