Measured temperature reductions and energy savings from a cool tile roof on a central California home
Introduction
The number and size of air-conditioned homes in hot climates has risen significantly over the past 20 years, increasing U.S. residential cooled floor area by 71% [1]. Boosting the albedo (solar reflectance) of a building's roof can save cooling energy in summer by reducing solar heat gain, lowering roof temperature, and decreasing heat conduction into the conditioned space and the attic ducts. It may also increase the use of heating energy in winter. Prior research has indicated that net annual energy cost savings are greatest for buildings located in climates with long cooling seasons and short heating seasons, especially those buildings that have distribution ducts in the attic [2], [3], [4], [5], [6], [7].
Solar-reflective “cool” roofs decrease summer afternoon peak demand for electricity [3], [8], [9], reducing strain on the electrical grid and thereby lessening the likelihood of brownouts and blackouts. Reducing peak cooling load can also allow the installation of a smaller, less expensive air conditioner. This is referred to as a “cooling equipment” saving [9]. Smaller air conditioners are also typically less expensive to run, because air conditioners are more efficient near full load than at partial load.
Roofs can cover a substantial fraction of the urban surface. For example, when viewed from above the tree canopy, roofs comprise about 19–25% of each of four U.S. metropolitan areas—Chicago, IL; Houston, TX; Sacramento, CA; and Salt Lake City, UT [10]. Citywide installation of cool roofs can lower the average surface temperature, which in turn cools the outside air. A meta-analysis of meteorological simulations performed in many U.S. cities found that each 0.1 rise in urban albedo (mean solar reflectance of the entire city) decreases average outside air temperature by about 0.3 K, and lowers peak outside air temperature by 0.6–2.3 K [11]. Cool roofs thereby help mitigate the “daytime urban heat island” by making cities cooler in summer. This makes the city more habitable, and saves energy by decreasing the need for air conditioning in buildings. Cooler outside air can also improve air quality by slowing the temperature-dependent formation of smog [12], [13].
Replacing a hot roof with a cool roof immediately reduces the flow of thermal radiation into the troposphere (“negative radiative forcing”), offsetting the global warming induced by emission of greenhouse gases [14], [15], [16]. Most recently, Akbari et al. [17] estimated that increasing by 0.01 the albedo of 1 m2 of urban surface provides a one-time (not annual) offset of 4.9–12 kg CO2. Substituting 100 m2 of cool white roofing (albedo 0.6) for standard gray roofing (albedo 0.2) would provide a one-time offset of about 20–48 t CO2.
The direct cooling benefits of increasing the albedo of a residential roof have been simulated or measured by several workers. For example, Akbari et al. [3] simulated with the DOE-2 building energy model the annual cooling and heating energy uses of a variety of building prototypes in 11 U.S. cities. They found that raising the albedo of an RSI-3.3 asphalt-shingle roof by 0.30 reduced the annual cooling energy use of a single-story home by 6–15%, and increased annual heating energy use by 0–5%.
Parker and Barkaszi [18] measured daily cooling energy uses in summer before and after applying white roof coatings to nine single-story Florida homes. Savings ranged from 2 to 40% and averaged 19%. In a home with RSI-3.3 ceiling insulation, increasing the albedo of an asphalt shingle roof by 0.44 (to 0.59 from 0.15) reduced daily cooling energy use by 10%, and lowered peak cooling power demand by 16%.
Miller et al. [19] measured cooling energy uses in three pairs of Northern California homes. Each pair of homes had color-matched standard (lower albedo) and cool (higher albedo) roofs. The first pair had brown concrete tile roofs with albedos of 0.10 (standard) and 0.40 (cool); the second, brown metal roofs with albedos of 0.08 (standard) and 0.31 (cool); and the third, gray-brown shingle roofs with albedos of 0.09 (standard) and 0.26 (cool). After adjusting for widely disparate occupancy patterns, summer daily cooling energy savings were estimated to be about 9% in the homes with the cool tile and cool metal roofs; savings for the cool shingle roof were unclear.
High thermal capacitance and/or subsurface natural convection (“above-sheathing ventilation”) in the roof system can further cool the building [20], [21], [22], [23]. For example, Miller and Kosny [24] measured the summer daily heat flows through an SR 0.13 flat tile roof on double battens and through an SR 0.09 shingle roof, each installed over a modestly insulated (RSI-0.9) ceiling in a test assembly. The heat flow through the tile roof was only half that through the shingle roof, even though the solar absorptance (1 – solar reflectance) of the tile was only 4% lower than that of the shingle. Note that above-sheathing ventilation (air flow in the space between sheathing, or roof deck, and the roofing product) is usually driven by buoyancy, rather than wind, because building codes typically require the air space at the eave (bottom edge) of the roof to be closed for fire protection [25].
Two of the most popular roofing product categories in the western U.S. residential roofing market are fiberglass asphalt shingles (hereafter, “shingles”) and clay or concrete tiles (hereafter, “tiles”). Surveys by Western Roofing Insulation & Siding found that shingles and tiles comprised 50% and 27% of 2007 sales, respectively, and 63% and 14% of projected 2013 sales [26], [27]. Substituting a light-colored tile for a dark asphalt shingle reduces the roof's solar heat gain, roughly doubles its thermal capacitance [28], and provides above-sheathing ventilation. In a mild-winter climate where heating is needed primarily in the morning, this substitution may even decrease heating energy use in winter. This is possible because increasing the roof's thermal capacitance keeps the attic warmer overnight, while high roof albedo has little consequence after sunset.
The present study compares two side-by-side, single-story, single-family houses in Fresno, California. Fresno is located in the state's Central Valley, a hot climate in which homes use air conditioning from approximately May to October. The first house has a standard dark asphalt shingle roof, and the second a cool concrete tile roof; they are otherwise quite similar in construction and use. The homes serve as show models and are open to the public every day from 09:00 to 17:00 local time (LT). By monitoring temperatures, heat flows, and energy consumption in these air-conditioned houses, we investigate the extents to which over the course of a year the cool roof reduces (a) roof and attic temperatures; (b) conduction of heat into the conditioned space and into HVAC ducts in the attic; (c) cooling and heating energy uses; and (d) peak-hour power demand. We also compare measured cooling energy savings to cooling energy savings calculated from heat flow and temperature measurements, in order to evaluate whether a simplified experimental configuration without power meters can be used in future cool roof experiments.
Section snippets
Theory
While the tested homes share similar floor and elevation plans, differences other than roof construction, such as those in plug load (appliances and lights), fenestration (window area, orientation, construction, and coverings), and occupancy, can influence building conditioning energy use. Here, we derive two ways to isolate the energy savings attributable to the cool roof.
Overview
Temperatures, heat flows, and HVAC (compressor + fan) energy uses are compared over the course of 12 months in two adjacent and similar homes in California's Central Valley, one with a standard roof and the other with a cool roof. Monthly rates of natural gas use for heating are obtained from utility statements.
Cool roof energy savings in the cooling and heating seasons are computed via both Method A (difference in ceiling + duct heat gain, divided by COP or AFUE) and Method B (difference in HVAC
Weather
6 July 2012 and 21 January 2013 were selected as representative sunny days in summer and winter, respectively. The maximum and minimum outside air temperatures on 6 July 2012 were similar to the average maximum and minimum values on July 6 from 1995 through 2011. However, the maximum outside air temperature on 21 January 2013 (sunny) exceeded the historical average for that day of year, because winter days in Fresno are often cloudy or rainy [40], [41]. On the summer day, about 2 weeks after
Cooling and heating energy savings
Following Method B, the cool home with the reflective tile roof (initial SR 0.51; thermal capacitance 40 kJ/m2·K) used 26% less annual cooling (compressor + fan) energy, 4% less annual heating fuel energy, and 3% less annual heating fan energy than the standard home with the dark shingle roof (initial SR 0.07; thermal capacitance 22 kJ/m2·K).
The Fresno home's fractional annual cooling energy savings (26%) were 2.6 times the 10% daily cooling energy savings that Parker and Barkaszi [18] measured
Summary
Temperatures, heat flows, and energy uses were measured for a year in two side-by-side, single-story, single-family homes in Fresno, California. One house had a reflective concrete tile roof (initial SR 0.51; thermal capacitance 40 kJ/m2·K), and the other a standard dark asphalt shingle roof (initial SR 0.07; thermal capacitance 22 kJ/m2·K). The flat tiles were mounted on battens, creating an air gap between tile and deck; the shingles were affixed directly to deck. The buildings were otherwise
Acknowledgements
This work was supported by the California Energy Commission (CEC) through its Public Interest Energy Research Program (PIER). It was also supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, State, and Community Programs, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We wish to thank Michael Spears, Woody Delp, and Charlie Curcija (Lawrence Berkeley National Laboratory); Victor Gonzalez, Tony Seaton, Terry
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2021, Energy and BuildingsCitation Excerpt :For example, there are well over 100 such studies just for cool envelope materials [183]. Individual-measure benefits explored in California (the venue of the current work) include those of cool roofs [34,184-194], cool walls [185,195,196], advanced windows [197,198], and shading [70]. From the perspectives of policy makers and residents, it is important to estimate the benefits and potential drawbacks of the proposed solutions quantitatively before selecting and applying them to DACs.