Analytical Study of Residential Buildings With Reflective Roofs
Robert R. Zarr
Annual heating (cooling) loads, peak heating (cooling) loads, and exterior roof temperatures for a small compact ranch house are computed using the Thermal Analysis Research Program (TARP) developed at the National Institute of Standards and Technology. The thermal performance requirements for the thermal envelope of the residence are based on prescriptive criteria in the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE) Standard 90.2-1993. The residential model, with minor modifications in the thermal envelope for different locations, is subjected to hourly weather data compiled in the Weather Year for Energy Calculations for in the following locations: Birmingham, Alabama; Bismarck, North Dakota; Miami, Florida; Phoenix, Arizona; Portland, Maine; and, Washington, D.C. Building loads are determined following a full factorial experimental design that varies the following parameters of the residence: solar reflectance of the roof, ceiling thermal resistance, attic ventilation, and attic mass framing area. Values of solar reflectance for the roof are varied from 0.1 to 0.8; ceiling thermal resistance, from uninsulated to R-8.6 m2 K/W (R-49 h ft2 F/Btu); attic ventilation, from 0.5 h-1 to 9.2 h-1; and, attic mass framing area from 31.0 m2 to 46.5 m2. Results of the annual heating (cooling) loads and peak heating (cooling) loads are illustrated graphically, bothglobally for all cities and locally for each geographic location. Optimum settings for each parameter are determined. A parametric study is presented that plots building loads as a function of roof solar reflectance for different levels of ceiling thermal resistances. Additional simulations are conducted for each location to determine the predicted exterior roof temperatures of the residential model for one year (8760 h) of weather data. For these simulations only the roof solar reflectance is varied from 0.1 to 0.8. The other building parameters are fixed base levels. For example, the ceiling thermal resistance is fixed at levels in accordance with prescriptive criteria in ASHRAE Standard90.2-1993. The effect of roof solar reflectance is again presented graphically. A box plot is used to summarize statistically all 8760 exterior roof temperatures, indicating a significant reduction in peak roof temperatures is possible due to increased levels of solar reflectance. Additional plots for the daily roof temperature profiles for a typical summer day and average monthly temperatures for one year are also presented. In summary,the report presents a simple economic analysis that examines cost savings for each geographic location. Local residential utility rates for summer 1994 and winter 1994-1995 are obtained from the National Association of Regulatory Utility Commissioners. Energy costs are computed by assuming local residential performance efficiencies for electric and gas heating equipment. The estimated annual energy cost for electric and gas heating is presented graphically by plotting cost versus roof solar reflectance for different levels of ceiling thermal resistance. For a residence without attic insulation in a hot climate, substantial savings are available by making the roof more reflective. At higher levels of ceiling thermal resistance, the savings is less.
Analytical Study of Residential Buildings With Reflective Roofs, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=860767
(Accessed March 1, 2024)