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Abstract
This paper presents an analysis of the effect of roof solar reflectance on the annual heating (cooling) loads, and roof temperatures of residential buildings. The annual loads, peak loads, and exterior roof temperatures for a small compact ranch house are computed. The thermal performance requirments for the thermal envelope of the residence are based on prescriptive criteria given in ASHRAE Standard 90.2-1993. The residential models, with minor modifications in the thermal envelope for different locations are subjected to hourly weather data for one year compiled for the following locations: birmingham, AL; Bismark, ND; Miami, FL; Phoneix, AZ; Portland, ME and Washington, DC. Building loads have been determined for a full factorial experimental design that varies the following parameters of the residential model: solar reflectance of the roof, ceiling thermal resistance, attic ventilation, and attic mass framing area. The effect of each parameter is ranked highest to lowest for effect on annual heating and cooling loads, and peak heating and cooling loads. A parametric study plots the building loads as a function of roof solar reflectance for different levels of ceiling thermal resistances for each geographic location. An analaysis of building loads is presented for the building model conditioned under a transient solar profile and steady ambient conditions of either -1.1C for heating, or 26.7C for cooling. The time of th year is specifically selected to provide equal hours of night and day. The site location for the model is arbitrarily selected. Values for roof solar reflectance are varied from 0.1 to 0.8 and the ceiling thermal resistance, from "uninsulated" to R-6.7 Hourly predicted profiles are presented for the cooling load, attic air temperature, and outside roof heat flux. The results are in agreement with the annual loads computed for hourly weather data and indicate that radiative cooling of the structure at night is significant. The presence of thermal insulation in the ceiling reduced the effect. Additional simulations are conducted for eqch location to determine the predicted exterior roof temperatues of the residential model for one year of weather data. For these simulations only the roof solar reflectance is varied. the other building parameters are fixed at "base" levels as specificed by ASHRAE Standard 90.2-1993. The effect of solar reflectance is presented graphically using a box plot to sumarize statistically all 8760 exterior roof temperaturs. The results indicate a signifgicant reduction in peak room temperatures is possible for high levels of solar reflectance. Additional plots for the daily roof temperature profiles for a typical summer day and average montly temperatures for one year are also presented. Included in this report is a simple economic analysis that examines cost savings for each geographic location. The estimated annual energy costs for electic and gas heating are plotted versus roof solar reflectance for different levels of ceiling thermal resistance. For a residence without attic insulation in a hot climate, substantital savings are available by making the roof more reflective. At hight levels of ceiling thermal resistance, the savings are less
This paper presents an analysis of the effect of roof solar reflectance on the annual heating (cooling) loads, and roof temperatures of residential buildings. The annual loads, peak loads, and exterior roof temperatures for a small compact ranch house are computed. The thermal performance requirments for the thermal envelope of the residence are based on prescriptive criteria given in ASHRAE Standard 90.2-1993. The residential models, with minor modifications in the thermal envelope for different locations are subjected to hourly weather data for one year compiled for the following locations: birmingham, AL; Bismark, ND; Miami, FL; Phoneix, AZ; Portland, ME and Washington, DC. Building loads have been determined for a full factorial experimental design that varies the following parameters of the residential model: solar reflectance of the roof, ceiling thermal resistance, attic ventilation, and attic mass framing area. The effect of each parameter is ranked highest to lowest for effect on annual heating and cooling loads, and peak heating and cooling loads. A parametric study plots the building loads as a function of roof solar reflectance for different levels of ceiling thermal resistances for each geographic location. An analaysis of building loads is presented for the building model conditioned under a transient solar profile and steady ambient conditions of either -1.1C for heating, or 26.7C for cooling. The time of th year is specifically selected to provide equal hours of night and day. The site location for the model is arbitrarily selected. Values for roof solar reflectance are varied from 0.1 to 0.8 and the ceiling thermal resistance, from "uninsulated" to R-6.7 Hourly predicted profiles are presented for the cooling load, attic air temperature, and outside roof heat flux. The results are in agreement with the annual loads computed for hourly weather data and indicate that radiative cooling of the structure at night is significant. The presence of thermal insulation in the ceiling reduced the effect. Additional simulations are conducted for eqch location to determine the predicted exterior roof temperatues of the residential model for one year of weather data. For these simulations only the roof solar reflectance is varied. the other building parameters are fixed at "base" levels as specificed by ASHRAE Standard 90.2-1993. The effect of solar reflectance is presented graphically using a box plot to sumarize statistically all 8760 exterior roof temperaturs. The results indicate a signifgicant reduction in peak room temperatures is possible for high levels of solar reflectance. Additional plots for the daily roof temperature profiles for a typical summer day and average montly temperatures for one year are also presented. Included in this report is a simple economic analysis that examines cost savings for each geographic location. The estimated annual energy costs for electic and gas heating are plotted versus roof solar reflectance for different levels of ceiling thermal resistance. For a residence without attic insulation in a hot climate, substantital savings are available by making the roof more reflective. At hight levels of ceiling thermal resistance, the savings are less
Date
10/1998
10/1998
Author(s)
Robert Zarr
Robert Zarr
Page(s)
76
76
Keyword(s)
ASHRAE; attic; cooling; energy; heat transmission; load; reflectance; temperature; ventilation
ASHRAE; attic; cooling; energy; heat transmission; load; reflectance; temperature; ventilation