Thermal Resistance of Enclosed Reflective Airspaces in Building Applications

Many parts of the building envelopes contain enclosed airspaces. The thermal resistance (R-value) of an enclosed airspace depends on the emissivity of all surfaces that bound the airspace, the size and orientation of the airspace, the direction of heat transfer through the airspace, and the respective temperatures of all surfaces that define the airspace. Assessing the energy performance of building envelopes requires accurate determination of the R-values of enclosed airspaces. In this paper, a comprehensive review about the thermal performance of enclosed airspaces is conducted. This review includes the computational and experimental methods for determining the effective R-value of enclosed reflective airspaces. Also, the different parameters that affect the thermal performance of enclosed airspaces are discussed. Finally, practical correlation for determining the R-values of enclosed airspaces of different inclination angles and directions of heat flow as a function of all parameters that affect the thermal performance of the enclosed airspaces, namely: average temperature, temperature differential, aspect ratio, and effective emittance is provided. This correlation can be used by modelers, building designers, and architects in the design for thermal resistance of building enclosures. As well, this correlation can be implemented in the currently available energy simulation models (e.g. ESP-r, Energy Plus, DOE, etc.).

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