Direct Sun and Occupant Comfort

Practical Applications of the CBE SolarCal Method in the Design Process





Direct sun on the body is a known potential cause of thermal discomfort for occupants, yet most thermal comfort simulations do not capture its effects, because the simulations run on engines that account for the effects of solar radiation on a space, but not the occupant within that space. The Center for the Built Environment (CBE) recently published a solar calculator (SolarCal) that establishes parameters to calculate the solar adjustment in mean radiant temperature (MRT) due to short-wave solar radiation on the body (dMRT). The SolarCal methodology accounts for the combined effect of the direct beam solar, diffuse solar from the sky, and reflected solar from surrounding surfaces. When integrated with existing thermal comfort analysis workflows, CBE’s calculations help paint a more accurate picture of the occupant thermal environment, which in turn helps set reasonable client expectations and influences building design development.

This study evaluates two thermal comfort analysis approaches that integrate dMRT calculations using the SolarCal model into existing thermal analysis workflows in practice. Each method was created for and applicable to different projects at different stages of design. The first approach is spreadsheet based, combining simulation results from an EnergyPlus model, epw data, spatial approximations, and Python scripting, to calculate annual hourly adjustments to hourly operative temperature. This approach is appropriate for early concept calculations and assists in setting reasonable client expectations and guides facade design. The second uses solar Radiance software simulations to account for complex geometry and sophisticated spatial conditions. The Radiance simulation approach proposed collapses the calculations established by SolarCal by simulating the incident radiation on a body. This approach accommodates more developed spaces and shading systems, which are regularly encountered in later design phases of a project. Both approaches account for the influence of direct sun on the human body, allowing this dynamic aspect of the thermal environment to become a performance metric that influences design decision making.


As energy performance requirements for today’s buildings become more stringent, design teams increasingly turn to passive design strategies and mixed-mode systems to maintain thermal comfort conditions in a space. And

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Thermal comfort is an extensively studied and regulated area of building design, but there are still factors that affect thermal comfort that are only now being incorporated into technical standards

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Two thermal comfort analysis methods were developed for spaces with varying complexity and different phases of design. The first method was applied to a space consistent in section, where solar

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Results & Explanation

Method 1

Figure 5: Top - Axonometric showing zone level operative temperatures for a point in time. Bottom - Annual hourly operative temperature results for top level.Figure 6: Top - Axonometric

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Conclusion and Future Work

The results emphasize the importance of solar radiation control methods, especially external shading and localized controls. The results derived from both approaches to calculating potential discomfort due to solar radiation

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Many thanks to Ed Arens for corresponding about the CBE approach to calculating dMRT.

Rights and Permissions

Arens, E., T. Hoyt, X. Zhou, L. Huang, H. Zhang and S. Schiavon. “Modeling the comfort effects of short-wave solar radiation indoors.” Building and Environment, 88 (2015), 3-9. http://

ASHRAE. “Standard 55-2013. Thermal environmental conditions for human occupancy.” American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), 2013.

ASHRAE. “Addendum g to Standard 55-2013. Thermal environmental conditions for human occupancy.” American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), 2016.

Hoyt, Tyler, Schiavon Stefano, Piccioli Alberto, Moon Dustin, and Steinfeld Kyle, “CBE Thermal Comfort Tool.” Center for the Built Environment, University of California Berkeley, 2013.,

Levitt, Brandon, M. Susan Ubbelohde, George Loisos, Nathan Brown, "Thermal Autonomy as Metric and Design Process." Proceedings from Pushing the Boundaries: Net positive Buildings, SB13 Vancouver, 2013.