Mean Radiant Temperature (MRT): Thermal Comfort Parameter

Mean radiant temperature (MRT) represents the uniform temperature of an imaginary enclosure in which radiant heat exchange between an occupant and surrounding surfaces equals that in the actual environment. Unlike air temperature, MRT captures the influence of warm or cold surfaces, solar gains, and radiant systems on human comfort. This article defines MRT, reviews its history, explains measurement and calculation methods, and demonstrates its role in building design, HVAC controls, and health and safety protocols.

Use MRT insights alongside heat-stress indices like WBGT, ventilation metrics such as SFP, and comfort calculators including the PMV/PPD tool to maintain healthy indoor environments.

Definition and Governing Equations

Radiant heat exchange fundamentals

MRT is derived from the Stefan–Boltzmann law, which states that radiant heat exchange between two surfaces depends on the fourth power of absolute temperature and the view factor between them. For a person approximated as a small sphere, MRT can be calculated using T_r = [Σ F_p,i · T_i⁴]^1/4, where F_p,i represents the angle factor between the person and surface i, and T_i is the surface temperature in kelvin. Standards often use simplified approximations, such as averaging surface temperatures weighted by area or using globe thermometer readings.

Relationship to operative temperature

Operative temperature combines air temperature and MRT, weighting each by their relative contributions to heat transfer. In still air, the operative temperature is approximately the average of air temperature and MRT. When air speed increases, convective heat transfer gains importance, reducing the weight of MRT. Understanding these relationships informs design strategies that balance radiant and convective systems to meet comfort targets defined in ISO 7730 and ASHRAE Standard 55.

Historical Development

Early research and standards adoption

The concept of MRT emerged from 1930s research by Bedford and colleagues investigating thermal comfort in workplaces. The Fanger comfort model, developed in the 1960s, incorporated MRT as a key variable influencing predicted mean vote (PMV). Standards such as ISO 7726 formalised MRT measurement techniques, specifying instrument accuracy, response time, and environmental conditions to ensure consistent data across laboratories and field studies.

Integration into building certification systems

Modern green building programmes—LEED, WELL, BREEAM—reference MRT when evaluating radiant systems, occupant thermal comfort, and daylighting strategies. Thermal comfort credits often require documenting MRT alongside air temperature, humidity, and air speed, demonstrating compliance with occupant satisfaction thresholds. This trend encourages design teams to model MRT during early concept stages rather than treating it as an afterthought.

Measurement Techniques

Globe thermometers and sensor arrays

The traditional method for measuring MRT uses a globe thermometer—a temperature sensor placed at the centre of a matte black sphere. ISO 7726 specifies globe diameters (typically 150 mm) and emissivity requirements to mimic human thermal response. Smaller data loggers, infrared thermography, and distributed surface sensors now supplement globe measurements, enabling high-resolution MRT mapping in complex spaces like atriums or manufacturing halls.

Computational modelling and building simulations

Building energy simulation tools calculate MRT by evaluating surface temperatures from heat balance equations. Radiant system designers use these models to predict comfort impacts of hydronic slabs, panels, or chilled beams. Coupling simulations with the seasonal performance factor calculator verifies that equipment selections achieve desired MRT profiles across seasons.

Applications

Building design and retrofit decisions

Architects and engineers evaluate MRT when selecting glazing, shading, and insulation to avoid cold downdrafts or radiant asymmetry. Radiant heating and cooling systems leverage MRT to maintain comfort at lower air temperatures, reducing energy use and improving alignment with high COP heat pumps. Post-occupancy evaluations track MRT to diagnose complaints and fine-tune controls.

Health, safety, and industrial hygiene

MRT influences heat-stress assessments in manufacturing plants, foundries, and outdoor venues. Pairing MRT data with WBGT calculations informs work/rest cycles, hydration policies, and protective equipment selection. Healthcare and laboratory environments monitor MRT to maintain sterility while protecting staff from radiant heat emitted by equipment or process loads.

Importance and Future Directions

Smart sensors and adaptive comfort

Emerging IoT sensors capture MRT, air temperature, humidity, and illuminance simultaneously, feeding adaptive comfort algorithms. These systems adjust set points based on occupant preferences, occupancy patterns, and energy costs, ensuring MRT stays within acceptable ranges while minimising HVAC energy.

Integrating MRT into resilience planning

As climate change intensifies heatwaves, resilience plans consider how passive design and backup systems maintain acceptable MRT during power outages. Pairing MRT modelling with the heat pump balance point calculator helps evaluate whether buildings can sustain safe conditions under reduced mechanical operation.