Mean Radiant TemperatureEdit
Mean Radiant Temperature
Mean Radiant Temperature (MRT) is a fundamental concept in thermal comfort and building physics. It describes the effective radiant environment experienced by the human body, accounting for the temperatures of surrounding surfaces and their ability to exchange radiant heat with a person. In its formal sense, MRT is the uniform temperature of an imaginary enclosure that would yield the same net radiant heat exchange between a person and their surroundings as the actual, potentially nonuniform space. This makes MRT a key modifier of comfort beyond the simple air temperature, since radiant heat transfer depends on surface temperatures, emissivities, and geometry rather than on air temperature alone. MRT is closely linked to concepts such as radiative heat transfer, surface temperature, and the broader study of the thermal environment radiant heat transfer surface temperature.
In practical terms, MRT captures how walls, floors, ceilings, and objects radiate heat toward or away from the occupant. In a room with warm sunlit surfaces, MRT can be substantially higher than the ambient air temperature, creating a sensation of heat even when the air feels only mildly warm. Conversely, cool surface temperatures can make a space feel more comfortable than the air temperature would suggest. Because MRT integrates the radiative influence of all surrounding surfaces, it often provides a better predictor of how people actually perceive a space than air temperature alone. MRT is therefore central to assessments of thermal environments in residential design, office spaces, and industrial facilities, and it interacts with related measurements such as the operative temperature Te, which combines radiative and convective effects to reflect overall comfort thermal comfort operative temperature.
Definition and physical basis
Mean Radiant Temperature is defined through the heat balance between the human body and its radiative surroundings. The body gains or loses heat by radiation from all directions to surrounding surfaces, and MRT embodies the effective temperature of those surfaces as if they were at a single uniform temperature. Because radiative exchange depends on surface temperatures and their emissive properties, MRT provides a compact summary of the radiant aspects of the thermal environment. The concept is foundational in radiative heat transfer theory and is connected to fundamental ideas about emissivity, surface temperatures, and the spectral properties of materials. For readers exploring the physics behind MRT, see treatments of radiative heat transfer and surface temperature in general radiant heat transfer emissivity.
Measurement and estimation
Directly measuring MRT is challenging, because it requires knowledge of the distribution of surface temperatures and their radiative properties throughout a space. In practice, MRT is often estimated or inferred using instruments such as a globed sensor, commonly known as a globe thermometer, which integrates radiation from the surroundings and convective heat transfer with the air. A typical instrument is a hollow globe with a thermometer at its center; the globe temperature (T_g) reflects both air temperature (T_a) and the radiant environment, from which MRT can be inferred given assumptions about convective heat transfer coefficients and solar/visible radiation. See the globe thermometer for details on how this device is used to approximate MRT in field measurements globe thermometer.
Because occupational and comfort assessments frequently rely on standards and models, MRT is also estimated from climate data, surface temperature maps, or energy simulations in building models. In such contexts MRT serves as a parameter in predicting occupant comfort in conjunction with other factors like air velocity, humidity, activity level, and clothing insulation. References to radiative exchange with surfaces and to the calculation of MRT from measured surface conditions are common in building physics and thermal environment literature radiant heat transfer thermal comfort.
Relation to other concepts
MRT is a core component of the broader concept of thermal environment. The operative temperature Te is a related metric that combines MRT with air temperature to yield a single value representing the overall thermal exposure of a person. In many practical applications, Te can be approximated as a weighted combination of T_a and MRT, with the weights determined by surface properties, air movement, and the relative influence of convection versus radiation on heat exchange. This makes Te a convenient surrogate for evaluating comfort in spaces where gradients exist or where measurements are impractical. Occupant comfort research often uses MRT alongside other indices such as PMV and PPD to describe the likelihood of comfort across populations and climatic conditions operative temperature PMV PPD.
Applications and implications
In building design and HVAC planning, MRT informs decisions about surface finishes, shading, daylighting, and the use of radiant heating or cooling systems. Materials with high emissivity can increase radiative exchange and raise MRT in sunlit rooms, while low-emissivity coatings and proper insulation can help manage radiant heat transfer. Shading strategies, reflective surfaces, and control of interior surface temperatures are all tools to modulate MRT and thereby influence occupant comfort without excessive energy use. Standards and guidelines for thermal environments, such as those adopted by professional bodies and regulatory agencies, frequently cite MRT, Te, and related indices as key criteria for acceptable conditions in workplaces and residences thermal comfort building physics.
Controversies and debates
There is ongoing discussion among engineers, architects, and researchers about the best ways to measure and apply MRT in diverse environments. Critics of overly simplified approaches argue that MRT can be difficult to estimate accurately in spaces with complex radiation patterns, moving occupants, or strong solar inputs, leading to uncertainties in comfort assessments. While globe-based methods provide a practical means of estimation, they rely on assumptions about convection, surface properties, and geometry that may not hold in every setting. As a result, some researchers advocate for using Te or more detailed radiative models in certain environments, especially where solar gains or dynamic surface temperatures dominate the heat exchange. Standards bodies remain attentive to these issues, balancing practicality with fidelity in thermal comfort assessments. See discussions in the literature around the reliability of MRT estimations and the circumstances under which different metrics best predict occupant perception globe thermometer thermal comfort.
See also