Thermal
Mean Radiant Temperature
MRT
The area-weighted average temperature of the surfaces around a person, which governs how much heat the body exchanges by radiation. It is not what a thermostat measures.
In more detail
Mean radiant temperature is the area-weighted average temperature of the surfaces surrounding a person, weighted by how much of their field of view each surface occupies. It governs radiant heat exchange between the body and the room, which accounts for a large share of a sedentary person's heat loss indoors.
It is not what a thermostat measures. A thermostat reads air temperature at one point on one wall. A person seated beside a large window on a winter night is exchanging radiation with a surface far colder than that air and losing heat accordingly: the air can be at 70°F while the experience is cooler. The same works in reverse near a sun-warmed floor or a heated surface.
It is measured with a globe thermometer, which responds to radiation and air temperature together, or calculated from surface temperatures and view factors. ASHRAE Standard 55 uses it as an input, most directly through operative temperature, which combines it with air temperature into a single figure.
Why it matters
Mean radiant temperature explains comfort complaints that thermostat settings cannot resolve, and it is why large glazing areas, uninsulated corners and thermally bridged slab edges produce complaints in houses that are nominally at temperature. It also explains why radiant floors and high-performance windows feel better than their energy numbers suggest: both raise the temperature of surrounding surfaces rather than of the air.
The common misunderstanding
That air temperature is what people feel. Raising air temperature to compensate for a cold radiant environment works poorly and costs more, because the body keeps losing heat to the cold surface. The more effective correction is to the surface itself — better glazing, insulation, shading, or a warm surface nearby.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.