Mean radiant temperature (MRT) is the uniform temperature of an imaginary enclosure with which an occupant would exchange the same radiant heat as the actual non-uniform environment. It accounts for radiation from warm and cool surfaces like windows, walls, and equipment. MRT significantly influences thermal comfort, especially in spaces with large surface temperature variations.
Technical Details
MRT is calculated by weighting the temperatures of surrounding surfaces by their emissivity and view factor from the occupant’s position. A single cold window can lower MRT by 5 to 10°F (2.8 to 5.6°C) in its vicinity. Standards like ASHRAE 55 specify acceptable MRT differences of no more than 14°F (7.8°C) between head level and ankle level to prevent discomfort. Thermal imaging and globe thermometers measure MRT in field conditions.
Applications in HVAC Design
Radiant heating systems raise MRT by warming floor, wall, or ceiling surfaces, allowing lower air temperatures while maintaining comfort. High-performance windows reduce MRT asymmetry near building perimeters. Interior surface finishes and coatings affect radiative properties.
Practical Significance
Addressing MRT improves comfort near problem areas without excessive air temperature adjustments, reducing energy costs.