The Mean Radiant Temperature (MRT) indoors is an important factor in determining if a home is comfortable or not. The MRT is essentially a measure of the average temperature of all the objects in a space, including the walls, windows, furniture, people etc. It, along with the ambient dry air temperature, determines how thermally comfortable a space is. A newer, well-insulated building with high-performance windows will have a higher MRT than a conventional older building.
The importance of the MRT can be illustrated by something we’ve all experienced. In an older home in the middle of winter, the ambient air temperature indoors can be 68 to 70 degrees, and yet we feel cold when we’re near a window. That’s because the cold interior pane of glass in that window is literally sucking warmth away from us, or, to put it more accurately, our bodies are emitting heat to that cold surface, which causes us to feel cold. This is the inverse of the experience of feeling warmed by the sun outdoors on a cold winter day. Our skin has high emissivity and absorptivity, meaning we’re very sensitive to radiant heat loss and gain.
You might have noticed that heat registers in older buildings are typically located along the exterior wall, at windows – that’s precisely to counteract this effect. In a high-performance building, such as a Passive House, the MRT will be higher. Their triple-glazed windows will ensure the interior pane of glass is warm, so you’ll be comfortable next to them even on a cold day. This leads to other benefits – for example, because the heating system doesn’t have to counteract the cold glass effect, the conditioned air can be delivered to the living spaces on the inboard side of the rooms, rather than at the exterior walls. In this way the size (both of the heating equipment, and the size and length of ductwork) and complexity of the heating system can be drastically reduced, thus driving down the cost of the mechanical equipment too.
We’re all well aware of the effect of air temperature on comfort, but once you’re cognizant of the importance of Mean Radiant Temperature on comfort you’ll become more aware of it too.
