A person feels very comfortable in his house in light clothing when the thermostat is set...

A person feels very comfortable in his house in light clothing when the thermostat is set at 22°C and the mean radiation temperature (the average temperature of the surrounding surfaces) is also 22°C. During a cold day, the average mean radiation temperature drops to 18°C. To what level must the indoor air temperature be raised to maintain the same level of comfort in the same clothing?