We all learned growing up that “heat rises”. This isn’t totally accurate, but “hot air” does rise. In general this is a good thing, especially for the hot air balloon industry. But in a home, it is problematic.
When hot air is pumped through a duct, it is typically between 120–140ºF (49–60ºC). This hot air rapidly rises and creates a heated layer of air near the ceiling. Stratification increases as the ceiling height increases, and temperature differences can be easily felt by those living in a two-story home, with the downstairs too cool and the upstairs too hot. When ceilings are hot and just below a cold roof, heat loss increases dramatically. In fact, this is precisely why ceiling and attics require so much insulation.
Does your home have high ceilings? You will spend a great deal of energy heating the space above your head. If you are in a two-story home, you will work to find a balance between being too warm upstairs and too cool downstairs.
Radiant floors stratify much less for two reasons. First, at least 50% of the heat is infrared, a form of invisible light. Like all lighting, its effect is greatest the closer you are to the source. In other words, it concentrates much of its output beneath the floor, where you are, not above you near the ceiling. Second, because the temperature of a radiant floor is quite mild (75-80ºF / 24-27ºC), it only warms air into the mid-70s, far less than the 120-140ºF (49-60ºC) temperatures of a forced air system. It is not unusual for the ceilings in a radiant home to be 10-20 degrees cooler than a forced air home.