Radiant Floor Heating - Floor Heat Types

Difference between in-floor and under-floor heating
The differences between a radiant system that is installed in a thermal mass, (commonly called in-floor), and a system that is installed under the floor are based on the storage of the heat energy.

In an in-floor system, the thermal mass can store heat energy like a battery. As the room loses heat energy through the walls, glass and ceiling, the floor can then radiate off an equal amount of energy to compensate. Once this energy is depleted, the temperature will drop and the thermostat will call for heat and fire the boiler system.

With an under-floor system, there is very little heat energy stored. The tubing is installed against the bottom of the floor joists and has to transfer the heat energy through the plywood using mainly conduction and convection heat transfer. The plywood is not a good thermal mass and can’t hold much heat energy. As the room loses heat energy the floor has no extra to give up so the thermostat has to call for heat much earlier. The net effect is that the boiler system has to run more frequently.

One other difference is that since the under-floor system has to “push” its energy through the plywood’s insulative resistance (U-value), it needs a higher temperature below the floor to give the same temperature at the surface of the floor. This means that the boiler has to heat the water longer to achieve the same heat output. It also means that insulation is far more crucial to send the heat where you want it.


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