Radiant floor heating has come a long way since its first use in post WW II home construction. Back in the day it was a system of copper pipes embedded into concrete slabs that received their heat from a small boiler in the house.
Today radiant floor heating uses plastic tubing in place of copper and new boilers and zoning technologies has made it one of the best heating systems around. In a standard installation the tubing is laid out and attached to rebar before a concrete floor is poured. The result is a concrete slab which absorbs heat from the water running through the tubes and then radiates it into the house.
What makes concrete floor radiant heating so efficient and comfortable is the thermal mass of concrete. Thermal mass is defined as the capacity of an object to store heat. The greater the thermal mass, the better a substance is in retaining heat through varying air temperature changes.
Concrete's thermal mass is such that once a slab has been heated to a comfortable temperature, the thermostat can be dialed back while the slab continues to radiate constant heat for hours afterward. This enables concrete floor radiant heating to be very efficient while delivering very consistent heat, unlike the continual off and on cycles of forced air systems.
Another benefit of the concrete floor system is the method by which human occupants are warmed. With forced-air systems the furnace heats the air which in turn warms the occupants. Heating the air removes its moisture, making you feel colder than it really is.
With concrete floor radiant heating, occupants are warmed directly as heat radiates up from the floor. The heat is more natural, it warms the body more quickly, and more moisture is retained in the air. Radiant floor heating is so effective, it's possible to keep the thermostat three to four degrees lower and still produce the same amount of comfort as a forced-air system.
Radiant floor heating can be installed without a concrete slab but it's usually not as effective. Electric heating pads are one way to do this, and although they have a much faster response time than concrete, they don't hold the heat nearly as long.
Another option is to run water tubes perpendicular to the floor joists, drilling holes in the joists to allow the tubes to pass through. Again, this is not as effective as using a concrete slab as it relies on heating the air under the floor. Designers have recently come up with radiant panels and sub-floor materials to make this method more efficient, but the results won't be known until the materials have been deployed in the field for a length of time.
If you're thinking of radiant floor heating for a new build or a remodeling project, it seems a good way to go. Talk to your contractor about your options and then make the choice that's right for you.