Question: Radiant floors are touted as saving energy, although they’re fairly expensive. A 2002 BuildingGreen.com article, Radiant-Floor Heating: When It Does-and Doesn’t-Make Sense, suggests that radiant floors aren’t necessarily a good option in passive solar or superinsulated homes. Is that still accurate today or have there been recent developments we should know about? – Cynthia Free, Portland, Oregon

Answer: Radiant-floor heating is a great heat-distribution option … for lousy houses. In new construction, I argue that it makes a lot more sense to take the $10,000 or more you would spend on a radiant floor slab and put it into the house envelope (R-40 walls, R-60 roof, R-20 foundation, triple-glazed, double-low-e windows, etc.). Get the heating load so small that just a small woodstove or electric mini-split heat pump would provide all the heat that’s needed; in a compact house you won’t even need distributed central heat. It’s not only a matter of economics, but in a superinsulated house, a radiant slab isn’t a good thermodynamic fit, because to avoid overheating the space the slab temperature has to be kept so low (just a degree or two above the air temperature) that it won’t even be warm to the touch.

Solar thermal radiant floor heating is an old idea that has been made new again. Many years ago, both the Greeks and Romans heated the floors of baths with wood fires. The concept makes perfect sense – heat rises, after all. Today, solar energy is used to heat water that runs through piping in the flooring. Floors have a thermal mass, so they act as a sort of battery, storing heat for long periods of time.

adiant heating systems involve supplying heat directly to the floor or to panels in the wall or ceiling of a house. The systems depend largely on radiant heat transfer: the delivery of heat directly from the hot surface to the people and objects in the room via the radiation of heat, which is also called infrared radiation. Radiant heating is the effect you feel when you can feel the warmth of a hot stovetop element from across the room. When radiant heating is located in the floor, it is often called radiant floor heating or simply floor heating.