Every ad for radiant flooring shows the same thing: kids, dogs, cute couples lying on the floor looking happy. And what's not to like? It really is comfortable to have warmth underfoot, and many companies claim that it saves a lot of energy by putting the heat where you want it. But is it really green?
In his book Your Green Home, Alex Wilson wrote "it's a great heating option for a poorly designed house.... For the radiant floor system to provide enough heat to feel warm underfoot (the feature everybody likes with this system) its going to be cranking out more heat than the well insulated house can use, and it will likely cause overheating. A radiant floor heating system also has a very long lag time between when the heat is supplied to the floor and when the slab begins radiating heat....If there is a component of passive solar heating in the home, it will cause overheating because you can't turn off the slab when the sun comes out."
The thermal lag issue was the one that always concerned me; when the sun goes down and the thermostat kicks in, how many hours does it take for the floor to warm up enough to do anything?
Martin Liefhebber of Breathe Architect Inc. loves the stuff and puts it in all of his houses. I asked him about the thermal lag issue and he told me that the controls are much more sophisticated than just a thermostat. There are sensors in the floor as well as the room, so it can compensate for differences in slab temperature and room temperature. He often ties the floor into a solar thermal system.
But he also noted that he usually installs a wood stove that can be fired up instantly to fill in the gaps when the floor has not heated up quickly enough.
Alex Wilson has updated his thoughts on radiant flooring in a recent edition of Environmental Building News, complete with babies. He is even blunter, when talking about highly insulated homes (which most green homes are by definition):
While radiant-floor heat makes sense in certain buildings, it is not well-suited to highly insulated green homes for a number of reasons. First, in a home with a tight envelope and a very small heating load, even a small amount of heat can cause overheating, and the thermal mass in a radiant floor system (especially with concrete-slab systems) increases the risk of overheating. This is particularly true in buildings with some level of passive solar gain--the radiant floor may still be delivering heat even after solar gain raises the air temperature.
He goes on to note that the slabs have to be kept perhaps too warm simply to remain comfortable; "A slab maintained at 74°F (23°C) will be cooler than an occupant's skin, so bare feet will conduct heat into the slab. "
He then notes that they are expensive and that they might not even save energy. Read more at Environmental Building News