Radiant Floor Heating: Wrong Choice for Green Homes?
Radiant-floor heating is popular for some good reasons. It provides very comfortable, uniform heat, owing to the relatively low temperature and the large surface area from which the heat is radiated. It does not interfere with furnishings in a home as most other heat distribution systems do. It’s quiet. And, according to proponents, it can save energy by warming people directly (rather than heating the air)—thus allowing occupants to keep the thermostat (air temperature) lower.
Indeed, people living in houses with radiant-floor heat are often effusive in their enthusiasm. Walking around with bare feet on warm floors is very appealing.
Radiant-floor heating systems commonly involve PEX (cross-linked polyethylene) tubing embedded in a concrete slab; hot water is pumped through the tubing. The slab warms up and slowly radiates heat into the room. Radiant-floor heating systems can also be achieved with tubing under wooden floors.
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.
Second, when the heating load is very small, the radiant slab has to be maintained at no more than a few degrees above room temperature to prevent overheating, and this means that the slab isn’t likely to be warm to the touch. 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.
Third, radiant floor slabs and the mechanical equipment needed to heat them are expensive. For a typical house, such systems often cost well over $10,000. Again, in a highly insulated house, that is a lot to spend for a few hundred dollars worth of heat per year.
Fourth, there is little if any evidence that radiant-floor heating actually saves energy. The argument that homeowners will keep their thermostats set lower with radiant heat is not supported by (admittedly limited) research. And with slab-on-grade homes with typical levels of insulation beneath the slab—rarely more than two inches (50 mm)—there may be significant heat going into the ground.
In short, radiant-floor heating is a great heating option for homes with average or below-average levels of insulation, but it is not well suited to highly insulated homes, especially such homes with moderate solar gain (passive solar heating). For more on these issues see “Radiant Floor Heating: When it Does—and Doesn’t—Make Sense” in EBN.