Radiant Floor Heating 101

Radiant floor heating, popular in new construction, has become a viable retrofit option.

Radiant Floor Heating

Photo: Sun Climate

Radiant floor heating is arguably the ideal home heating system. It’s comfortable, efficient, unobtrusive, quiet, and does not blow dust and allergens around the way forced hot air systems do. Instead of overheating the room’s perimeter in the hopes that the warm air will travel throughout the space before rising, subfloor heating serves up heat from below. The result is a more even overall heat that warms everything in the room, including surfaces, furnishings, and, most importantly, you. Radiant heat is similar to the heat you feel when you stand by a window on a sunny cold day. Your face feels warm, but the sun didn’t need to heat the air outside to make you feel that way.

For the record, subfloor heating has been around for centuries, from the hypocausts—a floor raised on pillars where heat could circulate below and radiate through layers of tiles and stone—of the ancient Turkish and Roman baths, to Frank Lloyd Wright’s turn-of-the-century adoption of more modern Japanese examples. And while the decision to install radiant heating used to be a pre-construction call, today’s innovations make it feasible—and, even DIY-suitable—for existing home retrofits.

Radiant floors are heated either with electric resistance cables or hot water flowing inside tubing.

Electric systems are typically supplemental, not meant to be the sole heat source for a room. The cables, which are often pre-attached to mats for ease of installation, are installed over the subfloor in a bed of thin-set mortar. Ceramic or stone tile are popular finished floor choices. There are also radiant electric floor heating pads that can be installed under laminate and other floating floors, such as engineered hardwood. One manufacturer, Thermosoft, makes pads that produce 31 BTUs per square foot. Installation is simple. Just roll it out, tape it in place, cover with floating-type flooring, and make the electrical connections. No mortar is required.

Don’t want to pull up your existing flooring? Companies such as SunTouch make electric radiant pads that fit in joist bays under the subfloor. You will, of course, need access to the bays from a basement or crawl space. Batts of fiberglass insulation are installed under the mats so most of the heat goes up, not down.

Radiant Floor HeatingHydronic systems are usually designed to heat an entire house. Water is heated to between 100 and 120 degrees Fahrenheit by a boiler and circulated through tubing under floors. The tubing can be installed in several ways: embedded in a concrete slab, installed over an existing slab in cement, stapled under subflooring, or fitted inside the channels of specially designed subfloor panels. Any kind of finished flooring, including hardwood strip flooring, vinyl, or carpeting, can be installed above it. (Note: Some installers may recommend engineered wood rather than solid wood flooring products in homes with high moisture levels. Otherwise, changes in moisture content can cause wood planks to cup, bow, or warp.)

Radiant heating is more comfortable than other systems for a variety of reasons. First and foremost, it feels warmer because the heat is delivered where you live—near the floor. Since all surfaces in the room are also being heated, there are no cold objects to draw heat from you and make you feel cold. In addition, radiant heat does not constantly cycle on and off, causing you to be too hot one minute and too cool the next. Nor does it dry out the air that in turn dries out nasal membranes. Plus, radiant heat is relatively draft-free. There are no supply and return registers or convection-reliant radiators, and there is less air leakage around doors and windows. Finally, the air inside the home tends to be cleaner because dust and allergens are less likely to be stirred up by air currents.

Because electric heat is expensive, electric radiant floors are typically limited to small areas, such as a bath or kitchen. Programmable thermostats with both air and floor temperature limits are recommended with such systems, to save on energy costs. Hydronic radiant floor systems save energy and lower fuel bills because radiant heat feels comfortable at lower air temperatures, enabling you to lower the thermostat. Further savings can be realized because running a high-efficiency boiler at lower temperatures will increase its lifespan. In addition, hydronic radiant heat is more efficient than other systems because it uses relatively low water temperatures to heat your home. In effect, the entire floor is a radiator, so it doesn’t have to be as hot as conventional radiators. Boilers can heat water to lower temperatures more efficiently than they can heat water to higher temperatures.