Radiant floors circulate air, electricity, or most commonly warm water through tubing embedded in the floor of your home. The heat radiates up through the floor, warming the people, furnishings and air in the room. The warmth stays down around where the people are, not up at the ceiling or lost to the outdoors every time a door or window is opened. It's a comfortable, even heat where cold spots and drafts are eliminated.

Planning, and using a knowledgeable contractor, is important keys to success with radiant heating. First, the contractor should perform a detailed heat-loss analysis, which will form the basis for the placement of the tubing. For example, tubing might be placed closely together near a window to create a zone of warmth.

Radiant floor heating systems are more expensive to install, but advocates say it's an investment in comfort. The systems provide uniform heat and allow homeowners to heat specific "zones" of their homes.

Types of Radiant Floor Heating

There are three types of radiant floor heat: radiant air floors (air is the heat carrying medium), electric radiant floors, and hot water (hydronic) radiant floors. All three types can be further subdivided by the type of installation: those that make use of the large thermal mass of a concrete slab floor or lightweight concrete over a wooden subfloor (these are called "wet" installations); and those in which the installer "sandwiches" the radiant floor tubing between two layers of plywood, or attaches the tubing under the subfloor (dry installations).

Because air cannot hold large amounts of heat energy, radiant air floors are not cost-effective in residential applications, and are seldom installed.

Electric radiant floors are usually only cost-effective if your electric utility company offers time-of-use rates. Time-of-use rates allow you to "charge" the concrete floor with heat during off-peak hours (approximately 9 pm to 6 am). If the floor's thermal mass is large enough, the heat stored in it will keep the house comfortable for eight to ten hours, without any further electrical input. This saves a considerable number of kilowatt-hours compared to heating at peak electric rates during the day.

Hydronic systems are the most popular and cost-effective systems for heating-dominated climates. They have been in extensive use in Europe for decades. Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern underneath the floor. The temperature in each room is controlled by regulating the flow of hot water through each tubing loop. This is done by a system of zoning valves or pumps and thermostats.

In a "wet" installation, the tubing is embedded in the concrete foundation slab, or in a lightweight concrete slab on top of a subfloor, or over a previously poured slab. If the new floor is not on solid earth, additional floor support may be necessary because of the added weight. You should consult a professional engineer to determine the floor's carrying capacity.

Some "dry" installations involve suspending the tubing underneath the subfloor between the joists. This method usually requires drilling through the floor joists in order to install the tubing. Reflective insulation must also be installed under the tubes to direct the heat upward. Tubing may also be installed from above the floor, between an old and new subfloor. In these instances, the tubes are often in reflective aluminum sleeves that spread the heat to the sides, away from the tubing, and direct it upwards. The tubing and its reflectors are secured between furring strips which carry the weight of the new subfloor and finished floor surface.

Although ceramic tile is the most common floor covering for radiant floor heating, a variety of finished floor surfaces can be used. The choices include vinyl flooring, carpeting, and wood. Carpeting and padding, however, insulate the floor and reduces some of the benefits of radiant floor systems. If you want carpeting, use a lower nap carpet and thin, denser padding. You will also need to increase the system water temperature to compensate for the insulating properties of the floor covering. Most installers and some wood floor manufacturers also recommend using laminated wood flooring instead of solid wood. This reduces the possibility of the floor shrinking and cracking from the drying effects of the heat.

Older radiant floor systems used either copper or steel tubing embedded in the concrete floors. Unless the builder coated the tubing with a protective compound, a chemical reaction between the metal and the concrete led to corrosion of the tubing, and to eventual leaks. Major manufacturers of hydronic radiant floor systems now use cross-linked polyethylene (PEX) or rubber tubing with an oxygen diffusion barrier. This material is much more durable and slows the effects of corrosion in the system. Additives and filtration systems also help protect hydronic heating systems from corrosion.

Advantages of Radiant Heat

Studies conducted by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) indicate that with radiant heating systems people can be comfortable at temperatures 6°F to 8°F lower than with convective systems. Forced-air and baseboard (whether electric or hot-water) heating systems are convective systems because they use air as the primary heat-transfer medium.

Typically, heating outlets or baseboards are placed on outside walls, and the system is designed to fill the area with warm air until the preset temperature on the thermostat is reached. The warm air rises to the ceiling until it cools, falling to the floor for return to the furnace or to fill the convective vacuum created by a baseboard heater. Air stratification and heat loss to the ceiling are significant with convective heat.

Air is transparent to the transfer of radiant energy, which occurs directly from warmer to cooler objects. With radiant ceiling heat, the temperature varies only about 2°F to 4°F between the ceiling and the floor, with the floor being about 2°F warmer than the air. And radiant floor heating results in reverse stratification.

Humidification is unnecessary with a radiant system because radiant heat does not alter residential air moisture content, which is generally adequate if the air isn't dried out by combustion or by increased infiltration of cold, dry outside air.

Glass, particularly low-e glass, reflects long-wave radiance produced by residential radiant systems. This greenhouse effect serves to contain radiant energy within the heated building cavity, reducing heat loss.

Air-infiltration heat loss is reduced with radiant heat. Air infiltration and exfiltration increase as the difference between inside and outside temperature becomes larger. When superheated air from a furnace or baseboard heater flows against relatively cold exterior walls, the increased temperature differential results in a stack effect that draws cold air into the house through any cracks. With radiant systems, the air is only warmed to the temperature of the thermostat setting (which is usually lower to start with), so the temperature differential at the outside wall is less, thereby reducing air infiltration.

When applied to the sizing of a radiant system, conventional heat-loss analysis often includes a reduction in design temperature from 70°F to 65°F and a 10% to 25% reduction in building air infiltration, exfiltration, stratification and glass heat loss. The average 65°F radiant comfort temperature with 59°F day/night setback should reduce building heat load by 25% to 35% over convective systems.

Another advantage of radiant systems is that they don't dry the air or cause the dust and noise associated with forced-air systems. They may also, in some cases, completely eliminate the need for air ducts and room registers.

Additional Benefits

Some additional benefits of radiant heating include: