Posted: October 2009

Imagine stepping out of bed in the morning and onto a gently warmed floor.  Imagine the joy of walking across your bathroom tiles without the usual harsh shock of frigid porcelain on bare skin.

Radiant floor heating provides this comfortable experience in a home, while—among the many benefits—using far less energy than baseboard or forced hot air heat.

A perfect time to install a radiant heating system is when new flooring is being installed or during a remodel when the floors are being revised. This article will tell you all about the types of radiant heating systems available to you and the options you should consider when planning to install radiant heating in your home.

Radiant Heating

Radiant heating systems most commonly involve supplying heat directly to the floor although there are radiant systems that deliver heat to panels in the walls or ceilings of a house. Our focus will be on floor systems since they may be easily installed during a remodeling project.

Radiant heat systems depend largely on heat transfer: the delivery of heat directly from the hot surface to the people and objects in the room via the radiation of heat. 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.

Radiant heating has a number of advantages: it is more efficient than baseboard heating and usually more efficient than forced-air heating because no energy is lost through ducts. The lack of moving air can also be advantageous to people with severe allergies. Hydronic (liquid-based) systems use little electricity, a benefit for homes off the power grid or in areas with high electricity prices. Hydronic systems can also be heated with a wide variety of energy sources, including standard gas- or oil-fired boilers, wood-fired boilers, solar water heaters, or some combination of these heat sources.

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 finished floor or subfloor ("dry installations").

Types of Radiant Floor Heat

Hydronic Radiant Floors: Hydronic (liquid) systems are the most popular and cost-effective radiant heating systems for heating-dominated climates. Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern underneath the floor. In some systems 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. The cost of installing a hydronic radiant floor varies by location and also depends on the size of the home, the type of installation, the floor covering, remoteness of the site, and the cost of labor.

Electric Radiant Floors: Electric radiant floors typically consist of electric cables built into the floor. Systems that feature mats of electrically conductive plastic are also available, and are mounted onto the subfloor below a floor covering such as tile.

Because of the relatively high cost of electricity, electric radiant floors are more cost-effective if they include a significant thermal mass, such as a thick concrete floor, or if your electric utility company offers time-of-use rates which allow you to "charge" the concrete floor with heat during off-peak hours (approximately 9 p.m. to 6 a.m.). 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 (particularly when daytime temperatures are significantly warmer than nighttime temperatures). This saves a considerable number of energy dollars compared to heating at peak electric rates during the day.

Electric radiant floors can be a sensible choice for additions onto homes for which it would be impractical to extend the heating system into the addition.

Air-Heated Radiant Floors: Because air cannot hold large amounts of heat, radiant air floors are not cost-effective in residential applications, and are seldom installed. Although they can be combined with solar air heating systems, those systems suffer from the obvious drawback of only being available in the daytime, when heating loads are generally lower. Because of the inefficiency of trying to heat a home with a conventional furnace by pumping air through the floors, the benefits of using solar heat during the day are outweighed by the disadvantages of using the conventional system at night. Although some early solar air heating systems used rocks as a heat-storage medium, this approach is not recommended. For further information, see the Federal government’s Energy Savers section on solar air heating systems.

Types of Floor Installations

Whether cables or tubing, the methods of installing electric and hydronic radiant systems in floors are about the same.

So-called "wet" installations embed the cables or tubing within a solid floor and are the oldest form of modern radiant floor systems. The tubing or cable can be embedded in a thick concrete foundation slab (commonly used in "slab" ranch houses that don't have basements) or in a thin layer of concrete, gypsum, or other material installed on top of a subfloor. If concrete is used and 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.

Thick concrete slab systems have high heat capacity and are ideal for storing heat from solar energy systems, which have a fluctuating heat output. The downside of the thick slabs is their slow thermal response time, which makes strategies such as night or daytime setbacks difficult if not impossible. Most experts recommend maintaining a constant temperature in homes with these heating systems.

Due to recent innovations in floor technology, so-called "dry" floors, in which the cables or tubing run in an air space beneath the floor, have been gaining in popularity, mainly because a dry floor is faster and less expensive to build and, during a remodel, one generally has more ready-access to the installation space.  In contrast, a slab-embedded system presumes the pouring of a slab and installation of the system prior to the pour.  However, because dry floors involve heating an air space, the radiant heating system needs to operate at a higher temperature.

Some dry installations involve suspending the tubing or cables 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 or cables may also be installed from above the floor, between two layers of subfloor. In these instances, liquid tubing is often fitted into aluminum diffusers that spread the water's heat across the floor in order to heat the floor more evenly. The tubing and heat diffusers are secured between furring strips (sleepers), which carry the weight of the new subfloor and finished floor surface.

Floor Coverings

Ceramic tile is the most common and effective floor covering for radiant floor heating, as it conducts heat well from the floor and adds thermal storage because of its high heat capacity. Common floor coverings like vinyl and linoleum sheet goods, carpeting, or wood can also be used, but any covering that helps to insulate the floor from the room will decrease the efficiency of the system.

If you want carpeting, use a thin carpet with dense padding and install as little carpeting as possible. If some rooms, but not all, will have a floor covering, then those rooms should have a separate tubing loop to make the system heat these spaces more efficiently. This is because the water flowing under the covered floor will need to be hotter to compensate for the floor covering. Wood flooring should be laminated wood flooring instead of solid wood. This reduces the possibility of the wood shrinking and cracking from the drying effects of the heat.

Clearly there are a number of options to consider regarding radiant floor heating but the benefits are substantial, from the comfort radiant heat brings to a living space to the decreased energy costs you will experience for years to come.

For more information visit the U.S. Department of Energy “Energy Savers” web site.