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 (sleepers) 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 to carpet, 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.
There have been recent reports of problems with rubber tubing produced by one chemical manufacturer. Leaks develop at the metal connections or fittings, and in some cases, the tubing becomes rigid and brittle. Tightening connections and clamps only temporarily fix the leaks. This situation is currently in litigation. Remember, this problem only concerns this specific brand of rubber tubing. It does not have anything to do with the PEX tubing, which has performed very reliably for many decades.