Heat for the Feet

The sensor probe of NuHeat’s new MatComfort 82F, a programmable thermostat, must be placed beneath the underlay. According to the manufacturer, keeping floor temperatures at or below 82 degrees Fahrenheit allows electric radiant heating to go under laminate and engineered wood flooring. Dual-function panel systems combine a structural subfloor with a hydronic radiant heating system.  Illustration courtesy of Warmboard. Rehau’s dry Raupanel system is placed directly on top of the subfloor. Raupex pipe snaps into the grooves. It does not need a concrete overpour. Rather than using a mat or mesh, the WarmWire electric system from SunTouch uses wire, held in place by the WarmWire strap, to heat the floor.

For centuries, people have been warming their homes — and their feet — with radiant floor heating. Warm, clean, and efficient, radiant floor heating is increasingly popular in new homes, but it remains a rarity in remodeling, as many contractors believe radiant heating is impractical to install in an existing home.

There's some truth to that. "Wet" systems are the oldest and most popular type of hydronic radiant heat. Unless you are doing a large addition to a home, a wet system isn't practical because it requires tubing to be embedded in the concrete foundation slab, in a lightweight concrete slab on top of a subfloor, or over a previously poured slab.

Dry systems, on the other hand, pump hot water through tubing positioned in loops beneath the finished floor and do not need to be planted in concrete.

"Radiant floor systems can go in just about any type of remodel, whether it's an addition or a remodel within an existing structure. It all depends on what you want your systems to do and where you want it to go," says Gary Fries, a business development manager for Uponor Wirsbo Inc., a manufacturer of radiant floor heating with North American headquarters in Apple Valley, Minn.

That's good news, because homeowner demand for radiant floor heating is growing. In 2003, the Radiant Panel Association reported 13 percent growth in sales of radiant heat tubing. It was the 13^th consecutive year that tubing suppliers saw increased sales.

"We have customers who swear they will never use forced-air heat again. That's how much they love having a heated floor," says Paul Izenstark, director of technical services for Warmboard Inc., a manufacturer of structural radiant heat systems in Aptos, Calif.

Why heat the floor?

Radiant floor heating systems use air, electricity or, most commonly, hot water to raise the temperature of the surrounding flooring material. The heat then is released into the room. There are no heat registers or radiators, making radiant floor heating more aesthetically pleasing.

Because radiant heat warms objects near the floor, homeowners may find it more comfortable than forced air heating. Izenstark says floor heating is much more appealing in homes with high cathedral ceilings, for example, since forced air heat can quickly rise above the occupants.

Radiant floor heating also eliminates the dust, draft and noise problems associated with forced air. According to a German study presented at the 1990 International Indoor Air Quality Symposium, households with radiant floor heating experience a 50 to 80 percent reduction in dust mites. Because radiant flooring doesn't blow the air around and because it can take the place of carpeting, many manufacturers claim that it can be helpful for allergy sufferers.

Homeowners may also enjoy greater savings on their utility bills: Because residents are in physical contact with the heat source and the heat tends to remain in lower levels of the room, residents are more likely to be comfortable at lower temperature settings. Savings can be increased further still by installing multiple zones and individual thermostats in different rooms, allowing heat in unused rooms to be turned down. The floor's ability to retain heat for off-peak storage can reduce energy bills as well.

Radiant heat also can reduce heat loss from the home. In forced-air systems, registers are placed along outside walls and under windows to compensate for cold surfaces. Vented hot air goes up those cold walls, across the ceiling, and down to the cold air return. The greatest heat loss occurs in these locations. Radiant heating, in contrast, directs heat to the home's interior.

The RPA estimates a radiant heat system can result in 10 to 30 percent energy savings in most residences, although many manufacturers claim their systems save between 20 and 40 percent on total monthly heating bills.

Although he believes those estimates may be high, Dana Bres, a research engineer with the Partnership for Advancing Technology in Housing, says, "PATH research has shown that radiant heat rivals the energy efficiency of conventional heat, with excellent comfort and homeowner acceptance."

Hydronic systems

There are three types of dry-system hydronic radiant floor heating to choose from: above-floor systems, below-floor systems and dual-function panels.

Above-floor systems use grooved wood panels installed above the subfloor and below the finished floor. Cross-linked polyethylene (PEX) tubing is inserted in the grooves of the panels and sits flush with the panel surface. The panels usually come in two sizes: 7×48 inches and 10×48 inches. Above-floor systems are most commonly used under hardwood, tile and vinyl, although manufacturers say their panels are compatible with a variety of floor coverings, including marble and carpeting.

The panels used in above-floor systems are ½-inch thick and covered with aluminum, so they raise the height of the floor. They can be purchased individually or accordion style. The accordion style is six panels of a desired width adhered together using fiber tape. The panels unfold to cover a larger surface area.

Below-floor systems are installed beneath the subfloor. These systems require attaching PEX-type tubing to the bottom of the subfloor or suspending the tubing from the subfloor, usually by drilling through the floor joists. Reflective insulation must also be installed under the tubes to direct the heat upward.

Below-floor systems may require a higher water temperature to perform as well as above-floor systems because the heat has to travel farther to warm the room. A slightly larger heater may be needed to ensure the correct water temperature, potentially entailing more initial expense. In most cases, the labor savings are significant enough to offset the cost of a larger heater if necessary.

The third option — dual-function panels — offers the structural capacity of a subfloor as well as a radiant floor heating system.

"It's a two-in-one product. The labor works out well because a contractor needs to lay down the subfloor anyway. So why not have it be a subfloor that is a radiant panel?" says Izenstark. Warmboard's engineered tongue-and-groove panels are 4×8×1 1/8 inches thick and designed to hold ½-inch PEX tubing. The tubing is covered by alloy aluminum bonded to the panel.

Electric systems

While hydronic systems heat water with natural gas, propane, oil, solar or geothermal energy, manufacturers also offer thin electric systems that are designed for convenient installation above the subfloor and underneath tile, stone, wood and carpet. Because they cost more than hydronic or forced-air systems to heat large areas, electric systems are frequently used in small spaces, particularly in bathrooms with ceramic or stone flooring, as a supplement to forced-air heating. Electric floor radiant heating is made of heating wire, usually attached to a fiberglass net, placed directly under the flooring material.

A hydronic system can also be used in combination with a forced air system, in which case one system should be dominant. Fries says that where the radiant floor system is dominant, forced air only kicks in at a predetermined temperature, which depends on the climate zone and the size of the system. Izenstark considers the use of two systems unnecessary, especially with an effective hydronic system in place.

Installation considerations

The easiest time to install radiant floor heating is when adding a large room or completely gutting the house.

"If you are planning on doing an addition, such as a master bath and a master bedroom, you have to install a structural subfloor anyway, unless it's a slab or grade-raised floor," Izenstark says.

In retrofit situations, above-floor systems may not be a good choice because they are costly and time consuming, besides adding a half-inch to the floor height. A better choice may be below-floor systems, which require drilling holes through the floor joists so that the PEX tubing can pass through.

"If you are going to go from underneath, then you don't affect the floor height of the room above, but if you are going to go above the floor, then you are raising everything roughly half an inch," Fries says. "That may affect door jambs and create other issues." Below-floor systems are usually less costly than above-floor because they don't affect floor height and save on labor hours.

Another important consideration is the placement of the operating systems. For a typical hydronic system, a remodeler will need to install a circulation pump, a manifold of water distribution control valves, a thermostat, and a boiler or water heater. Small systems, however, can sidearm off an existing water heater.

"You need to set aside some room and have access to it, and that can be an issue for people who are tight on space," Izenstark says.

To sub or not to sub?

Because of the complexities involved, some manufacturers recommend that general contractors hire a knowledgeable subcontractor with plumbing and HVAC experience rather than do the installation in-house.

"Because of the electrical and plumbing side of things — such as line voltage wiring that can come into play, and the sweating that occurs with the piping — you just have to understand the dynamics that are taking place, how flow is going to run, and how to control that flow," says Fries. "If you don't have a background in that, you can get yourself in trouble."

According to Izenstark, it should not be a problem to find a competent subcontractor with experience installing radiant floor systems. "Nowadays, all over the country, there are new tradespeople specializing in radiant floor heating," he says.

Radiant floor systems work for renovations of all sizes