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I hate to be a wet blanket about something that's really popular. But I'm about to point out some of the reasons radiant floor heating systems aren't always as ideal as people think.

Radiant Floor Overview

Radiant heating systems convert a floor into a large-area, low-temperature radiator. In most modern systems, warm water is circulated through closely spaced plastic tubing that's embedded in the floor slab or attached to the underside of wooden subflooring. Underfloor insulation is a critical component. Zoning depends on advanced manifolds that regulate flow or modulate the water temperature in different tubing runs. Sophisticated controls regulate all this -- often with temperature sensors in the slab -- in the rooms being heated and outdoors.

Adding Up the Pluses

Radiant floor heating systems offer a range of benefits.

Comfort. The large floor area warms people by direct radiation instead of with heated air currents. Homeowners can walk around barefoot, even in the dead of winter -- a very popular feature. "Until you've lived with this form of heat," says Radiant Panel Association executive director Larry Drake, "it's hard to understand how comfortable it is."

Energy savings. The floor's "radiant shine" boosts the mean radiant temperature of a space and prevents temperature stratification. Proponents argue that this allows homeowners to keep their thermostats lower. Someone normally comfortable at 72°F would be comfortable at 68°, they suggest, offering significant energy savings. There are other potential sources of savings: Boiler temperatures can be kept lower, trimming heat loss from the boiler and pipes. And hydronic systems don't affect house air pressures; forced-air heat, by contrast, can pull heat-wasting drafts through walls and ceilings when the supply and return pressures are not in balance.

Empty Picture Box

Potential for use of solar energy. The relatively low temperature of water required for radiant floor heating systems is well suited to solar water heating. In addition, the concrete slab can store daytime solar gains for nighttime use.

Quiet operation. Radiant floor heating is very quiet, with no fan or duct noise and little of the gurgle or creaking sometimes heard in hydronic baseboard systems.

Flexible room layout. With no baseboard radiators and no floor registers, radiant floors don't restrict furniture placement or interior design.

Improved air quality. Radiant floors don't blow dust around like some ducted heating systems do. And, unlike electric baseboard systems, they don't use heating surfaces hot enough to burn dust particles (which can cause respiratory irritation).

So What's Not to Like?

In the right application, radiant floor heating is a superb heat-delivery system -- perhaps the best. You pay more for it, but the comfort, savings, and other benefits can justify the extra cost. Even so, it's my opinion that it's seldom worth the price premium to put radiant floor heating into new homes.

In most cases, it's wiser to put those extra dollars into the building envelope. In fact, once you have a highly energy-efficient house, a radiant floor heating system doesn't add a lot even if money is no object -- sometimes it can make things worse.

Economics

Granted, I'm a believer in super-high-performance buildings. I think homes in moderate-to-cold climates should go the extra mile: They should be insulated to at least R-25 in the walls and R-40 in the ceiling or roof, tightened to very low leakage levels, provided with very high-performance windows (unit U-factors below 0.3), and designed to take advantage of passive solar gain or suntempering.

Homes like that require very little heat, often using less than 2 Btus of heat energy per square foot per degree-day. The walls and window details, however, are not cheap. So why spend even more on expensive heat? As radiant maven Larry Drake admits, "The tighter the envelope, the less the amount of savings of a radiant system." Engineer Marc Rosenbaum, P.E., of Meriden, N.H., says, "It just doesn't make sense to put in a $10,000 heating system to provide $100 worth of heat per year."

When you splurge on high-end heating, you throw away the chance to offset those extra envelope costs with savings on mechanical equipment. That offset is one of the key principles of advanced building design. In houses with state-of-the-art envelopes, you can equal the comfort provided by radiant floor heat with one or two small, quiet, through-the-wall vented gas heaters (such as those made by Rinnai Corporation, Peachtree City, Ga., 800/621-9419, http://www.rinnaina.com) or with short sections of electric baseboard. At $1,000 to $2,000 apiece for Rinnai heaters or a few hundred dollars for electric baseboard vs. $10,000 for a typical radiant floor heating system, you could realize a savings of from $6,000 to more than $9,000 -- enough to pay for most of the envelope upgrades.

Too Hot to Handle?

Beyond the economic doubts, there are building science reasons to question the indiscriminate use of radiant floors. Each square foot of radiant slab pushes about 2 Btus of heat per hour into a room for each degree of temperature difference between the slab and the air -- the hotter the floor relative to the air, the faster the heat flows in. Well-insulated tight homes need only a trickle of Btus per square foot per hour, even during the peak heating periods. That means the slab should be only a few degrees warmer than the rest of the room, or the room will overheat. Given moderate solar gain, the slab's required output will be even less.

But for a concrete slab to feel warm underfoot, it needs to be about 80°F. Thus, for most of the heating season, the greatest feature of radiant floor heat -- a warm floor -- won't occur. Because the floor is insulated underneath, it will be more comfortable to walk on than most slab floors; but that benefit comes from the insulation, not the hydronic heat.

Vicious cycling. In a very well-insulated house, the time lag of heat movement through concrete can create its own problem: It can drive overheating, particularly if there are other heat inputs, such as passive solar energy. If a concrete slab is warmed during the early morning hours enough that the surface cannot readily accept more solar heat, extra solar gain will overheat the air. Energy consultant Andy Shapiro, of Montpelier, Vt., calls hydronically warming the slab in a passive solar house "a waste of energy."

Heat Loss to the Ground

With hydronically heated slabs on grade, there is potential for significant heat loss into the ground. According to Paul Torcellini, Ph.D., P.E., of the National Renewable Energy Laboratory in Golden, Colo., even with insulation under the slab, 20% of the heat is often lost to the ground. Typical manufacturer recommendations for 1 inch of extruded polystyrene (XPS) insulation beneath a radiant slab are clearly inadequate. Even 2 inches may not be enough; Shapiro recommends as much as 4 inches in cold climates.

It's ironic that most people want radiant floor heat because they don't like a cold floor, yet people have long resisted insulating beneath slabs -- which would dramatically reduce the cold-floor problem. They call for costly radiant floor heat (including rigid insulation under the slab) when the insulation alone would go a long way toward solving the problem.

Overpromising, Underperforming

Finally, it appears that much of radiant's assumed energy savings may not occur at all. There's not much data to back up the common claim that people who have warm-floor systems keep their thermostats lower and thus save a lot of energy. In fact, a recent study found no support for the claim ("Canadian Study Contradicts Radiant-Heat Claims," Notebook, 12/01). Researchers from the Canada Mortgage and Housing Corporation (CMHC) visited 75 houses (50 with radiant floor heating and 25 with other heat distribution systems) in Nova Scotia during the winter of 2000-2001. Thermostat settings in the houses with radiant floor heating averaged 68.7°F, while those in the control houses averaged 67.6°F. CMHC's Don Fugler, who managed the study, cautions that this superficial study examined only a small sample of homes; but he notes that it points up the need for additional research into the supposed savings.

Better Than Air? Maybe Not

Many people who opt for radiant floor heating do so because they don't like forced-air heat. It's widely believed that forced-air heating systems dry out air and generate dust. "Nothing could be further from the truth with a properly installed forced-air system," says Betsy Pettit, AIA, of Building Science Corporation in Westford, Mass. She argues that forced-air systems can provide heat, air conditioning, ventilation, and filtration -- all with shared fans and ductwork. But radiant floors do only one thing, she says, and at greater cost. "For me it's just a hard sell," she explains. "If you insulate the slab and if you build your building envelope correctly -- that is to say, leak free -- you can be more comfortable for less money with a ducted distribution system."

Where Radiant Floors Shine

So do radiant floors make sense anywhere? Yes, they do. For example:

* in buildings with conventional levels of insulation and standard windows -- especially in climates with minimal cooling loads -- where the extra comfort is desired and the budget allows

* in buildings with large open spaces and high ceilings

* in buildings where air flushing is common, such as garages, fire stations, airplane hangars, and industrial spaces (because the large-area radiant floor allows quick recovery)

* when system installation cost is not an issue, and satisfying most or all of the heating load with solar energy is a high priority

* when building occupants are acutely sensitive to dust

Last Word

One of the reasons radiant floor heating is so popular is that it's so much more comfortable than what most of us have experience with: older, drafty houses where there is significant floor-to-ceiling temperature stratification. If more people realized that the same -- or at least a similar -- level of comfort could be achieved simply by creating a really well-insulated, tight building envelope, we could be keeping a lot of people extremely comfortable while also saving a huge amount of energy, without needing radiant floor heat. "A house with a good enough envelope to be called green -- well insulated and tight -- will have a very high level of comfort no matter what type of heating system is used," says Shapiro, "as long as that system is well designed."

Alex Wilson, of Brattleboro, Vt., wrote the "Focus on Energy" column in JLC (then New England Builder) from 1982 through 1988. In 1992 he founded Environmental Building News, which just celebrated its tenth-anniversary issue. A longer version of this article appeared in the January 2002 issue of EBN. For more information, visit http://www.buildinggreen.com or call 802/861-0954.