In his new book, Your Green Home, Alex Wilson (the executive editor of Environmental Building News) refers to radiant floors as “a great heating option for a poorly designed house.” The heating requirements of an extremely well-insulated home with a properly airtight envelope, even in most cold climates, will most likely result in an overheated house if the radiant floor is warm enough to actually feel warm underfoot. If passive solar is a design feature, the slow response of high-mass radiant floor systems can also contribute to overheating. The expense of a wall-to-wall radiant floor system would generally be better spent on improving the envelope’s insulation and airtightness, and downsizing the space conditioning systems, particularly in new construction.
Yet radiant floor heating—in addition to being quieter and potentially providing better indoor air quality and greater comfort than bad forced-air systems—can be an economical choice under the right circumstances. In residences and small commercial structures, radiant floor heating makes the most sense for buildings with standard levels of insulation and typical double-glazed windows—particularly when they’re located in climates with small cooling needs. That is, retrofits of older houses in cold climates.
Underfloor radiant hydronic heating retrofit systems that install between the joists below existing wood floors are available from a number of sources. They should incorporate metal plates or fins, usually aluminum, that fit tightly around the tubing and make continuous contact with the floor to improve heat transfer. One good option of this sort is Uponor-Wirsbo’s Joist Trak, available for 1/2-inch or 3/8-inch tubing (see www.wirsbo.com). It comes in 4-ft extruded aluminum sections that screw or nail below the subfloor. The 4-inch width accommodates two runs on 8-inch centers between 16-inch on center (OC) joists. After the heat transfer tracks are installed, the tubing snaps into channels in the tracks—no additional fasteners or strapping are needed.
The other retrofit option for radiant hydronic floors is installing the heat delivery system above the existing floor (which raises issues about doors that will have to be adjusted, cabinets that end up too short, and trim that needs to be dealt with). This is often accomplished by embedding hydronic tubing in about 11/2 inches of self-leveling, cementitious material (such as Gyp-Crete from Maxxon)—a process that, depending on the product, can add up to a gallon of moisture loading to the building per installed ft2, and could potentially require some temporary or permanent structural augmentation to accommodate the weight. In addition, some of these flowables have admixtures that may release volatile organic compounds (VOCs). While wet installations provide extra mass for thermal storage, the extra mass means that they react more slowly to changes in thermostat settings as compared to a lower-mass system—which may or may not translate to reduced energy use, depending a good deal on how the occupants operate the system.
There are a number of above-the-floor, dry-install, lower-mass options for radiant hydronic floor retrofits. EasyFloor, by Florheat, takes a rather novel approach (see www.florheat.com). It uses lightweight, 16-inch-square interlocking modules made of recycled plastic that have molded-in, 8-inch OC guides for 3/8–5/8-inch tubing, allowing straight lines, 90° or 180° turns, and even S’-curves, with no special end pieces. Aluminum plates extend from the channels to help distribute the heat. A radiant barrier—an aluminized sheet of poly—is laid out prior to installing the modules; because there’s an air space, this is an application where a radiant barrier really will work to “reflect” heat. The downside to this system is its depth; the modules are 1 inch thick, and a thermal mass layer (typically 1/2-inch fiber cement board) must be laid prior to installing the finish flooring.
Another option is Warmboard—structural, aluminum-topped, 11/8-inch-thick, 4 x 8 sheets of tongue-and-groove plywood subfloor with sunken channels on 12-inch centers to accept 1/2-inch PEX tubing (see www.warmboard.com). Special sections with U-shaped channels are used for the loop ends. While Warmboard can be used in retrofits, it’s almost as thick as wet installations and has only slightly less depth than EasyFloor. Unless a remodel is going down to the joists, or extra floor stiffness is desired, other options may be more attractive.
Similar to WarmBoard, but nonstructural—and smaller in all dimensions—is ThermalBoard, which comes in 5/8-inch-thick, 16 inch x 48 inch medium-density fiberboard (MDF) panels made with recycled wood; its channels are on 8-inch centers for 3/8-inch tubing (see www.thermalboard.info). The panels weigh 2.5 lb/ft2. Third-party testing by Environmental Analysis, Incorporated, indicates no measurable off-gassing, a nice bonus.
No matter what radiant floor heating system is used, there are some things to keep in mind. The better the envelope, the less sense radiant floor heating systems make. For radiant floors to work well and efficiently, there needs to be insulation underneath; simply insulating a floor can provide rich rewards without installing a heat delivery system. If central A/C is in the scheme of things, ductwork—which can also be used for fresh-air delivery and whole-house air filtration—will still be required. In general, the more layers there are between the radiant heat source and the occupants, and the less thermally transmissive those layers are (think wall-to-wall carpeting), the more diminished the warm-floor effect will be.
Thinking back to those frustrated occupants, alternately hot and cold, cussing out the furnace, a heat pump—or just a better furnace and thermostat—along with some attention to the envelope, would probably make that desperate crowd just as happy as a radiant floor system… saving at least as much energy, and at less expense. But sometimes people just know what they want—and sometimes it’s a warm floor.
Mark Piepkorn is the lead products researcher for Building Green, Incorporated. He is coeditor of the GreenSpec directory, and the book Green Building Products. He is an associate editor for the monthly journal Environmental Building News.
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