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Of all the finishing materials that can be used with radiant heating systems, none are as durable as ceramic tile. Perhaps even more important, though-in light of increasing energy costs-no finishing material provides greater radiant heating efficiency than ceramic tile. Even in moderate climates, the allure of walking on warm tiles is a powerful incentive that makes this type of installation a red hot must-have. At the time this article was written, there were easily a dozen different brands and numerous methods for applying heat to a floor tile installation. There are numerous systems for heating (and cooling) wall, countertop, and ceiling tiles, too, but the focus of this article will be on floors, and I will begin by grouping all the systems into two main divisions: space heating and tile warming. Space heating systems are designed and built to provide heat for an entire living space (Photo 1). Tile warming systems are designed primarily to take the chill out of, and to gently heat, the tiles, and to supplement or extend the existing space heating system. The difference is significant, especially in cold and very cold climates where the relatively low output of the floor tile warming system might be inadequate to meet the heating needs of the living space above the floor.

Movement Joints: the most overlooked part of a radiant heat tile installation

A series of movement joints, required around the perimeter of any tile floor, is even more important around a radiant installation whose on/off cycling causes repeat expansion and contraction movement within the installation. Without the protection provided by such joints, many radiant systems fail when the electrical resistance wiring or hydronic tubing is pinched or damaged.

For best results, the 2003-2004 Handbook For Ceramic Tile Installation contains a wealth of information regarding movement joints and the most common types of radiant tile installations. For movement joints, see EJ171, Handbook pages 44 and 45. For hydronic or electrical resistance systems installed with concrete, see RH110 and RH115 on page 20; for electrical resistance systems installed over double-layer EGP plywood floors, see RH130. RH135, on page 26, covers radiant tile systems installed over cement backer boards (both fiber-reinforced and fiber cement types). Although most tile warming and space-heating systems share many similarities, all radiant tile systems are different and each requires specific installation and application instructions: make certain that the instructions you are following apply to the unit you are installing.

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Not all electrical resistance systems have been tested for use with ceramic tile. Before you buy, ensure that the unit you select has been through the ASTM C627 test (Commonly known as the Robinson Floor Test) and is rated for your application. Observe your local building code requirements, and follow the recommendations of the TCA Handbook and the relevant ANSI A108 specs.

Space Heating Systems

With a few exceptions, most space heating systems are called hydronic and are composed of a heat source, a series of tubes or pipes, and a liquid-usually water treated with an anti-corrosive and an anti-freeze - to carry and distribute the heat. The hydronic tubing or pipes must be carefully spaced to avoid the creation of unwanted hot spots while still delivering ample heat to warm the space. This can be a problem when large rooms connect with narrow hallways (Photo 2). Lateral spacing is important for the distribution of the heat while vertical spacing determines how quickly the tiled surface can respond to the need for additional space heat. Vertical spacing can also affect the strength of the floor: tubes placed very close to the bottom of the tiles assure the most rapid heat transfer, but they also reduce the compressive strength of the tiles and the installation. Generally, tubes or pipes should be completely encapsulated by the concrete or approved self-leveling compound, and be located in the upper 1/3 of the mortar or SLC bed.

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Traditionally, small pillow blocks were used in an attempt to support the tubing prior to encapsulation with lightweight concrete; however, pillow blocks are notorious for slipping and too many for practicality are required for adequate vertical spacing. Also, the tile industry has warned against the use of lightweight concrete for years.

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Photo 3 shows part of an integrated system for dealing with hydronic tubing: the lightweight molded foam boards serve as an insulator, a tubing spacer, and a screed for finishing with mortar. The system can also be used with made-for-tile self-leveling compounds (Photo 4). Whether you use a manufactured product like the system shown here, or a site-made system for spacing the tubes, the entire system - all of the zones being installed with radiant heat and tiles-should be pressurized, at least to normal operating levels, until the mortar bed or SLC has cured. This prevents any tubes from collapsing under the weight of the concrete or SLC, or the feet of any workers placing the material (Photo 5).

Hydronic systems are usually found on new construction or larger remodeling projects. To prevent contaminating the domestic water system, radiant hydronic tubing should be connected to a boiler dedicated to providing space-heating capacity. When maintaining a hydronic system in freezing conditions, an anti-freeze additive is highly recommended in the event of a power outage.

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Floor Warming Systems

Floor warming systems for tile vary in the way heat is brought to the tiles (or other finish), but all are based on the fact that electrical resistance produces heat. Some systems arrange the resistance wiring in a sheet-like carrier that is unrolled and fastened to the floor (Photo 6), while others are based on a single wire which is then evenly laced across the floor to be warmed (Photo 7). All such systems should be connected to a dedicated breaker by a qualified electrician. For this portion of the article, photos were taken during the installation of a system that uses thin plastic netting to arrange and space the resistance wires. Also included are photos of a single-wire system. When planning any electrical resistance (ER) system, the entrance of the power supply cable and thermostat wiring must be considered in the initial design of the floor (Photo 8). Usually, to result in a flat floor for the tiles, a groove must be chiseled into the setting bed for the power supply cable, which is considerably thicker than the heating wires (Photo 9).

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In this example, the netting carrying the hot wires was nailed flat over a layer of galvanized reinforcing mesh. The galvanized mesh is used to reinforce a layer of self-leveling compound (SLC) that will be used to level the surface of the wires. As you might imagine, care must be taken to avoid mashing the resistance wiring into the metal reinforcing mesh before the SLC has cured. Note the use of kneeling boards in Photo 10, as I use hot glue to flatten a portion of the mat.

Depending on the size of the tiles and the spacing of the resistance wires, there are numerous ways to handle the tile installation. Tiles large enough to span three resistance wires can be installed in one step, using a medium-bed latex thinset mortar. Smaller tiles tend to tip over the resistance wires, sink into the soft thinset, and cause unacceptable lippage and risky repair strategies. When installing tiles that span less than three wires, I recommend a made-for-tile self-leveling compound that can be used for a radiant installation (Photo 11). Yes, less expensive featheredge compounds can be used, but they come with a substantial increase in the cost of labor.

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Even more importantly, though, trowelling, screeding, and finishing featheredge mortar over resistance wiring greatly increases the odds for a wiring problem while the use of an SLC reduces the potential for problems.

Some SLC and featheredge compounds are not approved for use with tile, so it is important to check the material you use to see if it is rated both for tile and for radiant heat use. I prefer to single-source surface prep materials along with the thinset, grout, and movement joint sealant. When the tiles are to be installed in a two-step installation-covering the wires first and then installing the tiles-there is a bit more work, but far less stress on the resistance wires and the installer.

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The one-step method works, but it requires patience, careful workmanship, and often, considerable back-buttering. On all radiant installations, I aim for minimum 95 percent adhesive coverage to maximize heat-transfer, compressive strength, and impermeability (85 percent is the industry minimum for dry-area floors, 95 percent for wet area tiling). This is not too difficult to achieve on a hard, flat surface, but it's a challenge over a somewhat loose network of 1/8-, 3/16-,

or 1/4-inch wires.

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To assure good coverage when employing the one-step method, I test a variety of notch trowel sizes on-site and add 1/8-inch (total depth of thinset mortar) to account for slight irregularities in the floor plus the varying thickness of the resistance wires and matting. For most 12-inch tiles, I use a 1/4-by-1/2-by-1/4-inch U-notch trowel to spread thinset over the matting, and for best results, I back-butter each tile with a 1/4-by-1/4-by-1/4-inch square-notch trowel. This provides maximum adhesion, and it gives me ample room for tile adjustment.

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Movement Joints

Regardless of how the tiles are installed, a minimum 1/4-inch gap should be maintained between all the materials installed over the subfloor and the restraining walls. This joint should be free of any adhesive or grout residues and filled with a resilient caulk or sealant. Refer to the TCA Handbook for complete details regarding movement joints, their placement, and how to fill them.


TCA Handbook, ANSI A108 Specifications:

Suntouch Electrical Resistance Systems:

Watts Radiant Hydronic Systems:

Schluter BEKOTEC System:

Warmly Yours: