Step 1: Overview
This in-the-floor heating system consists of one thin continuous cable heating element woven into a mat that you install under the tile. This makes it a project best done when overhauling or changing the floor covering of an existing room or when adding a new room. It can be installed as supplemental heat to take the chill out of the floor or as space heat to warm the entire bathroom. It’s also a great project for warming entryway and kitchen floors.
- The benefits?
- It’s easy to install. You embed a cable-laced mat in the mortar when you lay the tile. If you’re not comfortable with the wiring portion, hire an electrician.
- It’s safe. Once the heating system is installed, it’s nearly impossible to damage. The GFCI-protected thermostat instantly cuts power in the event of a short or other problem.
- It’s inexpensive to operate. At 12 watts per square foot, our 30-sq.-ft. mat drew 360 watts of power— about the equivalent of an electric blanket or large TV.
- It takes up zero space. Got a big, clunky radiator? Remove it and gain valuable square footage by installing this stuff.
- It’s versatile. If your existing furnace or boiler doesn’t have enough oomph to heat a newly remodeled or added space, floor heat can do the job.
- It’s really, really comfortable. When your feet are warm, your entire body feels warm. You’ll find yourself reading and playing games with your kids on the bathroom floor.
The downside? It can’t be retrofitted under existing tile floors, the total initial cost of materials is high, and you’ll most likely need to run new wiring from the main circuit panel to the bathroom.
Step 2: Find electrical power
For a heated floor area less than 20 sq. ft., you could (in most cases) draw power from an adjacent GFCI-protected outlet without overloading the circuit. (If the thermostat you purchase is already GFCI protected like ours, you can use any outlet. In any case, the mat must be GFCI protected.) But a larger mat on an existing circuit— a circuit that might also accommodate a 2,000- watt hair dryer—can cause overloads and nuisance circuit breaker trips. For our larger mat, we elected to install a dedicated circuit with its own wiring and circuit breaker. Both 120-volt and 240-volt mats are available.
A programmable thermostat that turns the mat on during busy times, then off when you’re sleeping or away, costs more initially but will save energy and money in the long run.
Step 3: Special-order your custom-size mat
Photo 1: Test the mat for continuity
Test the heating cable for manufacturing or shipping damage with a volt-ohm meter. The resistance reading on the mat label and the resistance registered by the meter should be within 10 percent of each other. If not, see the manufacturer’s instructions. Digital volt-ohm meters like the one shown are inexpensive and are easy to operate.
Photo 2: Add cement board to the subfloor
Install cement board over the existing subfloor. Trowel on a layer of thin-set mortar, then secure the cement board with cement board screws. Cover the seams with mesh fiberglass tape and thin-set to create a “unibody” floor. Snap chalk lines on the floor to mark the tile layout.
A number of companies offer electric resistance floor warming systems. Standard sizes are available at home centers and tile stores. You can also special order custom sizes and shapes by sending a detailed drawing of the bathroom floor plan and location of fixtures. The mats come in 12-, 24- and 30- in. widths and increments of 5 ft. in length (10 sq. ft. minimum). When in doubt, the supplier will specify a mat smaller than you need since the mat cable can’t be cut.
Before installing the mat, use a volt-ohm meter (Photo 1) to obtain a resistance reading to make sure it wasn’t damaged during manufacturing or shipping. Prep your floor as you would for any tiling job. Install 1/2-in. cement backer board, securing it to the existing subfloor with mortar and cement board screws (Photo 2). Make sure no screw- or nailheads protrude above the cement board. A sharp edge can damage the cable. Tape and mortar the seams to create a solid, continuous surface. Snap tile layout lines on the floor once the mortar has dried.
Step 4: Test-fit the mat to avoid glitches
Photo 3: Check the fit
Test-fit the mat, keeping the cable 4 in. from fixtures and walls and 2 in. from one another. Give priority to those areas where you’ll stand barefooted the most. You must not cut or cross the cable, so make sure the mat fits.
Photo 4: Notch a path for the wires
Chisel a groove in the cement board for the enlarged portion of the power lead to nestle into. Notch the bottom plate of the wall to provide a pathway for the power lead, thermostat wires and conduit.
Before proceeding with the actual installation, do a test layout (Photo 3). Follow these basic guidelines:
- Install the mat up to the area where the vanity cabinet or pedestal sink will sit, but not under it; that can cause excessive heat buildup.
- Keep the mat 4 in. away from walls, showers and tubs.
- Keep the mat at least 4 in. away from the toilet wax ring.
- Keep the blue heating cable at least 2 in. away from itself (Photo 5). Never overlap the cable.
- Don’t leave large gaps between the mats. Your feet will be able to tell!
- If your mat is undersized, give priority to the areas where you’ll be standing barefoot most often!
Following your preliminary layout, mark the path of the thick “power lead” between the mat and wall cavity (Photo 4) and chisel a shallow trench into the floor. Notch the bottom plate to accommodate the two conduits that will contain the power lead and the wires for the thermostat-sensing bulb.
Step 5: Glue and tape the mat in place
Photo 5: Tape the mat to the floor
Install the mat, securing it lightly to the floor with double-face tape. Cut the mat (never the cable), then reverse direction at walls.
Photo 6: Glue loose wires to the floor
Secure individual removed cables to the floor using small blobs of hot-melt glue. Carefully cut and remove the orange mesh to free the cable. Do this to work around angles, obstacles and sections where full-width mats won’t fit. Do not overlap the cable. When the entire mat is fitted and installed, press it firmly into the tape and hot-melt glue any loose ends or humps in the mat. Perform a resistance test (shown in Photo 1) to check for damage.
Install the mat, securing it lightly to the cement board with double-face tape (Photo 5). To make turns, cut the mat between two loops in the cable, then flip the mat and run it the opposite direction. Never, ever cut, nick or stress the cable itself. Where the full-width mat won’t fit, or where you encounter angles or jogs, carefully cut the mat from around the cable, and hot-melt glue the cable to the floor (Photo 6). Continue using the full mat again when you can.
Install the entire mat complete with cuts, flips and turns to make sure it fits the space right, make any final adjustments, then press the mat firmly into the tape. Use hot-melt glue to additionally secure the mat. Don’t leave any humps or loose edges; you’ll snag them with your notched trowel when you’re applying the thin-set mortar.
If you’re not going to tile right away, lay thick corrugated cardboard over the mat to protect the cable. You’ll be glad you did when your kid walks in wearing baseball cleats.
Step 6: Wiring setup
Photo 7: Add the thermostat wires
Fish the power lead and thermostat wires through two 58-in. lengths of conduit and connect the tops of the conduit to a 4 x 4-in. electrical box. Position the lower end of the conduits in the notches and secure the electrical box to the studs. Weave the thermostat wire through the mesh so the sensing bulb is an equal distance between wires and 12 in. into the warming area. Use hot-melt glue to secure the thermostat wires to the floor and the power lead in the groove. Cover the notches in the bottom plate with protective metal plates. Do another resistance test.
Install conduit connectors to both ends of two pieces of 58-in. long 1/2-in. electrical metal tubing (EMT). Fish the power lead cable through one length of conduit. Hot-melt glue the power lead into the groove. Fish the thermostat wires through a second piece of conduit, then weave it 12 in. into the mat, keeping it equidistant from the cable on each side (Photo 7).
Secure the two lengths of conduit to a 4 x 4-in. metal electrical box. Secure this box to the studs so the lower ends of the conduits nestle into the notches you made in the bottom plate (Photo 7). Secure metal protective plates over the notches in the bottom plate to protect the wires and cable where they pass through.
Install the wiring from the area of the main circuit breaker panel (or nearby outlet) to the area of the wall cavity where the thermostat will be located. Don’t do any actual wiring in the main panel yet.
Step 7: Install the tile
Photo 8: Spread the mortar
Apply the mortar, first pressing it firmly into the mesh and floor with the flat side of the trowel, then combing it with the notched side. Try to “float” the trowel just above the cable. Use care not to snag the mesh or nick the cable.
Photo 9: Place the tiles
Lay the tile using the chalked lines as your guide. Wiggle and tap the tiles firmly into place to create a level surface. Readjust previously laid tiles so they remain in line and properly spaced; the thick mortar bed used to cover the cable and mesh allows for more movement than a standard tile installation. Grout the tile once the thin-set has properly set.
Select tile that’s at least 6 in. square so each tile will span two or more sections of cable. Smaller tiles are more likely to conform to the minor hills and valleys of the cable when you tamp them in place, creating a wavy surface.
Spread the mortar over a 5- to 10-sq.-ft. area of floor. Use the flat side of the trowel to press the mortar firmly through the mat and into contact with the cement board. You can establish a flat, uniform layer by lightly floating the trowel across the tops of the cable. Then use the notched side to comb the mortar to create ridges (Photo 8); a 3/8 x 1/4-in. trowel works well for most tiles. Again, lightly skim your trowel over the cable. The sheathing on the cable is tough, but you still need to avoid any “sawing” type action or jabs with the trowel. It takes a little trial and error to get a flat layer. The No. 1 goof that people make is slamming the edge of their trowel on the floor to knock excess thin-set loose—cutting or nicking the cable.
Place the tile, then tap it firmly into place with a rubber mallet. Do two resistance tests (Photo 1) while installing the tile to ensure you haven’t damaged the cable. (If the resistance test fails, see the manufacturer’s instructions to find the problem.) Once the mortar has dried, grout the joints.
Step 8: Final steps
Photo 10: Finish the wiring
Wire the thermostat according to the manufacturer’s instructions. Our thermostat had individual pigtails for securing the wires from the power lead and the cable running from the main panel. Have your electrician make the final connections in the main circuit panel. Power up the system for 10 or 15 minutes to ensure that the floor heat functions, then turn it off and keep it off for two to four weeks while the mastic and grout cure and harden.
The instructions that came with the mat and thermostat were so darn good we felt comfortable completing the wiring of the thermostat and mat. We left installing the new circuit breaker and final connections in the main circuit panel to a local electrician. We suggest you do the same. Once the wiring is complete, energize the system for a few minutes to make certain the controls work and resistance cable heats. Don’t put the system into full operation until the thin-set and grout have had time to properly cure and harden—usually two to four weeks. Then call the family together and play a game of Scrabble on your cozy, warm bathroom floor.
Required Tools for this Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
- Cordless drill
- Hot melt glue gun
- Utility knife
Volt-ohm meter (or continuity tester)
Required Materials for this Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
- Cement board underlayment
- Radiant heat mat
- Thin-set mortar