This story shows how to install above-floor electric mats for radiant heat.
Using your dimensional plan, mark the location of the thermostat on the wall and install a junction box. Use a flanged box in new construction and a retrofit unit for an existing wall. In both cases, install the box 60 inches above the floor. Using 12/2 cable, add a new circuit or extend an existing circuit but do not connect the circuit to the power source.
Unpack the mat. Check the resistance using an ohmmeter. The reading should be within 10 percent of the rating listed on the UL label. Write the reading on a piece of paper. You will need it when installing the mat to make sure that the heat cable has not been nicked during installation.
If the power lead is thicker than the mat (some come with flat ribbon leads) you can sink it into the substrate if you've used cement backerboard. Use a cold chisel to cut a channel for it. If you don't recess the lead, you'll have to make sure it's adequately covered with thinset.
If your system uses a thermostatic sensor, weave the sensor bulb between two heating elements. Adhere the bulb wire with dots of hot glue. Now check the mat resistance with an ohmmeter. If the reading falls outside the manufacturer's tolerances, find the damaged mat and replace it.
The heating element is only a layer of thinset away from the tile (shown), brick, or stone flooring. Power cables supply electricity; a sensor bulb relays the temperature back to the control. Insulation beneath the subfloor increases heating efficiency.
With the flat side of a 3/8-inch notched trowel, apply thinset over an area of the mat. Then turn the trowel over and rake the thinset to 1/4-inch uniform depth. Be careful not to snag the mat. Do not clean the trowel by banging it on the mat. Tile the area of the floor covered with thinset.
Using a jeweler's screwdriver, attach the two sensor wires to the screw terminals on one side of the control. Connect the ground from the mat power lead directly to the house ground. Wire the rest of the thermostat connections following the manufacturer's instructions.