You finished that long, hot shower. You’re
squeaky clean and cozy warm—until you step onto the tile floor. Then you get
another sensation—ice-cold feet. The solution isn’t to banish tile from
bathrooms—it’s too durable, water resistant and easy to clean to do that. The
answer is to warm up that tile from below with electric resistance
heat.
Project Facts
Cost: The floor heating mat costs $12 to $14 per
square foot; the thermostat costs $130 to $190. Add the price of tile
materials, cement board and electrical work.
Time: Mat installation added 6 hours to the floor
tiling project; the electrical box, conduit and thermostat installation added 3
hours.
Skill
Level: Intermediate wiring and tile-laying
skills.
Most
Common Mistake: Slamming the trowel down to knock off mortar, and in
the process, nicking the cable.
Lots Of Pros, Few Cons
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. If you operate it only during the high-traffic morning and
evening hours, this translates into 25¢ to $1 per day, depending on your
location, electrical costs and the season.
-
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 about $15 to $20 per square
foot, and you’ll most likely need to run new wiring from the main circuit panel
to the bathroom.
Where To 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.
Special-Order Your Custom-Size Mat
A number of companies offer electric resistance floor
warming systems. We ordered our electric mat, thermostat and installation
materials from Watts Radiant (see Buyer’s Guide, p. 64). We sent them the
required detailed drawing of the bathroom floor plan and location of fixtures.
In two days, they faxed back a proposed mat layout and bid. 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 company will specify a mat smaller than you need
since the mat cable can’t be cut. A good instructional video comes with the
materials. A few common mat sizes are beginning to be available at some home
centers.
When you receive 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). Tape
and mortar the seams to create a solid, continuous surface. Snap tile layout
lines on the floor once the mortar has dried.
Test-Fit The Mat To Avoid Glitches
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.
Do another resistance test.
Glue And Tape The Mat In Place
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.
Wiring Setup
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.
Tip
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.
Install The Tile
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.
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.
Final Steps
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.
Buyer’s Guide
SunTouch floor-warming mat and thermostat by Watts
Radiant,3131 W. Chestnut Expressway, Springfield, MO 65802; (888)
432-8932. www.suntouch.net
Click image to enlarge.
1. Test the heating cable for manufacturing or
shipping damage with a volt-ohm meter. The resistance reading on the mat label
(in our case, 31-35) and the resistance registered by the meter (in our case,
33.2) should be within 10 percent of each other. If not, see the manufacturer’s
instructions. Digital volt-ohm meters like the one shown cost less than $30 and
are easy to operate.
Click image to enlarge.
2. 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.
Click image to enlarge.
3. 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.
Tip
Make sure no screw- or nailheads protrude above the cement board.
A sharp edge can damage the cable.
Click image to enlarge.
4. 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.
Click image to enlarge.
5. Install the mat, securing it lightly to the
floor with double-face tape. Cut the mat (NEVER THE CABLE), then reverse
direction at walls.
Click image to enlarge.
6. Secure individual 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.
Click image to enlarge.
7. 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.
Click image to enlarge.
8. 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.
Click image to enlarge.
9. 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.
10. 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.
