Installing Radiant Heat in Floors


Add warmth underfloor by installing radiant heat beneath your floors

Instead of blowing hot air through a vent or pumping hot water to a baseboard radiator along the wall, radiant heat warms up the entire floor. The heat is made possible in two ways: either through pipes carrying hot water embedded in or directly below the floor, or via electric mats in or below the floor.

Types of Radiant Heat

Electric systems are thinner and good for retrofits. Some wires are designed to double as thermostats, simplifying installation. You do not need to buy a separate furnace, so the initial cost is lower and no pipes will freeze or spring leaks.

Hot water is the standard for larger installations. It is generally more economical and uses gas, propane, oil, solar energy or geothermal energy (a heat pump) as a heat source. Because the flow of water can be minutely controlled, this system provides heat in the right amount exactly when and where you need it.

A hydronic floor is no less complicated than a zoned-baseboard heat system. It uses a boiler heated by gas, oil or electricity, and requires valves and manifolds to distribute the water, as well as sophisticated thermostats to control the heat. While an electric system may be less complicated, certain systems involve heavy-duty wiring and in some cases a new electrical panel.

Where to Install

You can install radiant heat systems either above or below the subfloor. Ideally, it’s placed above the subfloor so the heat source is actually heating up the flooring and not the subfloor.

In a retrofit, this can raise the floor a couple of inches, creating problems with appliances and doors.

In an existing home, installation is usually below the floor — as long as the area beneath the floor is accessible. You can install the heating elements right against the floor.

With a hydronic system, you can attach aluminum plates beneath the floor to spread and store heat.

Radiant heating works well with ceramic tile and vinyl, but not with wood floors. Its success with carpeted floors depends on the length of the weave – thick carpets or those with thick pads won’t work well as heat generated by the system will stay trapped in the floor.

Installing Underfloor Electric Radiant Heat

There are two basic types of electric radiant heat systems: low-voltage mats and high-voltage cables. Cables are usually embedded in concrete and require some electrical skills to install.

Mats, however, are easy to install and wire. Some have wires that double as thermostats: As the floor temperature rises, the wires produce less heat. Mats made for above-the-floor installation need a layer of mortar for protection.

Underfloor installations have to be insulated but otherwise need no protection.

Before starting an installation, check with an electrician to be sure your existing electrical system can handle the new circuit, if it needs one. In general, you will need between 8 and 12 watts per square foot of heated floor.

The system shown in this guide is one of many available, and installations vary. Always follow the manufacturer’s recommendations.

Staple the mats in place

Unroll the mat material and cut it to length with scissors.
Strip the insulation off an end of the two wires that run the length of the mat.
Turn the mat so those ends are facing the wires that will bring power to it.
Have a helper hold the mats against the bottom of the subfloor while you staple them in place.
Do not install the 2 feet or so of mat to which you’ll be attaching wire.

Connect the wires

Crimp a black wire on one mat wire and a white wire on the other.
Slip the crimping sleeve over the wire at the end of the mat and put a 14-gauge stranded, tinned copper wire inside.
Squeeze the crimping tool to crush the sleeve tightly over the wires.
Cover the connection with electrician’s tape.

Install insulation

Once all the mats are in place, install insulation between the joists, keeping it the recommended distance from the heating mats.

Installing above the subfloor

If you can’t get to the base of a floor, install electric mats above the subfloor. Keep in mind that they will have to be covered by a protective layer of gypsum or concrete. Staple the mats on top of the subfloor and wire as directed.

While some manufacturers advise spreading thin-set over their mats, it can be difficult to keep the floor flat and level. Mats that include a self-leveling gypsum mixture over them is best. Let it dry the appropriate length of time, then cover it with flooring as if you were installing the flooring over a concrete base. In bathrooms and kitchens, install a moisture barrier to protect the gypsum.

Installing Hydronic Heat

Start at a corner of the room and run tubing to the other side between the joists. Feed it into the bay between the neighboring joists through predrilled holes. When you have snaked tubing from one side of the room to the other, hold it in place with aluminum plates stapled to the subfloor.

Drill the holes for tubing

Drill a hole at one end of each bay so you can feed the tubing into the neighboring bay.
Plan where each hole needs to be and drill them all before installation.
Drill an oversize hole, as directed by the manufacturer, using a Forstner or similar bit. Spade bits aren’t durable enough for a big job like this. Space will be tight and the drill bit will have a thick shank, so use a right-angle drill with a ½-inch chuck.

Thread the tubing

Put a coil of PEX tubing on the floor at one end of the first bay while a helper stands on the other end of the bay.
Walk over to your helper while holding one end of the tubing.
Put the tubing through the hole in the joist and pull it back to the other side of the room while your helper feeds it through the hole.
When you reach the other side of the room, feed the tubing through the hole in the joist.
Have your helper walk the tubing back to the other side of the room and feed it through the next joist hole.
Continue until you’ve run tubing under the entire floor.

Fasten the dissipation plaTes

Once the tubing is in place, go back to where you began the tube.
Hold the tubing against the underside of the subfloor and put an aluminum plate over it.
Staple the plate to the subfloor.
Space the plates along the tubing as directed by the manufacturer.
If you have multiple loops, put a piece of tape on the ends of the tubing and label which end is connected to the heat source and which returns to the heat source.
Have an installer connect the valves, manifolds and pipes required to finish the job.

Above the floor hydronic heat installation

Like electric systems, you can install hydronic systems in a concrete, mortar or gypsum bed.

It’s easier to install the tubing in specially made plywood with precut channels. Nail the plywood in place as you would a subfloor or underlayment. At the wall, install pieces with curved grooves that make a U-turn, and send the tubing back across the room in another groove.

You can install carpeting and wood flooring directly over the plywood. Vinyl requires underlayment, and ceramic and stone floors should be set in a mortar bed or on cement backerboard.