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Installing Radiant Heat in Floors


Installing Radiant Heat in Floors


Principle of Radiant heat: 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. No matter where you stand on a radiant floor, you are always directly above the heat source.


Energy-efficient system: In order for you to feel warm, the system only needs to heat the floor up to about 85 degrees. Studies show that your energy costs may be 25 to 50 percent lower.


Get heat in the floor two ways: Through pipes carrying hot water embedded in or directly below the floor, or via electric mats, also in or below the floor get the heat.



Electric systems are thinner and are good for retrofits. Some wires are designed to double as thermostats, simplifying installation. You need not 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 to operate and uses gas, propane, oil, solar energy, or geothermal energy (a heat pump) as a heat source. Because the flow 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 requires a boiler heated by gas, oil, or electricity. It 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 case a new electrical panel.


If you are hands on person, find an installer who will work with you. Because running tubing or installing electrical elements is simple and time-consuming, it is the perfect job for a homeowner. If you are interested in saving a little money then do the heavy lifting yourself.


Above or Below?


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 material 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.


Whether the system is electric or hydronic, install insulation beneath it. If you don’t, half of the heat you generate will seep away into the room below the one you are trying to heat.


Picking the Right Flooring


Radiant heating works great with some types of flooring, but not as well with others.


• Ceramic tile has long been a favorite candidate for radiant heat flooring. You can lay the wires or tubes 
  in a mortar bed underneath, and the tiles themselves conduct heat quickly. Wood has been considered 
  a problem flooring in this arena because heat dries out wood, causing cracking and creating gaps 
  between boards. Properly installed, wood does work over a radiant heat system

• Vinyl works well. It can be applied directly over a mortar, concrete, or gypsum base that covers the 
  heating elements. Because it is thin, the floor heats quickly

• Carpet may or may not work, depending on the type. Thick carpets, or those with thick pads, won’t 
  work well because they act as insulation. Heat generated by the system stays 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 ability of the wire to produce heat drops.

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. Make sure the floor you’re installing under is suitable for radiant heat, and make sure the system is designed to account for climate and building insulation.

The system shown here is one of many available, and installation procedures vary from manufacturer to manufacturer. The system shown here is low voltage and uses a wire that acts as its own thermostat. Other systems require either a wall thermostat or one mounted directly in the floor. Always follow the manufacturer’s recommendations.

Step 1: Staple the mats in place

Step 1: 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.

Step 2: Connect the wires

Step 2: Connect the Wires Crimp a black wire on one mat wire and a white wire on the other. You will need a tinned copper sleeve for the job and a tool called a crimper. 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.

Step 3: Install insulation

Step 3: Install Insulation Once all the mats are in place, install insulation between the joists, keeping it the recommended distance from the heating mats.

Installing Radiant Heat Above the Subfloor

Installing Radiant Heat Above the Subfloor

If you can’t get to the base of a floor, install electric mats above the floor. 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. Have the work done by a company that pumps a self-leveling gypsum mixture over the mats. 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, you’ll need to install a moisture barrier to protect the gypsum.

Installing An Under floor Hydronic System

PEX tubing, the heart of a hydronic heating system, carries hot water along the length and width of a floor. PEX is short for cross-linked polyethylene, a plastic that has been manufactured specifically to remain flexible and withstand heated water.

Unlike electric mats, hydronic systems aren’t prepackaged. Work in the room which is below the floor you are going to 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. Some companies prefabricate the plates; others require you to bend your own. It’s not all that difficult, but prefab is definitely quicker.

Have a professional design the layout: PEX comes in various diameters, and you want to make sure your system is properly sized. In addition, large areas or those with small spaces between the joists may require two sets of loops under the floor.

Step 1: Drill the holes for tubing

Step 1: 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 1/2-inch chuck.

Step 2: Thread the tubing

Step 2: 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.

Step 3: Fasten the dissipation plates

Step 3: Fasten the Dissipation Plates Once the tubing is in place, go back to the beginning of the job. 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 Installation of A Hydronic System

Above-the-Floor Installation of A Hydronic System

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 backer board.

Flooring Projects: Easy As 1-2-3

The information on this page is taken directly from our printed book Flooring 1-2-3®, which shows you how to plan and execute dozens of different flooring projects. Like our other 1-2-3® series books, this handy volume contains all the knowledge you need to do the job quickly...and get it right the first time. Detailed photos and diagrams, helpful tips from the pros, and step-by-step instructions let you learn and work at your own pace—and help you avoid common mistakes in do-it-yourself projects. Pick up your own copy at any location of The Home Depot or order it online.


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