All About Radiant Floor Heating

In-floor radiant heat is produced either with a hydronic system that pumps hot water through plastic tubing or with an electric system, where the heat is created with cables, similar to a heating pad. In a house that already has a boiler, hydronic tubing can often tie into existing equipment; it can heat a whole floor or even an entire house. Electric radiant can connect to any household’s service panel but can be costly to run, so it’s usually con#ned to discrete spaces, like a bathroom or mudroom. Climate is a factor, too. To realize the economic bene#ts of a hydronic system, it’s best in locations with winter temperatures that require consistent heating. It is radiant’s even, consistent heat that makes it so appealing. It also can be easier to live with than forced air, with no furnace #lters to change, no whistling ductwork, and no restrictions on where furniture can go to avoid obstructing the airflow. While some elements of a radiant heating system are DIY-friendly, it generally requires working with an experienced pro. Installations can range from a simple electric mat that heats a powder-room floor and is controlled by a single thermostat to a complex web of water-#lled tubing designed based on a house’s size, energy costs, and number of zones. Here’s what you need to know about the different options that are available and how you might be able to put them to use.

Radiant Floor Heating: Why It’s Worth It

Radiant Floor Heating: The Vitals

How much does it cost?

If you have a boiler, the materials, and labor to add a hydronic radiant floor system range from $6 to $20 per square foot. Expect to pay from $8 to $15 per square foot for an electric radiant system.

Is it energy efficient?

In a well-insulated home, a hydronic radiant system can be up to 30 percent more efficient than forced air. You can spend $1 to $5 per day to run either type of radiant, depending on your utility costs.

How long does it last?

Hydronic parts should be good for 40 to 50 years. While a boiler’s average life span is 15 to 20 years, it should last longer since radiant heat doesn’t require the very high water temperatures that radiators do. Electric radiant should last nearly as long as a hydronic system.

DIY or hire out?

Hydronic systems are supplied and installed by a plumbing contractor. DIYers can fasten aluminum transfer plates and clip in PEX tubing between ceiling joists but will need help designing the system and a plumber to connect the boiler. Electric radiant is more DIY-friendly, but you’ll want a pro to make the final electrical connections.

Is Radiant Floor Heat Right for You?

Here’s what to consider when deciding if a radiant system is a good fit for your reno.

Pros

QUIET: You won’t hear air blowing through ducts and out of registers. Hydronic is quieter than its cousins, radiator and baseboard heat, whose pipes can cause banging or metallic tingling as aluminum fins heat up. Electric radiant is silent. COMFORTABLE: With the temperature of the floor closer to your body’s preferred temperature of about 70 degrees F, you feel comfortable even when the air temperature in the house is cooler. ENERGY EFFICIENT: “Nothing beats water for the transfer of energy,” says TOH plumbing and heating expert Richard Trethewey of hydronic radiant. “Put a Btu of energy into water, and it all sticks. Do the same with air, using electric elements, or gas- or oil-fired coils, and it drops off by about two-thirds.” Though pricier to run, electric systems still benefit from radiant’s efficient heat transfer. FURNITURE FRIENDLY: With no radiators or registers to consider, radiant heat doesn’t affect furniture placement. EASY TO MAINTAIN: There are no filters to replace, and no dust gets stirred up. It’s a good idea to have boilers inspected annually and hydronic systems pressure tested for leaks. Electric radiant doesn’t generally require maintenance.

Cons

PRICEY: Whole-house hydronic systems are costly to install, particularly if you already have forced air and need to put in a boiler. Electric radiant is less expensive to install but is usually confined to discrete spaces such as baths. Depending on the size of the space, you might need an electrician to make room for the load in your service panel. HATES DRAFTS: Hot always goes to cold, as Richard says, so in an old house that’s not well insulated, the heat transferred by a radiant floor system is robbed by cold walls, and air leaks around doors and windows, making it harder to feel comfortable. SLOW TO RESPOND: Because of the floor’s thermal mass, it can take hours (or even days, for a concrete slab) to reach a comfortable temperature after you adjust the thermostat. Fine-tuning that setting—to make it cooler on days with a lot of sun shining through the windows—also takes a lot of time. Sudden temperature swings reduce the radiant’s ability to deliver a consistent level of indoor comfort. ONLY PROVIDES HEAT: Unlike a forced-air system, which can have a cooling coil to also deliver air-conditioning in the summer, radiant systems almost exclusively deliver heat.

How Radiant Heat Stays Consistently Warm

Different heating systems rely on different means of transferring energy. In a forced-air system, a furnace’s heat directly warms the air and fans push out the hot air to spread the warmth around. This creates inconsistent temperatures across a room, with the hot air rising to the top and the cool air falling to the floor. Radiant heat transfers its energy to objects, including people, through infrared radiation, like the sun. Surfaces that absorb infrared heat continue to radiate it out. As a result, not only is radiant heat more consistent and more comfortable, but it also produces a more efficient transfer of energy, since it suffers almost no heat loss between the heat source, the floor, and the rest of the room— unlike leak-prone ductwork. As the heat rises, the coolest air stays near the ceiling.

Radiant Heat Product Options

When it comes to warming floors, there are two options: water-filled PEX tubing or electric cables. Here are some of the most popular products and where you might use them.

Hydronic systems

Flexible, easy to work with, and available in large spools, the cross-linked polyethylene tubing known as PEX is the backbone of any hydronic radiant floor heating system. Because it can snake its way under a floor in continuous loops with interruption, it reduces the chance of leaks. Usually spaced about 9 inches apart, PEX tubing can go where electric cables aren’t allowed, including in walls and ceilings—and it’s usually less expensive to heat its water to warm large surfaces like a family room floor or even a driveway. PEX can be embedded on its own in concrete slabs; to heat other types of floors, it is typically part of a product system —generally plates or panels—that employs a medium, usually aluminum, to help transfer and evenly distribute the water’s temperature to the floor.

Aluminum over plywood panels

These ó-inch-thick, 7-by-48-inch plywood panels have an aluminum layer that runs under the tubing tracks. Designed for additions and new construction, they add a minimal ó inch to the floor height when fastened over an existing subfloor or slab concrete. Silicone sealant bonds the PEX in place and facilitates heat transfer to the aluminum and the entire floor. SHOWN: Uponor Quik Trak, about $6 per square foot (without PEX); supplyhouse.com

Aluminum transfer plates

This retrofit system is best suited to installing radiant above an unfinished space. The 48-by-3ó-inch, 16-gauge aluminum plate is designed to fasten to the underside of a subfloor—flat side up, tubing channel down—usually in a basement ceiling. The plates accept .025-inch-thick PEX tubing; the hot water running through it gives up its heat to the aluminum, which in turn warms the floor above. SHOWN: Uponor Joist Trak Heat Transfer Panel, from $9.44 each; supplyhouse.com

Aluminum over OSB panels

A panel product more suited to use in a renovation, this option has a layer of aluminum bonded on top of an oriented strand-board substrate. The finished product measures just $”⁄’( inch thick, making it easier to screw down over an existing subfloor (or in walls or ceilings). Adding it might mean trimming the bottoms of doors and adding transitions between dissimilar surfaces to bridge finished floors of different heights. SHOWN: Warmboard-R panel, about $21 per square foot, installed; warmboard.com

Electric systems

Electric cables are the newer form of radiant floor heating, with more DIY-friendly products. While the systems come in different forms—from a dimpled membrane to a mesh mat to a thin film—the delivery is the same: A home circuit delivers energy to the cables, exciting them and transferring that energy to the floor above it as heat. Some manufacturers will help you design a pattern for your project, or create a custom mat that doesn’t need cutting. Under tile or stone, the cable should be nestled in an uncoupling membrane, which allows for movement in dissimilar materials to prevent cracking. While a home may have the panel capacity to power multiple cables or mats, it’s usually cost-prohibitive to heat large spaces with electricity. Note that cables are prone to nicks during installation.

Electric cable with uncoupling membrane

Electric radiant is popular under tile and stone floors, but because these materials can crack with movement, the heating element is nestled in a spongy waterproof membrane that allows the substrate and the finished floor to move independently. Electricity flows through 120- or 240-volt cables, generally 1⁄4 inch in diameter and spaced about 3 1⁄2 inches apart. The cables are laid around the polypropylene membrane’s nodes in a custom pattern that accounts for obstacles, such as toilets and vanities. SHOWN: DITRA-HEAT Uncoupling Membrane, from $2.69 per square foot, and DITRA-HEAT-E-HK Cable, from $2.77 per square foot; schluter.com

Electric mat

Cables are captured within a mesh and sold as a roll. For layouts with a uniform shape, installation is straightforward, but you can also cut the cable free of the mesh to work around objects, like a sink pedestal. Mats come in a range of sizes that can cover more than 150 square feet, some with a hole to fit around a shower drain. Others stick down with double-sided tape and can be laid over plywood, backer board, or a concrete slab. SHOWN: SunTouch TapeMat, from $169 for 10 square feet; warmyourfloor.com

Electric film

The film encasing the electric cable in this format is designed to float under click-together floors—vinyl, laminate, solid or engineered wood, or floating tile—with an underlayment over the subfloor. Available in 36- and 18-inch widths, it can also be used in exposed joist bays underneath a floor. Some versions are wired on both ends, so you can cut it to fit and use both sections wired to the same thermostat. SHOWN: QuietWarmth Heating Film for Click-Together Floors, from $57 for 7 1⁄2 square feet; warmyourfloor.com

Electric peel-and-stick film

In this newer form of electric radiant, the cables are woven within a peel-and-stick waterproof, anti-fracture membrane, saving the step of having to fasten the heat source to the substrate. It is designed for use under tile or stone (though not in a shower) and glue-down floors, including laminate and engineered wood (but not vinyl or linoleum). SHOWN: QuietWarmth Peel & Stick for Tile and Glue-Down Floors, from $84 for 7 1⁄2 square feet; warmyourfloor.com TOH Pro Tip: “Because a tiny nick can cause a malfunction, it’s good practice to test the resistance of any electric radiant system and floor-temperature sensor with a multimeter before and after installation, and after covering it with flooring. Some manufacturers require a log of these tests for warranty purposes.” — Charlie Silva, Home Builder

Where to Use Radiant Heat

Unlike rigid copper pipes and hot air, flexible PEX and electric cables make it easy to add heat to a wide range of surfaces UNDER WOOD FLOORS: Water circulates under the flooring between 85 and 125 degrees F, while cables typically use 120 volts, or sometimes 240, to gently warm wood to around 75 degrees without drying it out. All wood floors require a plan for expansion and contraction—usually by leaving a gap around the perimeter covered by molding. But it’s even more important over radiant flooring, where the boards can move more. In general, the narrower the boards, the better. UNDER TILE FLOORS: The thermal mass of thinset and tile makes a bathroom ideal for radiant heat, which can take the chill off floors, the tubs sitting on them, built-in benches, and even shower floors and walls, in some cases. While hydronic radiant is an option in the bathroom when used elsewhere in the house, electric cables or mats are frequently used because they deliver the warmth without the need for upgrading the home’s heating equipment or adding significant height to the floor. Shower walls are one of the few places other than floors where you’ll find electric radiant, because of the walls’ thermal mass and the lack of mechanical fasteners needed. Near an exterior wall, “the idea is to warm the tile so that the insulation behind the wall prevents the cold air from feeling like a draft,” says TOH general contractor Tom Silva. UNDER CONCRETE FLOORS: Like a bathroom’s tile, the density of a concrete slab in a basement or garage is ideal for radiant heat, where it can ensure cars, workshops, or home gyms are comfortable year-round. While both forms can be used, hydronic is typically installed during initial construction; the electric option can be added easily to existing slabs since it doesn’t add much height to the floor. UNDER KITCHEN COUNTERS: Big slabs of stone are susceptible to absorbing cold temperatures, especially near a window. “Say you have a granite countertop in the kitchen with a big window overlooking the sink,” says Tom. “The cold air that comes off the glass hits the cold countertop, and that falls down to your feet and feels like a draft, even with radiant floors,” he adds. “But if you temper that countertop, you’re adding more surface area that’s closer in temperature to your body, and you stop the falling of cold air.” STAIRWELLS: Cold air can pool at a stair landing or along a stairwell with lots of windows. Hydronic heating behind a drywall, plaster, or wainscoted wall can temper the wall surfaces and take the chill out. In most cases, code prohibits running electric radiant heat behind walls, other than in a shower. But there are sleek electric radiant-heated panels that hang on the wall, plug into an outlet, and work the same way. OUTSIDE: Tom and Charlie are fans of using radiant tubing to melt snow off steps, patios, and walkways. Both electric and hydronic can be used, though the latter is more common since many cold-climate homes have boilers. It’s installed underneath the masonry; a switch or a temperature sensor controls the snow melting. Some pros prefer to use a PEX-aluminum-PEX sandwich, often called PEX-AL-PEX, in these applications. PEX tubing can also run under the root zone of a garden bed, extending the growing season in colder climates. TOH Pro Tip: “In-floor radiant can cause wood floors to expand and contract even more than usual. To reduce the chance of the boards cupping, we generally keep them less than 6 inches wide.” — Tom Silva, General Contractor

Energy-Efficient Upgrades for a Hydronic System

While a conventional boiler is commonly used to produce a radiant system’s heated water, there are better options. Here are two of Richard’s favorites

High-efficiency boiler

Also known as a condensing boiler, a high-efficiency boiler sends less of the energy you pay for up the flue. Where a typical boiler may be 80 to 85 percent efficient, a condensing boiler can be as much as 95 percent efficient. The boiler captures and uses heat from its own exhaust system; pumps push the water it heats through a manifold that distributes it through the PEX in each zone. Though condensing boilers can cost twice as much as standard models, they can significantly reduce energy costs in a cold climate.

Air-to-water heat pump

Powered by electricity, a heat pump can find heat outdoors on the coldest days. In an air-to-water heat pump’s closed system, the refrigerant expands as it circulates outside, where it gets very cold—sometimes down to minus 20 degrees F. So even on a 0-degree day, it’s warmer outside than in the refrigerant lines. Since hot goes to cold, the outdoor air transfers heat to the refrigerant. The refrigerant is pumped into a compressor, which raises its temperature; it then moves through the system as a gas, and a heat exchanger transfers its heat to a hot-water reserve tank. A pump pushes the water toward the radiant system’s manifold. While it’s a definite investment, this type of heat-pump system can also provide household hot water—and, by reversing the process, produce air-conditioning, too.

Important Things to Know About Radiant Floor Heat

While radiant heating has many benefits, there are some fine points to consider

Take care with wood floors

Wood’s natural expansion and contraction can turn to cupping when exposed to radiant heat that is set too hot. Keep the surface temperature of all wood flooring to a maximum of 80 degrees F to prevent warping. Generally, engineered flooring’s plywood layer makes it more stable over radiant heat than solid hardwood. But not all engineered flooring is approved for use over radiant heat, so check with the flooring’s manufacturer. Some wood species are more stable than others; cherry and Douglas fir handle heat better than beech, white oak, or maple.

Insulate to keep the heat

The space below a radiant system can sap its effectiveness, costing you money. Plan to insulate with batts or spray foam underneath aluminum transfer plates between joists to ensure the heat moves up, and not down into the space below. Likewise, if you’re pouring a new concrete slab for an addition, foam panels designed to hold the PEX tubing can insulate from below, ensuring the heat moves up and through the slab. Electric-radiant manufacturers also offer a thin polystyrene underlayment to install under cables, radiant mats, or radiant film products to prevent heat loss.

Skip thick floor coverings

The conventional wisdom used to be that putting a rug or carpet over radiant floor heat was like putting a sweater on a radiator. But with the right planning, you can combine radiant heat with floor coverings. Opt for lower-pile carpet whenever possible, and keep pads less than 3⁄8 inch thick. Regardless of the covering, the goal is to keep the total R-value at less than 4—aim for 2.5. Since most carpets and rugs don’t come with a listed R-value, here’s a quick calculation: Measure the total combined thickness and multiply that by 2.6 to approximate the covering’s R-value. Carpet pads’ R-values range from .78 for 7⁄16-inch-thick waffled sponge rubber to 2.09 for 1⁄2-inch-thick rebond, or for bonded polyurethane made from scraps. Be aware that, for safety reasons, some electric radiant manufacturers prohibit the use of their product under carpeting.