Underfloor radiant heating, whether delivered through an electrical or hydronic system, is a luxury on those cold autumn and winter days that seem to fill up more and more of the calendar as the holidays get closer. Installers and contractors looking for new business opportunities may be tempted to start installing these systems immediately, but first it’s important to understand the basics.

We spoke to several installation industry experts about some basic questions installers might have when working with radiant heating systems. Our panelists this month are Russ Gaetano, Ardex’s senior marketing manager, and Thomas Utley, technical service consultant; Will White, director of technical communications and training for Custom Building Products; Monica Irgens, president of Electro Plastics/STEP Warmfloor; Arthur Mintie, Laticrete’s senior director of technical services; Deanna Summers, MP Global Products’ marketing specialist and account manager; Sean Gerolimatos, Schluter Systems’ director of research and development; Julia Billen, owner and president of WarmlyYours Radiant Heating; Janis Hubina, marketing coordinator for Warmup; and David Downey, sales and technical support for ACG Materials/AccuCrete.

What types of flooring are safe to use over radiant heating? What are the temperature tolerances?

Gaetano: Certainly tile is the most common floor covering used over in-floor heating systems, but manufacturers of wood, LVT and carpet are starting to approve some of their floor coverings for use over in-floor heating systems. The thermostats range from 82 degrees Fahrenheit—which is the default maximum setting for laminates—to 104 degrees Fahrenheit as the default maximum setting for tile. The flooring manufacturer will be responsible for providing maximum temperatures for their particular product offering.

Billen: Essentially every commercially available floor covering is completely safe to use over a properly installed radiant heating system. The most important consideration is matching the floor covering to the right floor heating system. 

For example, you wouldn’t want to pair our floor heating system typically used with floating wood or laminate floors (a cut-and-turn roll product called Environ Flex Roll) with LVP because the heat from the roll would cause the typically thin LVP to conform to the shape of the heating cables, which would be visible in the finished floor. The solution then is to completely embed one of our TempZone floor heating products (cable, mat or cut-and-turn roll) in self-leveling cement. This will ensure a perfectly flat surface on which to install the LVP.

Another point to consider is that some flooring types might have a set-back temperature limit, meaning that there is a threshold for how much of a fluctuation in temperature a flooring type can experience in a given timeframe. This information should be available from the flooring manufacturer. If you do have a floor covering with a set-back temperature limit, the best thing to do is to pair your radiant heating system with a nonprogrammable thermostat (like our nTrust model) so that you can simply set the thermostat and leave it running at a continuous temperature.

A few manufacturers of certain hardwood floor coverings do not recommend that you install them with a floor heating system. This doesn’t apply to the vast majority of wood floors nor is it always consistent based on the species of wood, so we highly recommend that you check with the manufacturer prior to installation. Another type of flooring that can be problematic for radiant heating are floor coverings with high R-values. One example would be carpet tiles with pre-attached insulating foam pads, which can prevent heat from effectively radiating into the room.

Mintie: Ceramic, porcelain and stone tile floors are often considered the best materials to use with underfloor heating thanks to excellent heat transfer properties. Meanwhile, thicker stone and marble floors have good thermal conductivity but may require more time to heat up. Hardwood, laminate or resilient finishes have inherent properties that could require a slightly different system.

Ceramic tile and stone installation materials are inherently more tolerant of applications exposed to heat. Typically, latex thin set mortars and grouts can handle upwards of 400 degrees Fahrenheit (205 degrees Celsius); electric radiant heat flooring systems usually operate at temperatures between 80 and 120 degrees Fahrenheit (26 to 49 degrees Celsius). Finishes and/or adhesives that are sensitive to this temperature range should more than likely be avoided when using radiant heating.

Irgens: The type of flooring will depend on the radiant heating system installed: water tubing, electric cable, electric thin film or low-voltage PTC poly-carbon. Since the maximum floor temperature is limited to less than 90 degrees Fahrenheit, most floor materials can be used. However, you may have cold and hot spots that would affect the expansion and contraction of some flooring materials, such as certain types of wood. Additionally, people may place heavy objects on the floor which will create overheating problems.

Hubina: Flooring materials suitable for use with a radiant heating system will depend on their thermal conductivity, which means how quickly and efficiently the generated heat transfers to the floor surface. The best flooring for underfloor heating is flooring with good conductivity as it heats up quicker, gives more heat output and is more efficient to run. However, this does not mean that underfloor heating cannot be used under less conductive materials—there are systems available for use with virtually any floor finish.

Gerolimatos: In general, ceramic and stone tiles are the floor covering materials most compatible with radiant heating. They have high thermal conductivity, which allows heat to radiate freely to objects in the space above. Tiles are also suitable for use in high-temperature applications far in excess of radiant heated floors.

While floor coverings such as engineered hardwood, LVT and others may be used, the maximum temperature limits are much lower than tile. It is essential to read the floor covering data sheet or installation instructions, or to verify directly with the manufacturer that the chosen floor covering is compatible with radiant heating. Carpet’s relatively low thermal conductivity makes it a poor choice from an energy efficiency perspective. Solid hardwood is generally not recommended due to the risk of damage caused by excessive drying while being exposed to the heat.

Summers: QuietWarmth Float is designed to be used under floating floors such as wood, laminate, WPC, SPC and EVP; our only requirement being that the overall thickness of the product is 4mm. This system will not exceed 85 degrees Fahrenheit without a thermostat controlling the temperature. This is due to the unique conductive ink technology which spreads out the watt density to the entire flooring area.

QuietWarmth Peek and Stick for Tile is designed for tile installations where thin set and mortars will be used. The same conductive ink technology is used as the floating floor application, but it is embedded into a watertight, peel-and-stick membrane that allows mortars and thin sets to be applied.

QuietWarmth Under Subfloor is designed for those areas that already have flooring on them but access to the joist underneath is available. Simply staple the product between the joist cavities and insulate below it to encourage the heat moving upward to the flooring materials. This would be the only system that can heat under carpet or sheet vinyl, as it is installed two inches below the floor decking and into the joist cavity.

We do recommend using one of our floor-sensing thermostats for optimal safety and operating efficiency. Each has a built-in GFCI to protect from unsafe surges and a floor probe to ensure your toes are the right temperature.  

White: Flatness of the substrate has been and always will be paramount to the flatness of the finished flooring system, regardless of the type of flooring materials used. A traditional approach would require installing repair materials first, allowing them to dry and then installing the uncoupling mat. Now there is a new approach, due to technological advancements in cement repair materials, that allows installation of the radiant uncoupling mat first. Then flatness tolerances can be achieved by the installation over the mat system.

Can radiant heating be used over any type of subfloor? What are some of the installation differences?

Billen: Yes, electric floor heating can be used over any type of typical subfloor including OSB, plywood and cement slabs. The primary difference between installing floor heating over a slab vs. a wood subfloor is that you should use an insulating underlayment over the slab and below the electric heating system. This is done to prevent heat loss (or heat sink) caused by the slab and to help direct the heat upwards into the room that it’s heating. Either cork or CeraZorb (a synthetic cork) can be used as an insulating underlayment but we do recommend CeraZorb, as it has double the R-value of natural cork.

Mintie: Most radiant heating systems are versatile enough to be installed over both plywood and concrete substrates. However, when installing over on-grade or below-grade concrete surfaces, additional precautions should be taken to help prevent heat loss through the concrete slab. Generally, a backer board with insulation properties can be placed over the concrete slab in order to assist and prevent heat loss from happening. For plywood substrates, installers should conform to the requirements set out in the Tile Council of North America (TCNA) Handbook when installing ceramic, porcelain or dimensional stone tile finishes over heated flooring systems.

Irgens: A water tubing system is normally installed in concrete, but some tubing systems are also routed using special boards. Most electric cables are installed on concrete or on plywood using thin set. Also, some electric mats go directly over the subfloor and under the finish flooring.

Utley: Ardex Flexbone Heat can be installed over concrete, wood, screeds, gypsum, cement boards or existing tiles. When installing over concrete, the substrate must be prepared according to the ASTM F710 standards and must be flat to a maximum variance of 1/4 in. in 10 ft. When installing over plywood, the subfloor must be constructed in accordance with the ANSI L/360 or ANSI L/720 standards. All plywood joints should be filled with a skimcoat such as Ardex SKM. When installing over plywood, the appropriate thickness must be honored dependent upon the on-center joist spacing.

Summers: QuietWarmth can be installed over any type of subfloor. When installing over concrete it is important to first install a 6 mil poly film to keep moisture vapor from causing issues at the thermostat and to protect the heating system. Additionally, adding an insulating underlayment over the concrete, such as our QuietWalk, will allow the heat to reach the flooring material instead of being absorbed by the heat sink of the concrete.

Gerolimatos: Radiant heating can be used over virtually any subfloor, the most common being concrete and plywood. Wood panel underlayment systems that are fastened to the subfloor and feature grooves to hold hydronic tubing are more compatible with plywood than concrete. It is simpler and easier to fasten the panels to plywood, and risk of moisture from the concrete causing problems with the wood panels is avoided. Lightweight concrete and gypsum pours are suitable for use over plywood or concrete, but adding thermal insulation to reduce heat loss to the concrete below may be desirable or even necessary depending on the particular project requirements.

Downey: Radiant heating can be used over any structurally sound subfloor material. Most pours of gypsum underlayment designed to go over radiant heating systems, such as our AccuRadiant, are formulated specifically to prevent the thermal breakdown of the underlayment.

Hubina: The only difference is that over a concrete slab, Warmup insulation boards are recommended to prevent downward heat loss.

What is your latest radiant heating product?

Gaetano: Ardex Flexbone Heat In-Floor Heating Systems feature unique bone shapes that deliver multiple advantages such as ease of pre-filling, less heat cable touch points to the plastic for better heat conductivity, less air space under the membrane so heat is transferred faster, and more energy efficiency to the floor covering. It is a three-in-one membrane for in-floor heat, uncoupling and waterproofing.

Downey: The AccuRadiant Floor Underlayment System is a fast-applied, quick-setting gypsum cement formulated specifically for radiant floor heating applications. AccuRadiant provides compressive strengths up to 3,500 psi, depending on the mix design. Non-shrinking and non-cracking, AccuRadiant will hold tubing or cables in place while providing the same sound and fire resistance qualities of our AccuCrete underlayment products.

White: While preparing many substrates for leveling is relatively simple and straightforward, plywood and oriented strand board (OSB) subfloors typically must be reinforced with metal lath. TechLevel WSF Fiber Reinforced Self-Leveling Underlayment from CustomTech eliminates this time-consuming and labor-intensive process. Fiber-reinforced TechLevel WSF is ideal for use with hydronic and radiant heating systems, as it uses its strong tensile and flexural strengths to accommodate expansion from thermal changes.

Irgens: STEP Warmfloor is a low voltage, AC/DC heating system based on a self-regulating, PTC nano poly-carbon technology. Because the heater acts as a sensor over the whole surface, it cannot overheat and can be placed under any type of floor covering.

Mintie: The Strata_Heat electric radiant floor heating system by Laticrete consists of the Strata_Heat Thermal Pack, a heat-conductive thin set additive that utilizes Thermal Diffusion Technology to uniformly distribute heat through the adhesive. This eliminates cold spots and quickly achieves the desired floor temperature. These Thermal Packs are mixed in with appropriate Laticrete latex thin set mortars such as 254 Platinum or 4-XLT. The system also includes the Strata_Heat high-performance floor heating wire, uncoupling mat and Wi-Fi enabled thermostat.

Summers: Our new touchscreen, Wi-Fi enabled thermostats allow customers to control their radiant heating system from their smart devices.

Gerolimatos: Schluter provides a modular screed system, Bekotec, for use with hydronic radiant heating systems. The Bekotec system is based on a studded polystyrene screed panel. The studs confine curing stresses to small modules, which eliminates curling and allows the installation of continuous screed surfaces without any wire reinforcement or control joints. The Bekotec system can also be used in conjunction with common sound insulation materials to produce a flooring system with excellent sound attenuation properties.

Billen: Our newest radiant heating product is our line of Shower Waterproofing and Floor Heating Kits. These new items combine all of the elements of a shower pan waterproofing kit with a pre-sized electric floor heating system. The integrated heating system will warm the tile shower floor evenly so that there are no cold spots around the perimeter. The shower pan, which is pre-sloped, eliminates the need for a traditional mud bed. Our shower kits feature an integrated hair trap in the drain assembly, a strong drain flange, a wide variety of available grate covers and a dense shower pan.

Hubina: Our 4iE Smart Wi-Fi thermostats are now offered in both horizontal (the original model) and vertical (the new 4iE Portrait) orientations, with the new thermostat designed to fit common vertical mounting boxes. Additionally, 4iE thermostats are compatible with Amazon Alexa, so controlling the floor heat is more convenient than ever. We have also introduced a peel-and-stick version of our DCM-PRO Membrane floor heating system. The membrane adheres directly onto plywood and concrete subfloors without the use of thin set.