When most consumers think of electric underfloor radiant heating systems, what often comes to mind is bringing warmth to a cold tile or stone floor. However, as the technology of these systems has advanced, more floor coverings can tolerate the heat—including hardwood, LVT, laminate and even carpet if desired. We spoke to a range of industry experts about installing these systems over different substrates and under various types of flooring, and what, if any, calculations need to be made about the thermal resistance of the floor.

For this roundtable we spoke with: Sean Gerolimatos, director of research and development at Schluter Systems; Julia Billen, owner and president of WarmlyYours Radiant Heating; Karen Dhanota, marketing and communications specialist for M-D Pro; SunTouch’s Tracy Hall, national sales manager, and Tara Sipika, marketing specialist; Monica Irgens, president of Electro Plastics/STEP Warmfloor; Wally Lo, Nuheat product manager; Mike Micalizzi, senior director of technical services for Custom Building Products; Arthur Mintie, Laticrete’s senior director of technical services; Deanna Summers, MP Global Products marketing coordinator; and Regis Verliefde, Warmup CEO.

Q: How do the installation methods vary depending on flooring type?

Billen: “The type of floor covering used in a project is one of the most important factors that will dictate not only what type of electric floor heating system is used in a project but also how it’s installed.

“For example, the most commonly heated flooring surface is tile, and for a radiant heating system to function correctly under this covering the heating element should be embedded beneath the tile’s surface in thin set or self-leveling cement.

“This essentially turns the entire embedded structure into one continuous heated surface which ensures the delivery of even heating. Additionally, heating cable can be paired with an uncoupling membrane, which will protect the floor from foundational movement and provide some degree of crack isolation.

“Embedding heating elements in a self-leveler is also important for thin, malleable flooring types like LVT, as the flat surface provided by the self-leveler will ensure that the flooring material doesn’t conform around the cable due to heat.

“The best way to heat nailed hardwood floors is to embed floor heating cable that is installed with fixing strips—which are plastic strips that can be trimmed to fit between sleepers. This way you’re able to maintain proper spacing of the heating element.

“For floating floors—typically engineered wood or laminate coverings—it’s recommended to use an adhesive-free heating element that doesn’t need to be embedded. These products are usually very thin, can be placed directly between an underlayment and the floor covering, and have lower-wattage outputs than embedded heating elements.”

Verliefde: “The installation methods are mostly the same for anything but nailed hardwood. With tile, we recommend either a cable-on-mesh system or the more popular cable-on-membrane applications. You then thin set and tile directly over it.

“All Warmup systems are approved to be embedded in levelers, so this allows installation of LVTs and floating floors—even carpet. Although for carpet and laminate, we recommend our non-thin set FOIL mats.

“Overall, a heating cable embedded in 3/8 in. of leveler can be covered with pretty much any floor covering material. Rubber floors and cork are not ideal given their lack of conductive properties, but most common floor types are good fits.

“When it comes to nailed-down hardwoods, we recommend a unique application that has been pretty popular as it keeps the floor height low. We utilize 3/8 in. thick wood sleepers run across the room, between which we will run the cable. You then fill the gaps with leveler to obtain a smooth heat mass, keeping the sleepers exposed so you can safely nail the wood floors to them.”

Irgens: “As you know, electric heating systems come in different versions. Most common are: cables on a roll, cables embedded in a mesh or mat, ink dispersion thin films and positive temperature coefficient (PTC) semi-conductive flat heaters. Each manufacturer will indicate the different installation methods depending on the product type and floor covering. Some heating systems are limited to the location or application and certain types of floor coverings.”

Sipika: “For all flooring types, mortar is first applied to cover the heating wire with a flat finish. This can be flat-troweled thin set or a self-leveler. With tile you would use the notched side of the trowel to add more thin set, preparing the surface for the type of tile selected. For other flooring types including resilient, laminate and hardwood, you need a smooth finished surface.”

Hall: “When it comes to hardwood or laminate, we like to limit the temperature of the settings. Many flooring manufacturers do have recommended temperature limits for the sensor. It also depends on where in the home it is being installed: vestibules, hallways, kitchens, bathrooms, basement, bathrooms in basements, etc.”

Mintie: “When installing electric radiant floor heating systems, the methods differ based on suitability. For example, 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 and resilient finishes may have other inherent properties that may require a slightly different system. The desired finish must be compatible and tolerant of the thermal fluctuations these systems can exert, to ensure there is no damage to the flooring.”

Micalizzi: “Methods for radiant heat installation are based on the heating systems’ configuration and materials. When only wires make up the system, they’re typically encapsulated by a self-leveling underlayment.

“When the product incorporates a mesh that holds the wires, the mesh can be adhered to concrete and plywood with a cementitious mortar like ProLite or VersaBond and encapsulated with the same product, or with a cement-based patching compound or a self-leveling product. 

“If the radiant heating system threads wires into a pre-formed plastic mat, a self-leveling product like CustomTech WSF can be applied over the mat as this product is fiber-reinforced.”

Gerolimatos: “The installation of different floor covering materials can influence the installation method for floor warming systems. Unlike tile, engineered hardwood and LVT have maximum allowable temperatures that must be respected. This can be accommodated by varying cable spacing and adding hard limits on the thermostat.

“Resilient floor coverings may also require a thicker application of leveling compound to ensure proper support when exposed to traffic. Always read and follow both the floor warming and floor covering manufacturers’ technical literature and written instructions, making sure to resolve any inconsistencies with the manufacturers themselves.”

Summers: “There are different technologies out there that are developed differently and may use more support materials to install than others. Various wire and cable products usually require a pour of self-leveling compound to embed the system and make the subfloor flat. It can also be something as sophisticated as a specialized mat that integrates the cable within it.

“Newer technologies like our QuietWarmth system comprise a thin conductive carbon mat that doesn’t require embedding or self-levelers. It has a low enough profile that you can skip the other support materials. It is possible with our product to install flooring the same day.”

Dhanota: “Tile is a product that will transfer heat and is installed using thin set mortar. Hardwood, laminate and resilient floors are types of flooring that do not transfer heat, so the manufacturers should be contacted to make sure they can go over heated floors. To put these materials over electric heat cables, you must have approximately a 1/2 in. of mass—i.e. concrete, concrete board or self-levelers—which will transfer heat from the cables.

“M-D Pro’s Prova Flex-Heat features a deep locking channel design suitable for multiple sizes of heating cables and is 6mm thick to aid in transitions up to higher surfaces like hardwood. The exclusive Vapor Management and Shear Stress Control systems provide unsurpassed anti-fracture and uncoupling benefits.”

Lo: “Nuheat Floor Heating Systems can be installed under different types of flooring in roughly the same manner. Apply a layer of thin set, then the Nuheat floor heating system; add another layer of thin set, and then install the desired flooring on top.”

Q: Will these systems work over both concrete and plywood subfloors?

Billen: “Electric floor heating systems can be installed over both plywood and concrete subfloors, but the installations will be different. When installing an electric radiant heating system over a concrete subfloor, it is always recommended to use an insulating underlayment with a high R-value [Editor’s note: The R-value is the measurement of a product’s thermal resistance]. This helps prevent ‘heat sink’ (or heat loss) to the subfloor and greatly improves the energy efficiency of the system. An insulating underlayment can also be used with a plywood subfloor if it’s above a poorly insulated area like a garage.

“Another consideration for contractors to keep in mind is, if they are embedding a heating system in thin set or self-leveling cement, they should always make sure that the adhesive is compatible with the subfloor type. There is a huge variety of adhesives available on the market, so it’s always a good idea to double-check that the product you are using is applicable to your project.”

Verliefde: “All systems on the market are suitable over common subfloors, like 3/4 in. ply, OSB and slab on grade. The main consideration for contractors is the heat loss to the subfloor.

“There is a misconception that heat rises. Heat goes in all directions—it is hot air that rises. This means that a heated floor on a slab will be significantly less efficient than on a more insulating plywood base. From our testing, heat loss on plywood is about 25%, while on slabs it can reach 65%. This is where insulating becomes part of the discussion, or cable spacing and output, so that the end results meet the consumer’s expectations.

“In general, over slabs, we recommend insulation. It will take three to four hours for a system to reach 80ºF on a basement slab without insulation. You can tighten up the cable spacing as well, and some systems allow higher outputs for that.”

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 sink or heat loss through the concrete slab. If heat sink or heat loss occurs, heat may not transfer properly or efficiently.

“Generally, a backer board with insulation properties, like Laticrete Hydro Ban  Board, can be placed over the concrete slab in order to assist and prevent heat sink or heat loss from happening. For plywood substrates, installers should conform to Tile Council of North America (TCNA) requirements when installing ceramic, porcelain or dimension stone tile finishes over heated flooring systems.”

Irgens: “Most electric heating systems can go on plywood or concrete. It is important to have thermal insulation under the heating system to push the heat in the right direction, as otherwise heat will go in all directions.”

Lo: “Nuheat Floor Heating Systems can be installed over plywood, cement board and concrete subfloors. For slab-on-grade concrete installations, we recommend installing a thermal insulating material (cork, synthetic membranes, etc.) onto the concrete slab before installing any floor heating system. This will ensure that heat produced by the floor heating system will radiate up to the flooring surface instead of heating the concrete below.”

Gerolimatos: “Electric floor warming systems can be installed over both plywood and concrete subfloors. Regarding the floor covering itself, there can be different requirements for the assembly design and installation based on the substrate. The same applies to the floor warming performance.

“Concrete substrates absorb more heat energy than plywood, which can increase the time for floors to warm up, particularly when there is no insulation layer below the concrete. Options to address this challenge include installing the heating cables at closer spacing for increased energy output, or installing a thermal break prior to the heating cable installation to reduce heat loss into the substrate.

“Schluter Ditra-Heat-Duo provides faster warm-up times and reduces impact sound transmission through floor-ceiling assemblies. The system features an integrated thermal break in the form of a thicker bonding fleece, which reduces the initial rates of heat loss to the substrate (especially concrete) and improves the floor warming response time during temperature ramp-ups at the standard cable spacing.”

Hall: “For concrete (slab on grade) we recommend a thermal insulation product (not just a thermal break product) with an R-value of 1. We may also recommend an output of 15 watts per sq. ft. versus the industry standard of 12 watts per sq. ft. Plywood is not as thermally conductive and can be at 12 watts per sq. ft.”

Sipika: “For tile or stone installations where substrate movement or water exposure is a concern, HeatMatrix Uncoupling Membrane would be used as a base with WarmWire installed in the channels. This process is the same for both plywood and concrete substrates.

“If moisture control and crack prevention are not a concern, WarmWire with CableStrap or TapeMat are typically selected. Double-sided tape can be used to attach CableStrap to both concrete and plywood surfaces. For plywood, CableStrap can alternately be installed using screws.”

Micalizzi: “Concrete may exhibit excessive moisture so it should be tested per ASTM methods or evaluated by placing a clear, 24 in. square sheet of polyethylene taped with duct tape and checked after 24 hours for condensation. A special treatment using CustomTech MVC may be required depending on the finished flooring.

“For tile work, a membrane for crack isolation or waterproofing may be needed to protect the tile from movement or retain water. Over a wood framed floor, a membrane such as Custom EasyMat could be placed prior to the heating system to serve as a thermal insulator from a cool basement.”

Summers: “While you can install a radiant heating system over both plywood and concrete subfloors, each subfloor has different considerations and code requirements. Ultimately you must follow any code recommendations for subfloor preparations that the desired floor covering materials indicate.

“For concrete installations, moisture protection will be important as well as understanding the limitation of the product over concrete environments. For wood subfloors, especially when using tile, ensure the subfloor meets TCNA guidelines so a stable subfloor is in place before the system is installed.”

Dhanota: “Yes, electric radiant heating systems can be installed over both plywood and concrete subfloors. Contractors should keep in mind that in either case the substrate must be clean, flat, dry and structurally sound.”

Q: Should contractors understand the R-value of a flooring material before installing it over radiant heat?

Billen: “Since a radiant heating system is below the floor covering, the R-value of that floor covering will of course have some impact on how efficiently heat is able to radiate upward into the room. The higher the R-value, the less heat a floor covering conducts.

“However, modern electric floor heating systems will be able to heat through the vast majority of commercially available floor coverings. A good rule of thumb is that if the R-value of the flooring material is less than 1, it shouldn’t pose any significant issues for a properly installed floor heating system. The R-value of a floor covering should be available from the manufacturer and is typically established by a national laboratory.”

Verliefde: “The typical UL testing is done at an R value of 1. Most common floor types don’t meet that as they act as conductors, especially tile. Even traditional 3/8 in. wood doesn’t meet that. So in general terms, there are very few flooring types that are detrimental to electrical floor heating.

“Thick berber carpet over 1 in. and cork floors are usually the ones we caution against, but everything else is fine. Especially since the cable system is typically embedded in a thin layer of thin set or leveler. So in the end, the heat will radiate through all materials, but some are obviously more effective than others at radiating the heat into the room.

Few flooring options come with an R-value tag, but this shouldn’t be of concern until you get involved in unusual situations. So my take is that 99% of the time, the residential contractor will not run into a situation where the floor covering will not be suitable for underfloor heating. Are some better than others? Yes. And for that we recommend giving us a call.”

Irgens: “R-values only indicate the insulation value of the product. For floor heating you should not have more than 85oF on the surface, which is a safe temperature for most materials. When higher thermal resistance is applied on the radiant floor, compensate by placing higher R-value insulation under the heating elements with a ratio of 4 to 1.”

Lo: “Almost all resilient floor coverings are able to be installed on top of radiant heat. The lower the R-value, the quicker the floor covering will heat up. In general, we recommend the R-value of the floor covering be R-1 or less for the efficient operation of radiant heat systems. Check with the manufacturer of the floor covering to ensure it is suitable for installation over radiant heat systems.”

Mintie: “Most electric warming and heating systems can predict their heat output. Concrete typically has an R-value of 0.002 to 0.005 for every inch of thickness, depending on the mix design. These calculations should be made by the project engineer to determine an exact figure.”

Gerolimatos: “A common practice to determine R-value is to use the ASTM C518 Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. In simple terms, the heat flow meter features a hot side and a cold side to promote heat flow through the test specimen. The R-value is calculated based on the measured rate of heat transfer.

“Different floor covering materials have different R-values. For example, tile has the lowest thermal resistance of floor coverings in general, which makes it ideal for floor warming applications. Wood flooring tends to have higher thermal resistance, but can still be used in floor warming applications successfully. Carpet will typically have the highest thermal resistance of flooring options and is not commonly used in floor warming applications.”

Sipika: “The R-value of flooring installed over radiant heating has a direct impact on the amount of warmth felt on the surface. If heat cannot radiate away from the system due to high R-value coverings, there is also potential for wire damage. The combined R-value of all added layers needs to be considered when evaluating a flooring type for use over electric radiant.

“A number of websites publish typical floor covering R-values, including the Radiant Professionals Alliance (radiantprofessionalsalliance.org).”