First, we must understand a few basics of radiant heat transfer. Heat always moves from a warmer place to a cooler place. Cold objects in a warm room will heat up to room temperature. For example, if a hot cup of coffee and ice cube is left on the table what would happen to them? The cup of coffee will cool to room temperature and the ice cube will melt and the water will warm to room temperature. We must remember heat transfers only in three ways: Conduction, Convection and Radiation.
Conduction is the transfer of heat from one molecule to another throughout the substance. Not all substances conduct heat in the same way or speed. Stone and metals are good conductors, whereas wood, cork and carpet are poor heat conductors. We also need to look at insulators, such as air, when locked in an enclosed space like in the cells of fir or oak flooring. In comparison, an exotic wood specie, where those same cells are filled with extractives/oils and no air, makes them better conductors.
How does conduction energy work? Take a metal rod and hold it over a fire. The metal will feel cool until the flame heats up the atoms, moving to adjacent atoms until the rod heats up and becomes too hot to hold. One question many people have is “Why does metal or ceramic feel colder than wood, if they are both at the same temperature?” The answer is simple. Metal conducts heat away from your hands where wood does not, making it feel warmer to the touch.
Convection is the motion of warm material rising upward then cooling off and sinking again, producing a continuous circulation of heat transfer. A good example of this is a kettle on the stove bringing water to a boil. The warmer water will rise to the surface while the cooler water sinks, creating the circulator motion of heat.
Radiant is, in simple terms, invisible electromagnetic infrared waves that heat objects, not the air. This is done by supplying heat directly from a source (radiators, floor/ceiling panels, etc.) and providing radiant heat transfer. The delivery of heat is from the source (hot surface) to objects or people within the room through inferred radiation.
Radiant Systems Types. Let’s look at the different radiant system types. First we have the old conventional steam cast iron radiators, baseboard units whether copper or cast iron, and in-floor/walls/ceilings (hydronic or electric). In all cases they are radiant systems. The most common of what the flooring industry refers to as “radiant” is in-floor heat systems.
Hydronic systems uses heated water supplied through Pex tubing and can be done in several different methods, each method having its pros and cons. For example, a system installed in concrete/gypsum or Quik Trak is twice as efficient as a staple up system mounted on the underside of the subfloor. The supply water temperature is dramatically reduced on a concrete/gypsum or Quik Trak system than the staple up system due to the R-value (a measure of thermal resistance; the higher the number the more insulating it is) of the subfloor. This is extremely important to moisture-sensitive flooring such as wood, cork, bamboo and laminate.
Electric systems can be either cable or low-voltage systems. They also have their pros and cons. The cable systems are designed more for tile or stone application by covering the cables with thin-set or self-leveling compound. The one drawback of this system is if the cable becomes cut or damaged the system will not work. Electric mesh or mat low-voltage systems work well for either wood or stone/tile applications. Lower heat temperatures are quite user-friendly for many wood installations, whether engineered or solid wood flooring. One drawback with low-voltage systems is they may not be the preferred choice of primary heat source in northern climates with sub-zero temperatures.
Radiant Heat Design. Is your project new construction or a remodel project? During the heat design stage you should work with the mechanical contractor to ensure he understands the floorcovering type(s) you are providing. Most in-floor heat system are designed to accommodate an R-value of 1.5. For example, if the specs call for carpet and wood/cork/bamboo on the same zone you could have a problem. Carpet/pad R-value is twice that of wood/cork/bamboo flooring. If the surface is kept at 82 degrees the carpet side may be too cool for the customer. The customer will then turn up the heat to make the carpet side comfortable, and that will overfire the wood/cork/bamboo side creating cupping, splitting, gapping, etc. This is why it’s vital to work together on design with the mechanical contractor.
Floor Selection Process. Under correct circumstances just about any floorcovering can be installed over in-floor heat systems. Every flooring type varies in thickness and R-value whether tile, stone wood or carpet, so it’s vitally important to understand their characteristics.
Engineered wood/cork/laminate are a multi-ply construction and are dimensionally more stable than solid hardwood. One advantage is they are thinner and therefore more conducive than 3/4” solid floors. However when using in a floating application one has to include the R-value of the pad which could restrict the heat output.
For example floating a 5/8” engineered floor with an R-value of .63 and a pad of .20 provides a total R-value of .83 which is less than the 1.5 R-value set point. Now, if we change the pad to an R-value of 3 for a total of 3.63, we have reduced the heat by 100%, simply meaning it will overfire the system and cause structural failure to the floor. So it’s extremely important to understand a few basic considerations of in-floor heat systems.
Solid wood flooring can also perform well with radiant systems as long as we consider the width of the flooring, keeping in mind wider planks will provide wider gaps than narrower planks. Also narrower planks are less prone to cupping than wider planks.
Wood characteristics such as knots, mineral streaks, checks, etc. are also something to be considered. The reason is the grade may allow knots/checks/splits and is part of the overall character of the floor; however, once heated these characteristics may change in shape/size and become a concern to the buyer.
Resilient flooring vinyl/rubber/linoleum is typically conductive, quite durable and radiant-friendly. However, certain sheet products expand and contract at different rates which may pose a threat to some large installations. Always check with flooring manufacturer’s installation recommendations over in-floor heat systems.
Carpet/padding and area rugs work well, but have their own challenges when being installed together with a moisture-sensitive floorcovering like wood. As mentioned earlier, both floorcovering types on the same zone can be deadly for wood installations. Always check with the radiant system installer to ensure flooring and heat performance. One consideration is area rugs over a wood floor—it’s not uncommon to see floor temperatures rise to 95 degrees under the rug, trapping the heat.
Radiant System Controllers. Humidity is required with moisture-sensitive floorcoverings and require humidification control. This can be accomplished through an add-on humidifier on the furnace or a portable unit. There are system controllers that will use the outdoor temperature and automatically adjust your humidity levels accordingly. Remember, 90% of wood/cork/bamboo failures with radiant heat are moisture-related.
Smart thermostats are a great option. Many of the new thermostats are programmable, read outdoor and indoor conditions, setup on a gateway for wifi capabilities and record a performance log. With moisture-sensitive flooring (wood, cork and bamboo) one does not want to use the setback capabilities due to fluctuating floor temperatures and shock to the floor. The floor should come up to temperature slowly rather than abruptly every morning.
Smart thermostats are also very useful when controlling humidity to outdoor temperatures and minimizing window condensation. The other great advantage is the wifi capabilities such as being able to control your home from your phone or computer. You can set up alerts with a monitoring station if temperature/humidity gets too low or if the system is in need of service.
In-floor sensors should be installed in allin-floor systems to ensure the floor temperatures are appropriate and not exceeding the levels provided by the flooring manufacturer.Electric systems are generally controlled with dual sensors where they combine the in-floor temperature sensor and the room temperature through the room thermostat. Other mat and low-voltage systems are self-regulating, and therefore may limit heat output to around 78 degrees.
Here are some final considerations before installing over an in-floor heat system:
- Regardless of the floorcovering always check with the floor manufacturer for in-floor heat special instructions regarding installation and maintenance.
- Work with your radiant heat technician to ensure the floorcovering requirements will be met.Most in-floor systems operate at 75 to 82 degrees with some up to 85, but again check with the floorcovering manufacturer.
- Understand your flooring limitations via the thermal conduction properties and R-values to ensure you are not restricting the heat system’s capabilities.
- In-floor systems may have an impact on adjoining floor heights, especially in kitchen and stairways.
- Expansion and contraction is a big consideration, especially when working with floating applications.
- Northern climates with large outdoor temperature fluctuations may require a secondary heat source to keep floor temperatures below recommended levels. Typically these regions require better relative humidity control due to lower outdoor temperatures, whereas moderate climates will require much less control.
- Suggested relative humidity for most wood/cork/bamboo ranges between 35% to 55% as required by manufacturers.
- A protective floor cover over a wood/cork/bamboo floor may prevent proper relative humidity from being achieved.
- The age of the home is important. Typically, older homes are less insulated, so heat loss is elevated and requires the in-floor system to work harder to supply heat.
- With floating floor applications be very aware of the underlayment pad. Use the lowest R-value possible and check with the pad manufacturer to ensure acceptability.
- For tile applications consider using crack isolation membranes.
Roy Reichow brings over 40 years of experience in the wood flooring industry as a wood floor contractor, consultant and educator. Roy is founder and principle of Reichow Parquet Flooring and National Wood Floor Consultants. He holds National Wood Flooring Association Certified Professional certification in Wood Floor Installation, Sanding, Finishing, Sales Counselor, Inspector and Commercial Inspector. Roy also serves on the National Wood Flooring Association’s Certified Professional Board of Directors and Marketing Committee.
Roy has authored articles published by the NWFA and the International Fraud Update, a publication of the International Association of Insurance Fraud Agencies. His wood flooring projects have been featured in American Woodworkers Magazine, Better Homes and Gardens, and Mpls/St. Paul magazine.
Roy has led educational seminars for the NWFA, wood flooring distributors, contractors, and home builders, and has been a featured speaker at the MPLS Home & Garden Show.
Roy is a regional trainer for the NWFA and is an instructor for the Wood Floor Maintenance Technician (WFMT) course for IICRC. He also conducts sales training for various wood floor retailers. You may contact him at Roy@nwfc.net.