Wood Floors and Radiant Heat

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Radiant (underfloor) heating is a type of heating system that is placed below the floor of a building. Radiant heating as a technology is the method of intentionally using the principles of radiant heat to transfer radiant energy from an emitting heat source to an object. Radiant heating may be either hydronic (water/fluid flowing through pipes) or electric (electric resistance heating elements).

The temperature of the floor surface is the key to determining what level of comfort is necessary. End-users may feel uncomfortable when coming in contact with floor surfaces that are too warm or too cool. Thermal comfort is defined by building scientists and health care professionals as “a condition of mind that expresses satisfaction with the thermal environment.” Radiant heating and cooling systems are commonly used when addressing human thermal comfort (even though radiant cooling systems are never recommended for use under wood flooring).

To provide an adequate thermal environment for the end-user, many factors must be taken into account, including the size of the room, the construction of the home, R-value of the windows, HVAC systems, the number and age of the occupants, and the interior finishes that may be directly affected by these requirements.

According to ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers), a floor temperature that creates optimal human comfort for most people lies just about at 75° F as shown in this graph.

Source: ASHRAE

Human comfort, however, is only one part of the equation. How the wood floor performs over the radiant heat is equally as important.

The ability/inability of a wood flooring material to transfer heat will directly affect the radiant heating system design. The ability of the radiant heating system to either adequately heat the facility, or to simply warm the floor, without damaging the wood, must be taken into account when specifying a radiant heating system within any facility.

These are some of the most important variables to keep in mind when specifying or installing a wood floor over any radiant heating system:

  • All wood flooring should be properly matched to the expected live-in conditions in which it will perform. With the heating source placed directly below the installed wood flooring, the moisture content will forcibly be reduced during the heating season if supplemental humidification is not added.
  • A supplemental humidity control system will often be necessary, and should be specified into the project, to properly support the wood flooring manufacturer’s environmental requirements, or when not specified, to support ambient airspace conditions between 30 percent to 50 percent RH, and 60° F-80° F, per NWFA Installation Guidelines. The wood flooring professional should confirm and document if a supplemental humidification system is present and operational before the specification or installation of the wood flooring.
  • In-floor or under-floor data loggers are recommended to be installed by the flooring contractor to ensure the temperature and humidity conditions do not exceed flooring tolerances. Placement of these sensors should be determined with the assistance of the radiant heating system installer to gain the most accurate floor temperature readings.
  • The end-user should be made aware of the importance of proper usage and maintenance of the humidification system and data loggers to ensure adequate temperature and relative humidity levels are maintained year-round.

The types of wood flooring best suited for under-floor radiant heat systems should be accounted for to ensure long-term performance.

Both in solid and engineered flooring options, certain species are known for their inherent dimensional stability such as mesquite, black cherry, black walnut, and others. Less stable species such as hickory, beech, and maple are less suitable for use over radiant heat.

Dimensional Stability Chart. Species with a higher number are less stable and are less suitable for use over radiant heat.

Wood is a hygroscopic and an anisotropic material, meaning it takes-on and throws-off moisture, and it shrinks and swells dependent on these changes in moisture differently in each direction. Wood shrinks and swells the most circumferentially around the growth rings (tangentially), about half as much across the rings (radially), and only minuscule amounts along the grain (longitudinally). The way in which it is cut from the tree for solid flooring is classified as plainsawn, quartersawn, riftsawn, livesawn, or end-grain. Quartersawn and riftsawn wood flooring are more dimensionally stable in width than plainsawn or end-grain wood flooring.

Wood changes dimension proportional to the width of the plank. Narrow boards expand and contract less than wider-width boards of the same species and cut.

Flooring Type
Engineered wood flooring is, in general, more dimensionally stable than solid wood flooring. However, not all engineered wood flooring is recommended or appropriate for use over radiant heating systems. Engineered flooring with a less-stable wear layer species such as hickory, beech, and maple are not normally best suited over radiant heat unless otherwise suggested by the flooring manufacturer. The cut and thickness of the wear layer lamina (peeled, sliced, or sawn) may also affect how the floor performs over radiant heat. Follow the flooring manufacturer recommendations for maintenance, environmental (T and RH) requirements, surface temperature requirements, and whether or not each specific product is intended to be used over radiant heat.

Wood flooring manufactured and expected to perform at MC levels higher than 9 percent or in conditions above 50 percent RH should not be used with radiant heating systems.

Most wood flooring can be installed over radiant heat, providing all of the necessary conditions are met. Successful wood floor installations occur when the radiant heat system design engineer, the radiant heating system installer, the wood flooring installer, and the end-user all communicate and fully understand what is required for the entire flooring system being installed. This communication should include which type of wood flooring to use, what installation method to use, understanding how this heat source may impact the wood flooring, what precautions to take before, during and after installation, and consistent communication among all parties when any changes take place to any part of the system. The end-user should also have a clear understanding of the flooring product use and maintenance requirements, as well as the radiant heating system features, limitations, and capabilities, and how all of these work together within the system, to stay
within the necessary parameters.

More information related to radiant heating systems installed below wood flooring is included in the upcoming updates to the NWFA Wood Floor Installation Guidelines.

Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at brett.miller@nwfa.org.

4 thoughts

  1. Brett, According to the chart White Oak is less stable than Maple.
    Is that data in the chart perhaps a bit questionable or at least evoking concerns about actual real life shrinkage performance?
    WO is trendy and more used these days, can you identify if that diagram is for plain-sawn wood or quartered or perhaps an avarage?
    In practice I always see Maple and Hickory move much more dimensional with changes in MC than WO, any thoughts about this?

    So when selecting a domestic ring-porous wood for flooring when using the data in the shown chart it indicates that Ash is favorable over White Oak (btw, I do like Ash myself a lot and it takes stains better than Oak imo).
    I always wondered if this chart was giving numbers from the Green/”Fiber Saturation Point” to Oven-dry. It would be good to know if there is a chart that gives numbers based on dimensional change from Air-dry to Oven-dry as I assume those numbers may be quite different than the Green to Oven-dry chart.

    Thinking this over, the chart shown here may create some concerns due to the fact that the shrinkage % numbers of WO, RO and Ash are close to obvious known unstable woods like Hickory and Maple Which we know for a fact move more in service than WO, RO or Ash.
    For our business (specially with hydronic heating) it would be more important to know the shrinkage coefficient from say 19% to perhaps oven-dry as that would reflect better to our trade conditions (with average MC ranges between 4-14% MC depending on geographic location.
    Most of all wood movement/shrinkage occurs when drying from Fiber Saturation Point aka FSP (say 28-30%) to about 14-18%, from 14-18% to oven-dry I feel the shrinkage is less dramatic. I’m concerned that the numbers in the chart are too generic, in other words covering too large of a MC range. We need to be able to identify the “working conditions” a wood floor will be exposed to and not a number based on what I referred to above.

    What is your opinion (hope it is not too confusing what I mentioned here?

    1. Hi Johannes- good comments.
      The shrinkage values referenced in this chart are, as you mention, referencing the amount (expressed as a %) that a wood sample shrinks when going from a green state (freshly cut), to an oven dried state (0% MC) in the tangential grain direction. Most applicable to a plainsawn board, referencing shrinkage in the board width.
      In order to determine approximately how much a given species may shrink/swell based on MC loss/gain going from FSP (28-30% MC) to an oven dried state, you should use the Dimensional Change Coefficient chart from the Moisture and Wood publication (#A100), or straight from the Wood Handbook (USDA, Forest Products).
      These values are given as helpful references to determine approximately how stable a given wood species may be when placed in service. The purpose of including reference to the characteristics of species dimensional stability in the radiant heat article, is to hopefully help the professionals, and consumers to make educated decisions when looking for a wood floor to install over radiant heating systems. Species that are less stable are likely to shrink/swell more than more stable species.
      Cut makes a huge difference as well. Quartersawn material will outperform plainsawn of the same species, and would be a better flooring selection. For example, plainsawn white oak may shrink/swell up to 10.5% its width when going from green to oven dry (10.9% when going from FSP to oven dry). The same white oak sample may only shrink 5.6% of its width when going from green to oven dry in the quartersawn cut (5.4% when going from FSP to oven dry).

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