Radiant (underfloor) heating is a type of heating system that is placed below the floor. As a technology, it is a 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).
Installation Conditions
There are a number of considerations 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. This will properly support the wood flooring manufacturer’s environmental requirements or, when not specified, support ambient airspace conditions between 30 and 50 per cent relative humidity (RH), and 60 F to 80 F (16 C to 27 C), per National Wood Flooring Association guidelines. The wood flooring professional should confirm and document if a supplemental humidification system is present and operational prior to specification or installation of the wood flooring.
In-floor or underfloor data loggers should 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 in order 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 RH levels are maintained year-round.
Wood Selection
The cut, width, species and type of wood flooring best suited for underfloor radiant heat systems should be accounted for to ensure long-term performance.
Wood is a hygroscopic and anisotropic material, meaning it takes on and throws off moisture, and it shrinks and swells differently in each direction depending on these changes in moisture. 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 plain-sawn, quarter-sawn, rift-sawn, live-sawn or end-grain. Quarter-sawn and rift-sawn wood flooring is more dimensionally stable in width than plain-sawn 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.
Both in solid and engineered flooring options, certain species are known for their inherent dimensional stability, such as American chestnut, black cherry and black walnut. Less stable species like hickory, beech and maple are not as suitable for use over radiant heat.
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 lesser 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 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 (temperature and RH) and 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 moisture content levels higher than nine per cent or in conditions above 50 per cent RH should not be used with radiant heating systems.
Communicating for Understanding
Most wood flooring can be installed over radiant heat, provided all necessary conditions are met. Successful wood floor installations occur when the radiant heat system design engineer and installer, wood flooring installer and end-user communicate and fully understand what is required for the entire flooring system being installed. This communication should include the type of wood flooring to be used, installation method, how the heat source may impact the wood flooring, and precautions to be taken before, during and after installation. There should be consistent dialogue between all parties when any changes take place to any part of the system, too. The end-user should also have a clear understanding of the flooring product used and maintenance requirements, as well as the radiant heating system features, limitations and capabilities, and how they all work together in order to stay within the necessary parameters.
Brett Miller is vice-president of technical standards, training and certification at the National Wood Flooring Association, an international not-for-profit trade association representing all segments of the hardwood flooring industry. Brett has been in the hardwood flooring industry since 1991. His passion began as an installer, sander and finisher of wood floors, and then progressed to being a successful business owner, wood floor inspector, commercial and residential project manager, regional manager for a large finish manufacturer, technical standards writer and an instructor. Brett can be reached at brett.miller@nwfa.org.