Wood Cuts: Tips for installing wood floor over radiant heat systems - July 2020
By Brett Miller
As consumer desire for home comfort and efficiency increases, the demand for radiant heat grows, as well. Radiant heat is a very efficient heating technology. Unlike conventional heating systems that heat the air, radiant heat systems transfer radiant energy from an emitting heat source to an object, like flooring.
Wood flooring can be installed over radiant heat successfully, as long as it is understood how radiant heat works, how it can impact wood flooring, what precautions should be taken and which type of wood flooring to use.
When radiant heat is used with flooring, the system is installed beneath the flooring material. Radiant heat systems may be either hydronic (water/fluid flowing through pipes) or electric (electric resistance heating elements). Both systems can be installed above, below or within the subfloor.
Subfloor materials may include concrete, wood, screeds/sleepers or a combination of any of these. The type of flooring specified, the flooring manufacturer requirements and the subfloor system will dictate the flooring installation method. Typically, radiant heat installation methods for wood flooring will include one of the following: nail-down over wood panel subfloor; direct nail to screeds/sleepers; glue-down; floating engineered; or direct nail through low-voltage radiant systems.
NAIL-DOWN OVER WOOD PANEL SUBFLOOR
When nailing down over wood panel subfloors, the subfloor surface temperature should not exceed 80°F at the time of installation. A Class II vapor retarder should be placed over a wood subfloor and below the wood flooring when being installed over an unconditioned space. Do not use asphalt- or bitumen-type vapor retarders over radiant heat systems.
Where a wood panel subfloor has been installed over a concrete slab, install a Class I impermeable vapor retarder over the slab, and under the wood panel subfloor in all on- and below-grade applications, and when calcium chloride readings are greater than three pounds, relative humidity readings are greater than 80% or calcium carbide readings are greater than 2.5%.
Fastener length should be taken into account to avoid penetration of the vapor retarder. Penetration of the heating tubes/elements must be avoided, as well. Length and placement of fasteners must be assessed and addressed prior to and during the install.
When the required fastener schedule can’t be followed because of the type of radiant heating system being used, an alternative installation method may be necessary. This may include any of the following:: nail-assisted glue-down; full spread glue-down; or floating installation methods.
DIRECT NAIL TO SCREEDS/SLEEPERS
The same rules apply regarding temperature and vapor retarder.
Hydronic tubing and electric elements typically are installed into the channels between the screeds/sleepers, and then embedded in a gypsum or lightweight concrete mix, which is poured even with the top of the screed/sleeper. This should present a flat surface that minimizes any air space between the subfloor and the flooring. The heating tubes/elements should be submerged enough below the surface of the screeds/sleepers that the wood flooring does not come into direct contact with the heating tubes/elements.
When nailing down a wood floor, consider the screed/sleeper placement in comparison to the angle of the fastener to avoid penetrating into any radiant heating tubes or elements. When the required fastener schedule cannot be followed due to the placement of the screeds/sleepers, an alternative installation method may be necessary. This may include any of the following: nail-assisted glue-down; full spread glue-down; or floating installation methods.
GLUE-DOWN
When gluing down, before installation begins, the heat will need to be reduced or even turned off for the duration of the flooring installation to avoid premature drying and skinning over of the adhesive. Check with the adhesive manufacturer for minimum and maximum subfloor temperature limitations during the installation. In addition, the channels containing radiant tubing should not be filled with adhesive, and temperature and vapor retarder rules stated previously apply.
Use an adhesive and moisture control system suitable for the flooring being installed and for the radiant heating system it is being installed over. Do not use asphalt- or bitumen-type vapor retarders over radiant heat systems.
When using the in-situ relative humidity tests (ASTM F2170), be extremely cautious when drilling into the slab where hydronic tubing or electric heating elements have been embedded, so as to not damage or puncture the heating system. Use of infrared cameras or heat-detecting devices may assist in identifying a safe location for testing.
FLOATING ENGINEERED
When floating engineered wood floors that will be edge-glued, ensure the glue recommended by the flooring manufacturer is approved for use with radiant heat, and that the subfloor surface temperature does not exceed 80°F at the time of installation.
The insulating properties (R-values) of flooring underlayment pads vary and may compromise the radiant heat efficiency. The pad R-value should be taken into account during the design and specification of the heating system. Do not use asphalt- or bitumen-type vapor retarders over radiant heat systems.
NAILING THROUGH LOW-VOLTAGE SYSTEMS
When nailing through low-voltage radiant heat systems, refer to the radiant heating manufacturer installation instructions prior to nailing through any electric radiant heat system.
Refer to the flooring manufacturer for proper installation methods over these types of heating systems and ensure that the subfloor surface temperature does not exceed 80°F at the time of installation.
The insulating properties (R-values) of flooring underlayment pads vary and may compromise the radiant heat efficiency. The pad R-value should be taken into account during the design and specification of the heating system. Do not use asphalt- or bitumen-type vapor retarders over radiant heat systems.
OTHER TIPS
Dimensional stability is an important factor in choosing a wood floor to be installed above 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 less stable wearlayer species are normally not suited over radiant heat unless otherwise suggested by the flooring manufacturer. The cut of the wearlayer lamina (peeled, sliced, or sawn) can also affect how the floor performs over radiant heat. Follow the flooring manufacturer recommendations as to 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 9%, or in conditions above 50% RH, should not be used with radiant heating systems unless otherwise recommended by the flooring manufacturer.
Both in solid and engineered flooring options, certain species are known for their inherent dimensional stability, such as American chestnut, black cherry, black walnut and others. Less stable species such as hickory, beech and maple 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 differently in each direction, dependent on changes in moisture. How the wood changes dimension is largely influenced by the species characteristics and the way in which the wood is cut from the tree. The way in which it is cut from the tree for solid or sawn flooring is classified as plainsawn, quartersawn, riftsawn, livesawn or end-grain. Quartersawn and riftsawn wood flooring is 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.
Radiant heating systems are designed to either be the sole heating source or a part of a larger heating system. Regardless of their intended use, when placed under a wood floor, they must do so without damaging the floor.
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.
The builder, the radiant heating system design engineer and the radiant heating system installer should be made aware of the type of floorcovering being installed over the radiant heating system in order to adequately pair the system with the flooring requirements, based on the environment in which they are being installed.
Unless otherwise specified by the wood flooring manufacturer, the heat loss calculations designed for wood flooring installations should restrict the operating temperature of the radiant heat system to never allow the surface of the installed wood floor to exceed 80°F. The entire wood flooring system, including the wood product, the installation method and the underlayment systems must be taken into account.
Separation of heating zones and thermostats, based on flooring type and temperature limitations, is required when multiple floorcoverings are being installed over any radiant heating system.
For electric radiant heating systems, a thermostat along with an in-floor sensor must be installed with the electric floor heating system to minimize the effects of rapid changes in temperature.
The end user should be made aware of the importance of proper usage of the entire radiant heating system by the radiant heating system design engineer and the radiant heating system installer as it directly relates to the floorcovering installed over it. The wood floor installer should provide maintenance instructions related to the heating and relative humidity requirements of the wood floor to the end user.
Radiant heating installed below wood flooring is becoming increasingly popular. Understanding how radiant heating works, the impact of heat on wood flooring, precautions that must be taken, and the type of wood flooring that works best with radiant heat will help to ensure a successful wood flooring installation over radiant heat.
The National Wood Flooring Association has detailed information about radiant heat available through its Wood Flooring Installation Guidelines. More information is available at nwfa.org.
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