In a previous article (FCIMarch 2003), we gave some general guidelines about solid wood flooring and radiant heating systems. We talked of the expectation of additional seasonal gapping associated with the heating system, about the need to consider the “off heating” seasonal expansion, to choose the flooring product wisely, and to consider all the system components when installing the flooring. In this article we will show some additional procedures and cautions that can help the flooring to perform even better over radiant heating.
With wood flooring and radiant heating it’s still all in thedetails. First, the wood flooring contractor should be involved with system design in order to comment and advise on the specifics of how the wood flooring system components will interact with the heating system. The specifics of that interaction will depend on the particular heating system used. The flooring contractor will have to advise what products are suitable, the specifics of flooring system components, and how and where to install them. The heating system may also have to be adjusted because of the different product thicknesses and the associated insulating values. This may require higher heating temperatures, more heating loops, or other modifications. Remember that temperatures above 135 degrees F can affect the strength of wood materials and require engineering modifications.
With systems where heating tubes are imbedded in the concrete slab, the type of floating subfloor system (two layers of ½” plywood or the ¾” x 16” plywood planks) has to be decided even before rough framing. The height of the system can affect the flooring height in relation to other flooring and associated stairways. The channeled modular board systems also have to be oriented properly with the tubes perpendicular to the flooring direction. Often the end tube loops are located in the same position across the floor, which may interfere with proper nailing. The end loops should typically be staggered in order to avoid a flooring run with insufficient nails; I’ve even seen cases of this where a run could have no nails! Also, the material used to make the channel systems should be assessed for nail holding. The panels should be equivalent in nail holding to a 5/8” or thicker plywood, made of veneers from group #1 dense softwood species.
Fine tuning also involves pre-installation procedures. The heating system should be on and running for at least five days before the flooring is even delivered to the jobsite. What if it is the summer time and only AC would be called for? Then the heat system will have to be run longer than five days-- probably 2 weeks-- and likely only at night when workers are away. This dries the floor system of extra moisture and allows movement associated with drying to occur before the flooring is placed. The flooring should not be stored at the jobsite while this is going on since over drying could result.
An example: If during winter heating the moisture content of flooring averages 5½% to 6% and in the summer it averages 8 ½% to 9%, the overall average is 7¼%. The target acclimation of the flooring should therefore be an average around 6¾% mc.
To figure the average moisture content of the flooring, check at least 20 different boards from different bundles for each 750 square feet of flooring. That’s 40+ boards for 1500 sq.ft. The overall range from driest to wettest should be 3%. One or two boards may be outside the 3% and present no problem. However, check an additional set if more than two high readings are recorded. A range of more than 3% with too many high readings indicates too much variation in moisture. The associated size variation from subsequent moisture change will likely result in excessive gapping.
- Know your flooring product and its specific dynamics.
- A gap-free floor is not a realistic expectation over radiant heating.
- White or light colored woods and finishes will show gaps more than darker products.
- Patterned floors will show gaps between board ends more than a unidirectional floor.
- Expect some seasonal cupping or crowning during those months when the heating is first turned on or turned off as the wood acclimates to the new season.
- Acclimation requires time, this can be days or even weeks to get the product at the correct moisture content.
- With wider widths back seal the boards before installation to mediate more rapid drying of the unfinished surface.
- Preplan the flooring system to have adequate subflooring conditions.
- Run the heating system before delivering the floor.
- Choose a NOFMA-certified product to assure consistency and protection should issues arise with the product.
For additional information on heating systems check with The Radiant Panel Association 800-660-7187 orwww.radiantpanelassociation.org.