The following photos are all over the same “staple up” heating system with ¾” plywood subflooring. This is a NOFMA 3.25” and 2.25” white oak floor in the great room. Note the gaps. Before installation the product had been stored for some time in the garage and was not properly acclimated. This resulted in expansion before installation and later permanent gapping. This gapping may be objectionable to some customers.

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.

This photo and the left close-up photo on the next page is a NOFMA 5” wide plain sawn white oak plank floor. Note the minimal gaps and that the flooring is very flat. This product is in the master bedroom which is not heated as much as the other floors. Also, particular attention was paid to acclimation (3+ weeks) and the flooring was back sealed before installation. Even though the plank is significantly wider than the 3.25 and 2.25 flooring, attention to detail resulted in less gapping.

With designs where a slurry is poured over tubes between 2” –by-4” sleepers forming a thermal mass, the flooring contractor is often not present when the screeds and tubing are placed and the slurry is poured. The contractor needs to layout and designate where the sleepers should be placed so that the nailing pattern of the flooring will not interfere with the heating tubes. Typically sleepers should be placed all along the perpendicular wall line for nailing along board ends, around the door jambs for nailing within the door way, oriented in the proper direction for the direction of the flooring, and proper spacing of minimum 12”O.C. with no additional subfloor. Where a subfloor is required, such as with 4” plank flooring, sleepers can be spaced on 16” centers which allows for greater flexibility of tubing placement. The location of heating tubing along wall lines should be varied in order that tube and screed intersections do not fall the same distance from the wall and interfere with nailing along a single run.

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.

This is a close-up of the 3.25” and 2.25” flooring showing the gaps. Note the wider gaps are generally associated with the 3.25” flooring. Some are 3/64” wide and will likely widen particularly if the weather is severe. These gaps do not close completely in the summer. The gaps in the plank and quartered floors do close in the summer.

Another heating system is the “staple up” system where the tubes are attached to the undersurface of the subfloor by staples or clips. The primary caution here is to not risk puncturing a tube with a too-long fastener. If clips are used and the tubes do not contact the subflooring the risk is minimal with ¾” subflooring and 1½” or 1-3/4” long flooring cleats. Wire staples present the possibility of run out or deflection as they go through the subflooring and may exit at an angle that can damage the heating tubes.

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.

This is the close-up view of the 5-inch-wide plank. The minimal gapping resulted from proper acclimation and back sealing before installation.

Properly assessing and acclimating the flooring is also part of pre-installation fine tuning procedures. Contractors should already know the expected average seasonal EMC for their geographic area. Since a floor heating system dries the system more than other heating systems, the target acclimation moisture content should be about 1/2% to 1% less than the normal average.

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.

The center is a standing view and at right a close-up of a NOFMA 2.25” quartered white oak strip in the entry. Note that even in the close -up, gapping is minimal. Quartered flooring is the best performer resulting in the fewest gaps under the same conditions.

Choosing a properly manufactured flooring product will make for easier and more consistent acclimation. NOFMA Certification Standards particular to moisture content assure that NOFMA certified flooring will have a narrow moisture content range when manufactured (6%- 9%). Certified flooring is therefore expected to perform in a consistent, uniform, and predictable manner allowing the contractor to more easily assess and accommodate the expected changes imposed by radiant heating. In addition, configuration of a certified flooring product assures good performance over any of the heating systems described. That is, minimum movement and noise that can be associated with the match fit and gaps.

The center is a standing view and at right a close-up of a NOFMA 2.25” quartered white oak strip in the entry. Note that even in the close -up, gapping is minimal. Quartered flooring is the best performer resulting in the fewest gaps under the same conditions.

Grain orientation and width choice will also affect flooring performance. When the geographical average seasonal variation is more than 3%, the associated variation can mean the difference in getting paid and or having to replace a floor. A quartered, vertical, or edge grained product can be expected to vary ¼ to ½ as much as a similar plain sawn or flat grained product. For instance quartered red oak will expand or shrink 20 percent to 40 percent less than plain sawn product. So, seasonal gapping will be 20-40 % smaller. Additionally, seasonal expansion during non-heating will be less so that cupping is less likely. The coefficient for movement of red oak is .00369 for plain sawn and .00158 for quartered (radial). A 3¼” plain sawn board over 4% moisture loss can shrink .00369 x 3.25 x 3 = .048”. A 5” quartered board with the same conditions can shrink .00158 x 5.0 x 4 = .032”. Even this wider board if quartered will move less than the narrower plain sawn board. Remember, in reality these numbers are averages and some boards will move more or less than others. In addition, no board in these widths will be 100 percent quartered, so figure for some additional movement.

  • 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