NOFMA Tip Sheet



Solid wood flooring and radiant heating systems can work well together with the correct choice of flooring materials, proper scheduling, and attention to installation details. The typical winter heating season is associated with low humidity and a dry environment. For all wood flooring products, during the winter a seasonal reduction in moisture content causes shrinkage and normal gaps.

With a radiant heating system, the heat source is near or actually touching the flooring and expected conditions can involve extra gaps between strips/planks and or cracks in the flooring itself. The heating system should be engineered for wood flooring with correct heat source temperatures and thermostatic controls. For information, check with The Radiant Panel Association, 800-660-7187.

The typical contractor wants to minimize his problems, and will try to reduce the drying effect to the flooring. A suggested procedure by contractors unfamiliar with the system is to acclimate the flooring product while the heat is running and thus dry it out before installation so gaps will not form.

This is only a good idea if the heating system is continuously operated throughout the year and environmental conditions remain constant.

What happens when the heating is turned off? Seasonal spring and summer humidity increases and can affect the flooring product dramatically.

The drying effect is reversed; the wood flooring absorbs available moisture and expands. If the flooring is installed when very dry and later expands, the result will be in the least cupping and may even buckle from the stress of expansion.

The key to solid wood performance is to take moisture readings of the site materials, the flooring to be installed, and to know from experience the normal interior environmental conditions of the geographical area. The ideal conditions would be to install the flooring at the average area moisture content over a subflooring system, also at the average moisture condition.

When it comes to the product choice, narrower is better. Wide width plank flooring 4 inches and wider will expand or shrink significantly more than the narrower product. Quartered/vertical grain product can also be a good choice as it is more stable than plain/flat-sawn material. Species with expansion factors similar to or less than Oak are suggested. Light colored and or white stained woods show gaps much more than naturally darker or site-finished brown stained woods; and so Maple and similar light colored species would elicit caution. When scheduling installations over radiant heating, it is important to condition the site. The heating system should be installed and have been running five days or more before delivery of the flooring. This removes any site-related extra moisture and relieves stress and movement associated with the previous uncontrolled environmental building conditions. It also goes without saying the site should be nearly complete and have near occupied environmental conditions well established. The flooring need not be installed with the heat running; in fact, it is preferred to not have it running during installation to maintain the recorded moisture readings. Install with the flooring at average area moisture readings. For average winter readings of 6-6 1/2 percent m.c. and summer 9-10 percent m.c., the average would be about 8 percent m.c. If the flooring is lower than the average expected moisture content, spacing may be required at intervals within the installed field. If higher, some drying may be necessary to reach the average.

A solid unidirectional 1/2-inch and thicker wood floor has to have a nailing base or subfloor. Gluing is not recommended for these products. It is important not to subject the heating system to harm by puncturing mats or tubes with fasteners. There are generally three types of heating systems. The "in slab" system, which has heating tubes imbedded in the slab; the "staple up" system, which has heating tubes attached to the underside of the subflooring on conventional joists; and the "thermal mass" system, with heat tubes embedded in a cement type material with sleepers or screeds as the nailing base.

For slab systems where the tubes are placed before the slab is poured (Figure 1), a floating subfloor is generally recommended as the nailing base. For 3/4 -inch thick flooring, this can be 2 layers of 1/2-inch thick plywood with the top layer installed at 45 degrees to the room dimension if the flooring is not to be on the same diagonal. The plywood should not be attached to the slab. Fasten the two layers together with 7/8-inch screws or staples on a 6-8-inch grid. Or use 3/4-inch plywood cut into 16-inch-by-8-foot planks scored 3/8-inch on the back (Figure 2), and lay in a staggered pattern (Figure 3). Also use necessary vapor retarders for on grade and first floor slabs.

Next are the sleeper systems with heating tubes between sleepers as the floor-nailing base in concrete or with reflective pans. It is important that: the sleepers be spaced on average no more than 12 inches on center for proper nailing frequency (Figure 4); sleepers should be a dense wood like southern pine or equivalent; the sleepers lay flat, and may require some kerfing (Figure 5); and sleepers be placed to support flooring at end walls and doorways (Figure 6). Also it is a good idea to mark where heating tubes cross sleepers.

The staple up system, where the heating tubes are placed on the underside of the primary subfloor between the joists, is another frequently encountered system. The major caution is to use fasteners short enough not to puncture the tubes. For 3/4-inch subflooring, 1 1/2-inch fasteners are generally recommended; 2-inch fasteners will exit the plywood and can puncture a tube (Figure 7). The longer fastener, 1 3/4 inches or 2 inches, may be used if the angle of nailing is greater. But, be advised that some fasteners, particularly those of wire, may not follow the angle of nailing and run out of the subflooring unexpectedly.

As has been stated in previous articles, know the wood product's dynamics; check its moisture content; know what expected conditions are present in the area; accommodate the wood product to those average conditions; and with these special installations, take more time to install carefully.