Radiant heat poses special problems for wood floors because the heat source is so close to the underside of the floor. This means that in the winter the flooring is likely to dry and shrink even further than it would with other forms of heating, such as radiant baseboard or forced hot air.

With radiant heat, the small seasonal gaps that normally appear between floorboards can become large gaps — especially when “panelization” occurs. Panelization is when several strips of flooring shrink and swell together as a unit, either because the finish has welded them together or because they are part of a glued flooring system. When this happens, the gaps that do occur tend to “accumulate” in one very pronounced gap every three to six boards. On a herringbone pattern floor, the large, zig-zag gap makes it look as if the floor has been “unzipped.” In general, glued flooring systems don’t work over radiant heat. The intense heat in the substrate causes the adhesive to fail, and this increases the risk of panelization.

Recommendations

As far as I’m concerned there are only two choices when faced with installing wood flooring over radiant heat: (1) Use a floating system or (2) use the highest grade quartersawn flooring available and install it with extra nails to prevent panelization.

Floating system. I recommend the metal-clip floating system manufactured by Junckers (pronounced “yonkers”), and imported from Denmark (Junckers Hardwood Inc., 4920 E. Landon, Anaheim, CA 92807; 800/878-9663). This product is technically more sophisticated than any of the edge-glued laminated systems. The underside of the Junckers flooring is covered with plastic to limit changes in moisture content. Metal clips mechanically fasten the entire floor together into a monolithic unit and eliminate panelization.

Nail-down floors. If you must use conventional strip flooring, there are three basic rules:
(1) Use boards less than 3 inches wide. If your customers insist on wide boards, let them take the responsibility for the shrinkage that will occur during the heating season.

(2) Use quartersawn flooring (edge grain in pine). In most commonly used hardwood species such as oak, quartersawn material (annual growth rings perpendicular to the face of the piece) expands and contracts only 30% as much as plainsawn (annual growth rings parallel to the face).

(3) Increase nailing to every 4 to 5 inches on-center. This is the only way to prevent panelization with nail-down strip flooring.