Options for Warm Wood Floors

The goal in designing a radiant system for hardwood floors is to provide continuous, even heat at as low a temperature as possible. There are several types of radiant floor systems, and some work better than others at meeting this goal.

Slab Systems

When I have my choice, I always prefer a system using thermal mass over a staple-up system. The mass helps even out temperature swings, which benefits the wood floor. I see no real differences in performance between full-thickness slabs and thin slabs poured over wood framing. With thin slabs, I like Gyp-Crete because it’s less likely to crack than a Portland cement mix, though it’s more expensive. Another thermal mass option we’ve been using is to substitute dry sand for the Gyp-Crete. The maximum fluid temperature I use in a concrete or Gyp-Crete system is 140°F, to keep thermal swings and possible fracturing in the concrete or Gyp-Crete to a minimum. The only times the design temperature would need to approach 140°F is if sleepers are laid on top of a slab and the hardwood flooring is installed on the sleepers, or if plywood is used to provide a continuous nail base. In general, I prefer to set the sleepers close together for good nailing, pour the thin mix in between, and lay the flooring directly over the slab and sleepers. Radiant Slab

Whether you install nail-down strip flooring (top) or a floating laminated wood floor (bottom) over a radiant slab, be sure to include a moisture barrier on top of the concrete as well as below the slab. For nail-down flooring, lay a single sheet of 3/4-inch plywood or a double layer of 1/2-inch plywood as a nail base.

Radiant Thin Slab

When installing nail-down strip flooring over a thin slab (top), the author recommends spacing the sleepers 8 inches on-center to provide good nailing. For wider sleeper spacings, you can use a plywood nail base, but the fluid temperature may need to be raised to overcome the plywood’s thermal resistance. A floating wood floor system (bottom) installs directly over the thin slab.

Staple-Up

In retrofit situations, when the wood floor is already in place, or when the designer, for whatever reason, doesn’t want to pour a thin slab, my second choice is a staple-up with a reflective insulating barrier below. This might be foil-faced rigid foam board or a reflective barrier with batt insulation underneath.

Reflective Staple-Up System

When installing underfloor radiant heating, the author staples the tubing directly to the subfloor, then adds an insulated reflective layer below — either foil-faced foam board, shown here, or a radiant blanket with batt insulation below.   Some installers use aluminum plates to help spread the heat sideways and prevent the "striping" effect. I don’t like the plates for a couple of reasons: They’re expensive — about $3 apiece — and they’re noisy when they’re walked over and when they expand and contract. I’ve found that as long as I use the reflective barrier, stapling the tubes directly to the subfloor without plates works fine. I’ve never had a complaint about striping as long as the floor is designed with minimum water temperature in mind. Another variation of this system, also designed to prevent striping, is the "suspended tube" approach — where the tubing hangs an inch or so below the subfloor with the reflective barrier below. This just makes it harder to install the tubing with little to gain for the trouble. I’ve seen this technique misapplied, where the tubing was stapled to the sides of the joists — which does a good job of heating up the joist. I prefer the direct contact of the tubing with the subfloor. Staple-up systems require slightly higher fluid temperatures, with the maximum I’d use being around 160°F. Usually, it takes about 130°F to 140°F in the joist bays to produce an 80°F surface temperature. A critical factor in designing any staple-up system is to make sure that the insulation R-value under the tubing is at least four times as great as the R-value of the materials above the tubing. The goal is to direct the heat up. Doug Mossbrook is president of Eagle Mountain HVAC in Canandaigua, N.Y., specializing in radiant heating applications. He has installed more than 100 successful systems with hardwood flooring. Thanks to Joe Jackson of Bristol Mountain Hardwoods for providing background information for the article.