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In this video This Old House general contractor Tom Silva demonstrates how to install radiant heating under solid wood floors.
Pro Tip: On a recent This Old House project, the crew discovered that prefabricating radiant floor panels in the shop dramatically cut installation time. As one installer noted, the shop approach would take about a day, compared to roughly three days for on-site installation—a significant labor savings that can meaningfully reduce overall project costs.
Steps for Installing Radiant Heat Under Solid Wood Flooring
- Fasten the heat transfer plates to the underside of the old floor with ½-inch screws. Install two parallel rows of plates between each pair of floor joists.
- Snap the flexible PEX tubing into the grooves of the heat transfer plates. When necessary, bore ⅝-inch holes through the joists to continue the run of tubing.
- Cover the tubing and transfer plates with aluminum foil.
- Spray open-cell foam insulation between the floor joists, covering the foil and insulating the tubing.
- Use a ⅛-inch notched trowel to spread flooring mastic onto the old hardwood floor. Work in small sections to prevent the mastic from drying out.
- Set the new hardwood flooring down into the mastic, then fasten it with an 18-gauge pneumatic nailer filled with 1-inch nails.
- Use a hammer and chisel to pry the floorboards tight together prior to shooting in the nails.
- Fill the nail holes, if desired, with wood putty, then sand the floor smooth.
- Apply at least three coats of polyurethane to the new floor.
Pro Tip: Richard Trethewey, This Old House plumbing and heating expert, explains why the aluminum plates are essential: “Plastic like PEX is not a good transfer medium. So the aluminum sucks the heat out and conducts it to the subfloor.” He also notes that the tracks keep the tubing evenly spaced at 6 to 8 inches to avoid cold spots.
When drilling holes through the joists for the PEX tubing, keep them at least 2 inches from the edges of each joist to preserve structural integrity. Also, avoid running tubing under cabinets or other built-ins. As Richard Trethewey cautions: “You don’t want to trap the heat under a sealed box.”
