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In this video This Old House general contractor Tom Silva demonstrates how to install radiant heating under solid wood floors.
Shop vs. Site: 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.
Hidden Cost Factor: If you’re installing radiant panels on top of the subfloor rather than underneath, be aware that the added floor height can trigger unexpected expenses. As noted in This Old House Magazine, building code requires that stair-tread height can’t vary more than ⅜ of an inch, so you may need to disassemble and rebuild a stairway — or trim doors throughout the space — adding to the overall project budget.
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.”
