1 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.
2 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.
3 Cover the tubing and transfer plates with aluminum foil.
4 Spray open-cell foam insulation between the floor joists, covering the foil and insulating the tubing.
5 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.
6 Set the new hardwood flooring down into the mastic, then fasten it with an 18-gauge pneumatic nailer filled with 1-inch nails.
7 Use a hammer and chisel to pry the floorboards tight together prior to shooting in the nails.
8 Fill the nail holes, if desired, with wood putty, then sand the floor smooth.
9 Apply at least three coats of polyurethane to the new floor.