1. Fasten the plywood-backed radiant-heat panels to the subfloor with 1⅝-in. screws.
  2. Stretch a continuous length of PEX tubing back and forth across the floor.
  3. Use a rubber mallet to tap the tubing into the grooves of the radiant-heat panels.
  4. Run the tubing ends down through the floor to the boiler.
  5. Mount the manifold to a wall near the boiler, then connect the tubing ends to the manifold.
  6. Run piping from the boiler to the manifold.
  7. Install finish flooring over the tubing.

Shopping list:

Radiant-heat panels used to hold PEX tubing in place in subfloor 1 5/8-inch drywall screws for fastening down the radiant-heat panels

Tools:

How to Retrofit Radiant Heat From Above - 23How to Retrofit Radiant Heat From Above - 17