How to Retrofit Radiant Heat From Above

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 team member noted, walking the tubing into the channels with your feet “certainly beats being upside down in the crawlspace trying to beat that tubing into that channel.” The prefab approach took about a day, compared to roughly three days for on-site installation — a difference that can significantly affect labor costs.

Pro Tip: One often-overlooked cost factor is stairways. According to This Old House Magazine, building code specifies that stair-tread height can’t vary more than ⅜ of an inch, so adding radiant panels on top of the subfloor may require taking apart and rebuilding the stairway to raise the treads — a labor-intensive addition to the project budget. Installing metal plates under the subfloor avoids this issue but means the heat has an extra layer of material to pass through.

Steps:

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: