Install a Heated Wood Floor
Many crawlspace floor joists are insulated with fiberglass batt insulation, which tends to absorb moisture and also can fall out if not properly installed. Insulating the perimeter walls is one way to avoid this. Another option is our method filling the entire floor joist bay and encapsulating the floor joist system (sills, rim joists and joists) with closed-cell, spray foam insulation.
The next step was to cover the floor with a crawlspace liner and open the crawlspace to the basement, which is HVAC conditioned. The goal was to build a sealed, conditioned crawlspace that would avoid condensation during humid seasonal conditions.
Once we were satisfied that we’d addressed those moisture issues, we added the electric radiant heat system.
Note: With a non-insulated crawlspace under the plywood, you should use insulation boards prior to installing the heating coils.
Using Electric Radiant Heat under Wood Floors
In our four-season porch, we decided to install 3/4-in. oak hardwood flooring to match the rest of the house flooring, but we had some floor height issues to deal with first. The porch floor was 1-1/2 inches lower than the finished hardwood floor height of the main house. We wanted the new heated hardwood floor to be seamless, and the same height as the main house.
Matching Floor Heights
In order to get our new porch floor the same height as the existing house floor, we had to add plywood to the porch floor. We screwed down a layer of 1/2-in. CDX plywood to raise the floor and then installed 1/4-in. plywood nailer strips (called “sleepers”) along the perimeter of the room and at 16-in. intervals to get to a total 3/4-in. height. The radiant heating cable measures 1/8 in. so we decided to use 1/4-in. plywood ripped to 3 inches for the sleepers.
Sleeper strips create lanes into which the heating cable will be placed and should be installed perpendicular to the hardwood flooring to be fastened into the sleeper.
Using 3-in. wide sleepers allowed us to space out sleepers to the standard 16-in. on-center spacing. This spacing creates a 10-in. wide space to run three loops of radiant cable spaced 3 inches apart.