How to Install Radiant Heat Transfer Plates

       Radiant heat transfer plates can be installed above the floor or beneath the floor in-between the joist gaps.  Many people, like myself, want to install radiant heat to an existing home where floors are already finished.  Installing radiant heat transfer plates and radiant tubing from beneath the floor is one of the most common retrofit installations of radiant floor heating.  Joist gaps usually carry a lot of dead (unutilized) space.  This space can now be used to run radiant tubing, heat transfer plates, thermostat wires, and provide placement for manifolds.  Some people have had radiant tubing previously installed within the joist gaps, but found that the temperature supplied by tubing alone was insufficient and would like to increase the efficiency of their system by adding heat transfer plates. Installing heat transfer plates is possibly the easiest part of the entire radiant heating system, and the most cost beneficial as well.
 

     Before pulling tubing, the subfloor needs to be inspected and all exposed screws and nails must be clipped or removed.

     Once tubing has been feed between each joist in a loop, there will be two lengths of tubing running in parallel.  The first heat transfer plate is snapped onto the tubing.  Typical joist spacing is 16 inches on-center, with an actual gap of 14.5 inches.

     Other consideration for the allowed placement are tubing size and the bend diameter that the tubing manufacture allows.  You don’t want to void your tubing warranty or kink a tube by bending it too tightly at the end of the gap.  For instance, 1/2” PEX tubing is typically not allowed to be bent tighter than a 10 inch diameter.  Therefore, 4 inch wide plates would be spaced 10 inches apart inside the joist gap.  Other types of radiant heat tubing such as PEX-AL-PEX allow a much tighter bend diameter and can usually allow spacing at an even 8” within the gap and across the floor (a big benefit to providing uniform spread heating).  However you plan your spacing, it helps to create a spacer block from wood to use during the installation.

     Plates are stapled up to the subfloor in progression, using 18 gauge (or thicker) staples or with pan head metal screws.  The staple leg length or screw length should not be longer than the thickness of the subfloor. For instance, use ¾ inch staple leg or slightly shorter for a ¾ inch thick subfloor.  Staple along the flat part of the plate at least every 12 inches, and on both sides of the plate.  A one inch gap should be placed between plates along the tubing to allow for the aluminum metal to expand with heat.    

To keep heat directed toward the floor above and not lost to the level below, the gap needs to be properly insulated.  Radiant heat is mostly achieved through conductive heat, but some convective heating can also occur when a 2” or larger gap is provided between insulation and subfloor.  For instance, a 12” high joist gap could have an R-30 fiberglass batt installed that is 10” high when expanded.  Other people have successfully used cut rigid foam sheets with a foil face to reflect the heat back towards the subfloor.

Feel free to comment for any questions you may have.  I am happy to help!