Retro-fitting an Existing Building with Radiant Heating
The most common way to retro fit a building with radiant is by installing aluminum radiant track below the lower floor. If the basement ceiling is unfinished radiant tracks and PEX can easily be installed to turn any floor into a radiant system. The old style method, that we do not use, required looping PEX between the joists then covering it with insulation. The PEX gives off the heat to the area (convection, not radiant), it rises and the insulation prevents it from warming the basement. We use only radiant track systems. While there is additional cost for the track, less PEX is required and the radiant heat is much more effective.
The PEX tubing carries the hot water through your system. It is important to have the correct type of PEX. It is not all designed for heating applications.
Installation of the track can be a money-saving DIY project. Two runs of t
rack are installed in each joist area. The PEX goes up one side and down the other where it connects to the main run to the manifold. Unless you have excellent plumbing skills you will want to get help 
at this point.
The aluminum tracks do an excellent job of drawing the heat from the PEX and passing it into the floor system.
Since heat naturally rises and the second floor gets some additional heat. If a more complete system is desired the flooring is generally removed and 4x8 sheets of radiant board are installed. These same sheets can also be placed vertically, they are only 5/8” thick, or low temperature radiators or baseboard can be installed.
R Value Chart.