Then I built a "hydronic sandwich" using aluminum heat transfer plates between 'sleepers' of 3/4" foil-faced isocyanurate insulation.
With Lisa's help, we routed 1/2" pex tubing in a serpentine pattern through the wall, across the doorway framing, through the smaller side wall, then back to the starting point...then we repeated it in the second bedroom with a separate run of pex...whew!
Finally, ready for drywall, which is screwed into the studs being VERY CAREFUL not to puncture the tubing runs.... The aluminum plates help conduct the heat out of the pex tubing and into the drywall surface. The insulation 'sleepers' act as spacers between the drywall and studs, and help direct the heat out outward to the room. The drywall surface readily conducts the heat and radiates it outward to the room. By using this method rather than the more tradition underfloor hydronic installation, more BTUs can be delivered to the room at lower water temps...and it was a much easier and less expensive method in this case. The obvious downside to this method is the risk of puncturing the tubing with fasteners. I took a lot of pictures of the assembly and documented the pex locations so if we ever need to hang a picture on the wall, I'll know where not to pound a nail!