I have hydronic radiant heat set up attached to the underside of subfloor and between floor joists spaced at 16". This set up sits above 1400 sq.ft. of unheated basement and needs to be re-insulated. Above it is the traditional subfloor with 3/4" engineered oak flooring immediately on top. I have been told many different methods to insulate this ranging from reflective bubble foil to standard fiberglass batting with a face to rigid insulation to any combination of 2 or more of these. How do I know how much R value I need and how best to achieve it efficiently (current and future costs) and safely (not prone to mold)?
Without having a basis to calculate, I was considering using a double bubble/double reflective foil stapled into the insides of the joists 4" or so below the radiant tubing (reported to be reflective and an R value of nearly 17) plus some rigid foam beneath that but still between the joists.