Which underlayment, if any, between 5" pine boards, and subfloor with radiant heat?
Nothing. You want the wood to breathe &amp; there are plenty of 100+ year old houses to prove &amp; approve this "method". If you're gluing &amp; face nailing the boards, then forget the nails. Screw down the subfloor &amp; then Finish Screw down the finish floor. You'll never have a squeak &amp; the floor will always be solid.