We're about to install 5" wide, tongue and groove, pine boards over the sub floor in our house. I've read differing reports on the proper material, if any, to put between the two.
Worth noting - these are relatively fresh beetle-kill pine boards. And the house sits at 9,000 feet on the Western Slope of Colorado. Relatively dry climate, but pretty snowy in March and April. There is a crawlspace underneath, which is dry dirt and rock. It also has radiant heat lines under the floor (which I'll be temp. moving before install as not to puncture).
Also - doing the upstairs rooms as well - different approach?