We've built a 1700 sq ft ICF house in Northern Michigan.  Another 1500 sq ft in a walk out basement.  Radiant heat in the concrete in the basement and staple up on the main floor.  We are planning on ceramic tile and plain cement  throughout the basement and I don't have questions about installation in the basement but I have a few questions on the main floor.  Where there will be soild hardwood on the main floor what would be a good surface to put underneath it.  We have OSB and engineered I beams that is extremely solid.  Half of the main floor will be ceramic tile and the other half will be solid hardwood.  We'll lay ceramic backer board type stuff where the tile goes, but I don't like the idea of felt paper under the hardwood.  how would the heat of the flooring effect this?  any smells over time?  What are current practices under hardwood and over staple up radiant floor?   Thanks

I have the same situation, but assumed that the installer would not put any underlayment. Many hardwood installers will install right over plywood or OSB. In my opinion, an underlayment (especially if it is thin foam) will impede heat flow. And furthermore, since it is above a heated space, the underlayment is not necessary from a moisture barrier standpoint.

Others on the forum may have a different opinion, and I am eager to see that they have to say.

the thin foam under floating wood floors is not thick enough to pose a problem, and is usually offset by the thinner nature of floating floors.

Felt is a non issue either way. Tar paper, however, will smell if heated to a high degree.