Updated: Apr 7
We found that the existing dark tile/concrete slab floor was always colder than the air, and was unpleasantly cold in the winter. It did not function as a thermal mass to heat the house because there is not sufficient thermal gain through the windows to bring such a large thermal mass above the room temperature.
So we added radiant thermal tubing to the floor, and in the living areas we added insulative wood on top. The house is already much warmer, without even running hot water through the hydronic system. Once we finish the flooring, the next step will be to add the equipment to heat and circulate the water.