Sounds like you want to hold-off any construction so try this: How are the basement windows? Most are single pane. Install some of the plastic window shield over them (the stuff you shrink with a hair dryer). Run the circulating fan from the furnace continually. This will keep the basement near the house temperature while refreshing the air there. Possibly add another heat vent - duct it over a window or to an outside wall. Our basement is finished with 2" XPS insulation against the concrete. It too drops to the low 60's but doing the above raises the temperature. We installed an electric fireplace as auxulary heat to make the space more comfortable in the spring/fall when the furnace is not running to heat the house. I don't believe there is a "good, inexpensive" solution. Water heaters are not efficient or intended for use as a boiler. Good luck.

One follow up to my post. Another thought would be to add an air return to the finished basement room. This would be relatively easy as the cellar w/the air handler is right through a wall - easier than adding a radiator with pex. Would adding a return to the basement (there are 4 small returns on first floor) make the room any (or much) warmer? Could this degrade the AC on first floor in summer? Thanks again.

We recently debated installing fiberglass insulation in a finished basement in another posting. After reading some of the postings, I am not sure what is the right thing to do in every situation. Part of my basement is finished with fiberglass batts in the walls. There is at least a 2 inch gap between the cinder block wall and the fiberglass. But my house has forced hot air heating where is basement is at 35-40% humidity in the winter. Your humidity is likely to be higher and your winters are colder than what I experience in central NJ. Maybe the right thing to do is no insulation. There is heat loss through the foundation, but it is much less than an exterior wall that is above ground. If the basement is comfortable in the winter then I question how much energy can be saved by insulating field stone foundation?

Julie - There really isn't that much temperature stratification to worry about. I have ducts in floors and ceilings of various houses and I don't really care where they are in heating or a/c season. And explain to me how radiant heat makes for a drier basement? Especially if you are talking about opening a/c vents? How does that correlate to radiant heat? If radiant were to leak, that would be better under basement slab - I suppose that is one reason. The diminished need for a/c in a basement would be another. Concrete slab is the cheapest way to do radiant is another reason. If you insulate the basement well (and you should), then the heat load is pretty low (few windows, conditioned space above, minimal air leakage). So the radiant is limping along and seems like a bit of overkill. Radiant is expensive. Better to spend that money to make the structure tight and barely need heat. More comfortable too. If you have endless money, then do both. But the best emphasis should be on a tight structure.