I hope nobody tries to ream me out... but I'm wondering why radiant is considered more efficient/green than forced air heating (meaning why is there a forum on this site of green building for radiant?).

With houses similarly sized & built, and side by side installations of geothermal with radiant in one house, and geothermal forced air in another, the sub-meters don't show that radiant is more efficient. In fact, it's much less efficient.

I'm not arguing anything about the comfort, rather, just the efficiency.

I would assume someone pooched the radiant system design, or that it's being operated incorrectly (setbacks with slabs, etc).

Or that the radiant panel is not properly insulated, perhaps.

I could see them being similar in a very tight envelope situation (though fans are inherently much less efficient than pumps), but very few cases where the radiant would be worse.

But part of my wondering is what makes radiant greener than forced air?

generally, it is more efficient (less stratification, greater comfort at a consistent temperature without constant thermostat fiddling). Generally, it is also more able to utilize alt energy sources. It also generally requires a lot less energy to move the heat around. It's also generally healthier.

But they are all generalizations, and there are always exceptions. Radiant is not greener than forced air just because it's radiant, it also has to be a decent system. I've seen (heck, in the past I've even designed) systems with a wall of pumps that would use as much electricity as any FHA fan system, for example. But a good radiant system is greener than a good forced air system, typically.

In some cases, the difference may not be very large though.

Built Green Colorado makes no distinction for this high(15) point item when using a GSHP.

They do give some extra points for radiant. See Built Green Colorado checklist(google it).