I was thinking about this more on the way home from work.
I think using water (or some liquid that holds heat well) is a better way to go all around. Water can retain heat much longer than air. With your design, it would get really cold at night. With radiant floors, it would stay warmer longer (possibly through the night?). I saw a design once in Colorado where they had built these window box type structures along the wall of the south facing side of the house. The tops of the boxes were clear (plexiglass maybe?) and the sides were wood. Inside the "window boxes", they placed very large drums of water, painted black of course. The side of the box facing the house was left open. The tubs would heat up through the day and slowly release the heat into the ambient air. Just another idea.
It would be nice to see the breakdown of efficiencies with these different systems. Perhaps one of the physics wiz-kids will come along and help out with the equations and such. I'll try to look around a bit, but it usually takes me a long time to calculate these things.
