Do you just absolutely *have to* build a sub-floor for your house??? I mean are there any structural ( or other ) reasons why you *must* do this ??? We have a dream to build a Passive Solar house. We don't have tons of money so we need to be careful how much we spend. I figure one method would be to use the Thermal Mass of the foundation. Wouldn't have to build a sub-floor, and as long as we insulate the foundation, it might do a respectable job of providing alternative heating during the winter. Of course we would have to calculate window, surface area and take into account all the other details that we need to think about when building passive solar. We wouldn't install any flooring of any kind on the first floor. So I just don't see any need to have a sub-floor ... Is this OK or am I just completely off my nut ??? Thanks So Very Much !!!