I am planning an insulation project that would give me about 16 inches of insulation. Only 5.5 inches of it will fit between the joists, so I'm going to be installing R-19 batts between the joists and then R-30 unfaced batts across the top. That puts the top of the insulation 10.5 inches above the tops of the current joists.

I want to build a floor above this insulation; I was thinking at maybe 14" above the tops of the joists. So I am thinking that I need to build a second set of joists directly above the first set, and plywood on top of that.

I want this newer floor to be able to support my body weight (150lbs) walking around on it occasionally, and hold light seasonal storage items and spare/extra building supplies. (ie. rolls of carpet leftovers) Another major purpose of this floor is to allow continued access to all electrical fixtures and ductwork, without crushing the insulation batts above the tops of the existing joists.

Am I correct in my assumption that it would be bad to support this newer floor with vertical 2x4's attached to the center of the existing joists? (I'm guessing that it would be better to put the vertical supports over walls in the house underneath so the walls are supporting the additional weight instead of the existing ceiling joists??)

Considering the low weight requirements, would 2x4 joists on the new floor be sufficient? Would I have to put them over each of the existing joists (16" apart), or could I get away with putting them over every-other-one of the existing joists? (32" apart) Could I get away with 1/2 inch plywood, or would I need thicker? (Or could I get away with 1/4 inch?)

I would welcome any suggestions or references to online articles describing how to do this, since I've never built a subfloor and I am very concerned about damaging my house's existing infrastructure.