I would create a support frame work under your home. This would hold the R28 insulation required, the non-perforated soffit holding the insulation, and the vapour-barrier would be on the warm side of the insulation. I would have this hang about a minimum of 12” to 16” below the home’s frame. This will allow you to supply heat and have cold air return to this space so you will have a much drier and warmer home this winter.