Expert: Bruce E. Johnson - 9/27/2010

Question
I am adding a narrow room (4 by 14 feet) to our house which will sit over an existing concrete garage floor.  The joist plate will be 6 1/2 inches from the floor, using 2 x 10 joists (to match the house joists).  They will run the short way (approx. 4 feet).  I thought I would use bricks or some type of block to build a perimeter support for the joist plate to sit on and then seal the sills well to prevent vermin and bugs from entering under the new floor.  
I was considering how to insulate the new floor since it will be exposed to the unheated garage air.  I thought about making a trough out of visqueen by stapling it to the wood sills and then pouring cellulose blown in insulation to the top of the joists.  If I do this, the void under the sub floor would be totally filled with no ventilation.
Does that sound okay or totally wrong?
Would it be better to build the joist supports elevated (supporting the corners and a couple supports in the middle) and seal the bottom of the floor and then put fiberglass insulation into the joist voids?  This would leave the entire bottom elevated and suspended above the concrete floor.  Air would circulate under the floor and bugs and mice would have access (hard to keep them out of the garage in the fall).
Or what would you recommend?  

Answer
Hi Bob, provided your garage slab has a vapor barrier under it (which is common practice).  Moisture shouldn't be a big concern.  I would build the floor system tight on the exterior to keep out the critters and then insulate the joists with fiberglass batt insulation with a paper or foil backing facing up into the living space and then apply my floor decking or sheathing.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com