This thread is a spin-off from the thread entitled "Frank's Workshop Construction Project" (http://www.sawmillcreek.org/showthread.php?t=7769).
The workshop that I am building has 2x8 pressure treated joists on top of beams on top of piers. Thus, it is open below, so insulation becomes important. I had planned to install two floors. The lower one would be 3/4 inch pressure treated plywood nailed onto the joists. On top of that, I was planning to put 3" of rigid styrofoam insulation within structure of nailers. One top of that, I was going to put a plastic vapour barrier then 1/2 plywood.
Three problems with this are:
(1) it raises the floor about 2.5" more than I would like,
(2) the styrofoam insulation is expensive,
(3) I only get R15.
It occurred to me a couple of days ago that I could suspend fiberglass batts within the joists before installing the plywood. I could support the batts with 1x3 cross pieces every foot. The batts I have in mind are the same ones that I will use in the walls -Roxul Flexibatt which is moisture resistant and which is rated at R21.5. I would then put plastic on top of the 3/4 " plywood and 1/2 plywood on top of that.
The advantages of this are:
(1) the floor would be at about the height that I like
(2) the cost of material is about 1/3 the original plan
(3_ I get R21.5 rather than R15.
I have never seen this technique described anywhere so I don't know if there is some problem with it. I wrote to the manufacturer of Roxul Flexibatt insulation asking them if this is a suitable application but have not heard back from them.
I would appreciate hearing from anyone who can either tell me that the technique is OK, can tell me that it is OK with modifications, or can tell why it is wrong.
In case the suspension technique that I described is not obvious, I mocked up a small section and took some pictures.