I am getting ready to lay/staple 3/8" x 3" Bolivian Rosewood; first floor, over a crawl space. The existing subfloor is 3/4" OSB. Here is my plan:

1. Screw Down Loose OSB with 2-1/2" Deck Screws (counter sunk).

2. Sand (belt sander) all OSB seams and any surface imperfections.

3. Lay 3/8" CDX plywood as an underlayment.

Here is where I need some help...proper install of the underlayment. Just to confirm with the 3/8" CDX I should:

a. Maintain a minimum 1/4" expansion gap between the drywall and edge of the CDX

b. Maintain a minimum 1/8" expansion gap between CDX seams.

c. Run the CDX perpendicular to the direction (length) of the OSB.

d. Screw or nail the CDX with 1' inch deck srews counter sunk direct to the OSB

e. Use a 2" nailing pattern (Screws 4-inches okay?) around the CDX edges (max 3/8" from edge) and then one every 6-inches in the field in a diagonal pattern.

f. Maintain a minimum 8" off-set from any OSB seams.

g. place machined egdes on wall side.

In addition to the questions above....I have another general question: My crawlspace is rather moist (likely adding a .12 mil vapor barrier before winter)...would there be any benefit to sandwiching a .6 mil vapor barrier between the OSB and CDX? Then follow up with a 15lb felt over the CDX?

Thanks for the help.

Your basic plan is fine. It will be slow driving all those screws. I prefer 1&1/4" construction staples. But screws actually hold better. It just takes forever, IMO. If your crawl space is damp, you need to locate the source of the moisture and eliminate it. Re-grade, re-direct downspouts, check for leaks, etc. And covering it ( the earth ) with a polyethylene plastic vapor retarder is called for and recommended. You should use 6 to 8 mil; .12 mil isn't even good enough for trash bags. You could lay a layer of 15 lb. or 30 lb. asphalt saturated felt over the OSB and UNDER the 3/8 plywood. Then use Fortifiber Aquabar "B" over the plywood underlayment. This will help considerably with the moisture vapor but not eliminate it. You could still have a moisture problem. That's why we suggest checking with a moisture meter. Then you know for sure. Putting plastic between layers of wood will accelerate dry rot of the subfloor as it will trap moisture in it.