I'm rebuilding a room over a crawl space,unvented, I have 6 mil on the ground and will have ins. Between the joists.
I have access to the tops of the block that makes up the walls, 2 rows on a footing.
Should I 1--just leave them as is 2-- fill them with the sandy soil from the crawl space 3--fill them with styrafoam beads that I have?

Thanks for your time---Art

Filling the voids in the block with strofoam certainly will help but not as much as venting the crawl. Which do you think is your bigger issue, humidity or insulation from the cold?

Everything looked good under the floor,all was dry. So I'm mostly looking for insulation.
Living in Northern Wisconsin.

Insulating the holow block that makes your foundation will give minimal insulation. You do plan to insulate the cavity between the floor joits and will produce the most benefit. If you have the foam and the time, go ahead but again minimal help. Tell me about the insulation between the joists and is there a vapor barrier, which side?

Going to nail furring strips to the bottom of the joists. This will hold up 1/2" blue styrafoam cut 14 1/2" wide between the joists.
Then I'm going to fill the space with insalation, batts or sty. Beads or blown in cel. Not sure yet.
Then a 6 mil. Plastic cover and 3/4" T&G plywood.

Rigid foam board insulation does not seal well between joists so taping is required, or use batts which fit snuggly.

Only using the foam board to hold up the rest of the ins.

Furring strips could do that, some folks even use poultry cloth.