We have back porch built on a 4” concrete slab. I would like
to enclose it and make it a storage room. It has a 7” step down
from the house floor. I was thinking of just covering it with
plastic 6mil, putting 2x8 (and shims) across. Put some
fiberglass (Upstate NY) insulation and nail sub-floor, done.
Then I got wondering if I should vent it to my basement. What
would be the recommended way of raising this floor and avoiding
rot?

Thanks
TP