What do you need to put in a sub floor in a basement?
The short answer:
The long answer:
When the foundation of your home was dug, there was a gap between the outer foundation walls and the inner walls. This space was backfilled with soil, but the soil will always be fluffed up and lighter than the virgin soil around it. When it rains, this soil naturally holds much more water than the rest, and a false water table is created around your home.
How does this affect your basement floor?
Your basement is made out of concrete, or perhaps, stone, or brick. Regardless, concrete, mortar, and brick are porous, and water vapor easily travels through as water pressure from the false water table presses on the foundation. Generally, the inside foundation surface won't look damp, because the drier air will suck the moisture out right away and turn it into humidity in the basement.
If you lay wood or carpet on the basement floor or drywall on the walls, it's going to get damp. It's damp, it's warm, and it's organic (drywall has layers of thick paper on its surface. That means mold, rot, mildew, smells, and dust mite feces for your basment. Even "mold resistant" drywall has no warranty against mold.
As a final strike, the air in your home moves upwards. This means that as warm air exits from the attic and upper levels, your basement air comes upstairs. If you have mold spores, odors, and dust mite feces in the basement air, you'll be breathing it in as well. And replacing the moldy, rotting floors will be VERY tough on your bank account.
Find some waterproof sub floor to place on your basement floor that can serve as a vapor barrier to prevent moisture from collecting under your carpet and starting a vicious cycle.
I write the content for Total Basement Finishing. We have some great info about sub floors there as well, if you're interested. Check out http://www.totalbasementfinishing.com/basement-floor.php .