The house I just bought is 8 years old and I want to finish 1/2 the basement. Do I need the 6-mil plastic under the subfloor? Do I need 1 or 2 layers of plastic in the walls. I was thinking of just using one layer under the drywall but my book tells me to put one under the insulation also. My basement is dry and i have never had moisture in the past. It is solid except for hairline cracks I did seal in the floor. I also used a sealer on the cement block.