Vapor barrier under carpet pad

Is it a good idea to lay down a sheet of plastic (polyethylene) under the carpet pad when installing carpet on basement concrete floor? The pad I am getting has vapor barrier between the carpet and pad.

Another possibility is to coat the floor with drylock. I am incliened towards the plastic sheet because of less labor, but if it turns out that sheet is not a good idea, I will use drylock.

Thanks!

I would advise against any type of pad/vapor pad in a bsmt situation. Its a given fact that all bsmts below grade will get wet. By placing a pad/vapor pad under the carpet will contribute to larger cost in restoring.

If your inclined to pad your basment drylock the cement. Leave the pad with the vapor barrior for a pet situation. If you have moisture under the vapor barrior wouldnt you want to know about it? Not only will it keep water from soaking into the pad it will keep water from coming into the carpet.

IMHO Dryloc and carpet glue down.