All we install is solid flooring. We install a lot over a slab. We rip out a LOT of junk that other "floor" guys install in customers houses. Here we have an interesting situation, the customer has 3/8" engineered and we are installing 3/4" 4" wide oak. We need to first put down another layer of plywood, 1/4 or 3/8, but we are debating on whether to put any moisture barrier under that plywood. We normally put tar and plastic, but not sure that would be the best idea here. Thought about 30# felt, but not sure. In the past we have just put plywood on top of it and had no problems. Just thought to get your opinions.