I'm building a riser that's 12 1/2" tall. I'm doing a first level using 2x6s and 3/4 inch plywood. Then I'm doing a second level using the same materials, but pulled in a little for the step up. I will ultimately carpet everything. I know that having some insulation in the second layer would prevent any hollow sounds from walking on it, but wondered if having insulation in that bottom layer (on my basement floor) was necessary. Thanks!