The builder wants to put up a second wall and then fill the space between the two walls with cellulose insulation. That would be an excellent solution. However, I would have him do 2 things... Fill the first wall with insulation too, and use 5/8 inch minimun thickness drywall on the second wall. The 2nd wall should not be attached or touching the 1st wall at all. It should be touching the floor and ceiling only.