ok so im rebuilding the walls in my basement and it is half underground. so i have a short wall to ground level that is concrete(foundation). but when they built the wall above it instead of coming out to the top of the concrete there is a small 5 inch "shelf" built in to the middle of the wall. my plan is to build the wall straight up without the shelf. but doing this i will have a 5-6 " space area between the old wall and the new wall above the foundation concrete wall. there is what appears to be drywall, and 3.5 in thick fiberglass insallation on that top part. so my question is. am i supposed to fill that space between the old wall and the new wall or is it ok to leave that an empty area? i plan on doing a 6" staggered stud wall with drywall on the inside(closest to the old wall) and double on the outside using probably some sound board or double drywall. thanks for the help.