I have a 1/2 bath right inside my back door. The floor leans backwards giving a 1/4 inch to 3/4 inch gap at the back wall. Cold air come in the bathroom. I temporarily put some insulation in the gap but want to fix the floor. In the basement underneath I can crawl under an exposed hole in cinder block wall. The space is dirt floor and only 3 ft. high. Do I put up a jack to make it even? Would this affect the structure? Also the cinder block is exposed. Should I cover that as well while I'm underneath? What would I use?