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?
Leveling any floor is a tricky project. On one hand, yes, the floor would be flat, but jacking up anything in your crawl space risks the possibility that the shifting can cause damage included pipe breaks, wires being pulled too tight and even wall or floor cracks.
The first step is to inspect the area under the sagged floor to determine why it has settled. If termite damage or decay is discovered, for example, that would have to be fixed before anything else to make sure the structure is stabilized.
Assuming the stricture is intact, and then I'd suggest leaving the floor as is and using a floor leveling compound on the top side of the floor to level out the surface before adding any new type of flooring over that. Floor leveling compounds are thick liquids that self-level and then dry solidly in place.
As far as the foundation is concerned, yes, it is normal to see the exposed concrete block. Some people insulate these with a foam board to other foundation insulators when they are above grade. However, having fiberglass insulation in the floor cavity is most important in a crawl space structure. If you have uneven gaps to seal, use and expandable insulator like Great Stuff.