I am starting to look into replacing all subfloor in my 1000 square foot house that is closing in on 100 years old. I have hardwoods underneath the carpet and then it appears as though I have 2 layers of subfloor. I know I have atleast one layer of 1x6 diagonal subfloor. I wasn't expecting two but regardless its old. When I bought the house 6 years ago they redid all the floor joists by sistering in new joists that are notched and land on the stem wall and cut the ends off the old original joists and sistered them. They did a pretty bad job sistering them. They didn't clamp or glue them but it appears like they just used a nail every few feet. Since the repair my floors are getting creakier every month and I can't live with it anymore. I plan on opening up the floors and replacing the 3 layers with 1 layer of 1 inch tongue and groove plywood. I will use adhesive as well. My question is when I am dealing with walls that run parrelel with the joists do I just block the joists underneath them to support them? I do NOT want to replace all the subfloor under existing walls or around the rim of the house although it appears that I may have to do some rot repair in those areas regardless. Any suggestions are welcome. I just planned on blocking all the joists that run underneath the parallel walls. The ones perpendicular should be ok from what I read. Thanks in advance.