Like the additional joist idea (sister). Use adhesive as well as nails or screws.
You could also box the joists (every other pair) with 1/2" or 3/4" plywood, again using adhesive and nails or screws.
Either of these choices should stiffen the floor considerably. The adhesive is important. Both ideas requires floor to be open underneath.
You could also box the joists (every other pair) with 1/2" or 3/4" plywood, again using adhesive and nails or screws.
Either of these choices should stiffen the floor considerably. The adhesive is important. Both ideas requires floor to be open underneath.