I have 12" I-joist at 16 oc. They span 17 ft and there is one row of blocking. The floors are a bit too bouncy for my liking so I’ll be adding additional blocking to reduce vibration is certain areas.
My question is: if I’m adding additional blocking, do I have to commit to adding them to each joist bay, or can I be selective and only add them to problematic areas?
Main reasons are high costs of lumber and labour.
Check with the manufacturer
This appears to be a construction question, not an inspection question. Consider another forum more suited to the topic?
adding blocking will do nothing to stiffen up the floor, you will just weaken the I-Joist putting nails through it. look into adding a LVL beam in the middle of the span. LVL beams are not that expensive. If you have a Menards in your area check with them.
In regard to the I joist being bouncy William answered correctly . Adding blocks will do nothing . You can add a girder under the joist to shorten the span but depending on the length of the girder you may need a supporting column in the center of the girder to stiffen the floor properly . If you don’t want a column cutting the space , then you need to add additional I joist at list every other one but then again in order to make the right call you need to see the length of the I joist . If around 20’ you may need to double up everyone . My call is that those joist were undersized or shouod have been installed 12" on center .
Very possible those are over spanned for 11-7/8" I-joist.
One would have to know the series and manufacture of the I-joist to know for sure.
Some manufactures I-joist are rate for L/480, L/360, and L/240. Some call for 19/32" OSB rated sheathing, glued and nailed, which makes a difference in the stiffness.
I would recommend a qualified licensed contractor that might have an in house PE to evaluate the floor framing to see if the bounciness is relevant to over spanned I-joist and provide a recommendation to a proper solution of repair.
Alex, the answer to your question is blocking helps distribute the load on a single joist to the adjacent joists. The farther away from the concentrated load the less the other joists can help. The bounce you feel when walking on the floor is associated with the floor geometry. If you saw the subfloor between each I-Joist you basically have a T shaped cross section. How well the subfloor is attached to the I-Joist is a critical component, however the depth of the I-Joist contributes the most when it comes to floor deflection or bounce when walking on the floor. You can install an I-Joist that meets all the criteria for bending and deflection, but feels bouncy when you walk. I-Joist manufactures have figured this out and now recommend deeper I-Joists in their joists tables in their published literature. To correct this bounce after the fact requires making the I-Joists stiffer. This can be accomplished by making the web thicker and/or making the bottom flange larger. Adding blocking will likely not solve your bounce issue. Shortening the I-Joist span by adding another support wall in the basement is another solution if it fits within your floor plan. Gluing and nailing a 2x4 or 2x6 to the bottom flange can be a solution that would stiffen the I-Joist, but that may require some engineering to get it right without too much guessing or trial-and-error.
Adding blocking will probably add squeaking that will increase over time.
I think that’s how Squeaky Fromme from the Manson family got her name. She was a Blockhead. 
Marcel, do you think nailing a block into top or bottom joists damages them?
Only if they don’t toenail properly and split the wood if the I joist is not LVL top and bottom.
