Vented crawlspace insulation in coastal climate zone 3

I'm looking at ways I can partially insulate my crawlspace. The main motivating factor is to cut the way low rumbles from a major street resonates through the house via the crawlspace, with thermal properties being a nice extra benefit. I'm hesitant to go big on the thermal side because I don't want to cause moisture problems.

As a starting point, I am thinking of building some DIY baffles around the vents closets to the street with Rockwool in them, to capture the sound and redirect some of it. After that, I'll still have a mostly full bag of Rockwool, which I'm debating how to put to use. So I'm thinking that I can put some of the left over batts (3.5 inch) into the joists (7.5 inch), flush with the bottom instead of with the subfloor, and picking joist bays near the vent.

Since I'm near the coast and humidity is generally high normally, what's the likelyhood putting batts in the joist bays with spacing like that will cause me grief down the line?