I get where you’re coming from with the foam board—definitely more durable than cardboard. One thing I’d toss out there: sometimes the really cheap foam can off-gas or get brittle over time, especially in hot attics. I’ve seen folks use pre-formed polystyrene baffles for a bit more money, but they hold up better long-term. As for blow-in over old batts, just watch for any spots where the old insulation might’ve gotten damp—it can trap moisture and mess with airflow if it’s not bone dry. Balancing intake and exhaust is always a moving target, especially in places with wild weather swings... I’ve had to tweak vent setups a couple times after unexpected drafts cropped up.
You nailed it about the cheap foam—seen way too many folks cut corners there and regret it a couple summers later. I’ve had better luck with those polystyrene baffles too, especially for tricky rafter bays. And yeah, attic moisture’s sneaky... I’ve pulled out insulation that looked fine on top but was nasty underneath. Sometimes just getting up there with a flashlight and poking around makes a world of difference before adding more blow-in. Vent balancing is always a moving target, especially with unpredictable weather—sometimes you think you’ve got it dialed, then a freak windstorm proves you wrong.
Those baffles really do make a difference, especially in older houses where the rafters aren’t spaced perfectly. I’ve had to double up a few times just to get a decent channel for airflow. One thing I learned the hard way—if you’re using blow-in, make sure the soffit vents don’t get buried. I thought I was careful, but ended up with condensation issues after a cold snap. Sometimes it feels like you fix one thing and another pops up... but at least the attic’s not a sauna anymore.