Polyiso IS rigid foam, foam that has a higher service operating temp than CPVC vent pipe. The same can't be said for EPS bead-board, or XPS (pink, blue, green, whatever.)
Rigid polyiso is good to about 250F.
CPVC is good to about 200F.
Expanded or extruded polystyrene is only good to about 165F.
Spray polyurethane is good to about 175F.
It's fine to put the iso next to the vent- if it's hot enough to degrade the iso the PVC venting is already melted. (And even when it's fully aflame polyiso doesn't melt- it chars.) EPS/XPS, hard to say- it can be degraded by extended exposure to temperatures well below the max operating temp for the CPVC, but there has to be something seriously amiss for the vent to reach the service temp limit of EPS or XPS. But if in doubt, use only polyiso next to the vent pipe, since it can take the heat better than the vent itself.