The radiator is vented to the overflow. The overflow is not vented. "Most" overflow tanks are not completely sealed. Their caps fit loosely, so they are not completely sealed and they do vent, even if not designed to do so. There are some aftermarket tanks like the little plastic ones Summit sells with the drain valve on the bottom are not vented at all but are completely sealed, but the radiator vents TO the overflow tank, so the radiator in those cases is vented, just not the overflow.
