Wow, a lot of bad information here. You cant really go to school for home building so information gets passed down...

You want your thermal barrier and air barrier on the same surface. Previously that was the attic floor, now it is the attic ceiling.

The heat from the whole house is rising up through the vents, and that is pulling cold air in down at the low points.

You want to seal the vents. It might not be a bad idea to install a fan to the outside up there that can be sealed or a bathroom exhaust fan. Your house sounds unlikely to become too airtight, but if it did the answer is controllable mechanical ventilation.