While pex does not ordinarily burst in the cold the water in the pipes can still freeze.
Insulation for water pipes in attic.
Process for weatherizing pipes in exposed areas of the home such as the garage basement and exterior walls.
If you have used a 14 inch flashing around the vent pipe it should be easy to insulate to this level while still maintaining a 4 inch gap at the top.
It is important to insulate pex pipe in an attic particularly in cold weather areas.
For other homes the pipes might be located in the walls or the roof rafters.
For some homes the pipes will run underneath a subfloor or at the bottom of the attic.
The drywall ceiling was insulated and the pipe was separately insulated even further as i say 360 degrees.
Find out where the pipes are located to see if a ladder is needed to reach them or if you will have to work on your knees installing the insulation.
Shop pipe insulation here.
Insulation will help maintain water temperature levels in the pipes.
Otherwise a cold water line run a long distance through a hot attic can take several.