The recommended depth for attic insulation is usually 10 14 inches.
Insulation for water pipes in attic.
While pex does not ordinarily burst in the cold the water in the pipes can still freeze.
For other homes the pipes might be located in the walls or the roof rafters.
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.
Water inside the pipes can still freeze.
Ref insulate cold water pipes thoughtfully the frozen pipe was in the attic of the garage.
Shop pipe insulation here.
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.
Insulation will help maintain water temperature levels in the pipes.
For some homes the pipes will run underneath a subfloor or at the bottom of the attic.