Air tight seal around anything that gets cold enough to condensate.
Condensation happens when moisture in the air we breath, condensates onto a cold surface. Like when you remove a coke can from the fridge.
If you can seal off the cold parts from air, then no moisture can form.
The thickness of your insulation depends on how cold it is, it must be thick enough to stop heat transferring through or condensation will form on the outside of the insulation.
Read some
stickies, there are heaps, start with
this one.