What are you actually trying to do ?
Insulation resistance is just resistance, ac or dc makes no difference, and neither does the actual test voltage. The result will be so many tens of Meg ohms, or whatever.
A high potential test (hi-pot) is different, it ensures that a circuit will not break down at a specified elevated voltage, but that really has nothing to do with insulation resistance. Exactly how to do the test will be laid out in the relevant standard you are trying to meet.
Then there is the non destructive method of measuring the actual breakdown voltage (ionisation test) which is different again.