Ideally insulation resistance measurement on ship electrical machine is while the machine is hot?
Insulation becomes more leaky (its IR value falls) at high temperatures.
So testing while hot shows the realistic IR value at, or near, its working temperature.
Insulation resistance can vary considerably with changing atmospheric conditions. A single reading gives little information. However, the regular recording of test results may show a downward trend which indicates impending trouble which can be remedied by preventive maintenance.