If the insulation is for heating purposes, use the minimum ambient temp, especially if freezing is a concern. If it is for cooling, use the maximum ambient.
Suggest approaching this by looking at the required plant availability.
Say the annual plant downtime is broken up into 5days unplanned shutdown, 10days annual maintenance. Of these 5days, we may assign say 4days for equipment breakdowns and other operating incidents, and leave one day for plant shutdown due to low ambient temp - excessive heat loss througn insulation.
1 day out of 365 = 0.27%
Then look at the tabulated data for daily min temp over the last 20years say, and find the daily min temp, below which the cumulative annual occurence is less than or equal to 0.27%.
It must be celarly shown that this excessive heat loss does lead to a plant shutdown due to high pressure drop, low operating temp or some other abnormality.