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External insulation of supply side plenum on hvac (located in attic)

Trying to identify the best/safest material for externally insulating the sheetmetal plenum. Found one entry ref. Using foil backed fiberglass also another using foil backed duck board. My concern is the temp. Of the metal plenum. Styrene duct board is rated to 400 F while the foil backed fiberglass I was looking at was rated to 250 F. What temp does a supply side plenum see during the winter?

What is the correct material to use?