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I wouldn't recommend fiberglass insulation, I'm not sure of the temp of the outside of the smoker and the safe temp limit. There is ceramic batt insulation, but more expensive and not very common to find in the stores.
Maybe something to insulate your HVAC plenum but you would need to read the labels.
I could have sworn that I saw an insulated cover for a smoker. With the colder weather coming, its a bit tough to keep a constant temp in the smoker. GI joes didn't have one.
Has anyone seen one, or know where I can get one?
If all else fails, I'll try to get my wife to make one.