Furnace stack frost

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I have recently started getting frost build up on the outside of my cinder block furnace stack and roof in the attic. This appears to have started after house was wrapped and insulated in foam /along with new windows. This happens during the very cold days when the furnace runs more than usual. The inside of my stack is clear. But on the upper stack in and above the insulation gets frosted up along with the attic roof (I tried a fix last year by pulling back the blown in insulation and filling the voids around the stack. I wrapping the stack on all 4 sides with foam insulation from the ceiling boards to just short of a foot of the roof line - with no success this year). And then when warmed up by the lower air in house melts and you get the smell of wet wood that is around and below the insulation. And in the one area that I can see the lower cinder blocks they are wet from the run down. Whats the solution I can use to solve this problem,is it better ventilation / insulation or installing a tin stack inside the old stack.