Instead of sealing up the poles, I'd be far more inclined to box them in and allow ventilation of the chase. There'd be a two fold advantage. First, allowing airflow will prevent any possibility of your sealing job not being completely effective. Second, it prevents any possibility of moisture building up and beginning the possibility of rotting out. Just insulate around the interior faces of the boxed in portion, with 2+ inches of polyiso board. Place vents to the exterior at the bottom and top of the chase.

I wouldn't hesitate to live in a home done this way, even if the poles are creosote infused.

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