Tom, The installer didn't cut anything when he installed the stove. He simply slid the 1/2 liner into the damper and up into the existing 10" metal liner already in the chimney. The he stuffed the damper area with insulation. I tried stuffing the outside vent with fiberglass insulation and it made no difference. I also tried incense into the vent and no detectable smell came into the house. I will try to seal up the outside vent better with ceramic blanket insulation.