Ok. We got called out for heavy smoke coming from someone's chimney last night. The resident said his wife was burning duralogs all week, he got home, threw one in, then threw 3 pieces of wood on top. (Older osburn stove i think). Said he saw thick black smoke pooring out of his chimney, so thought he had a chim. fire. We got there, and the smoke was coming out very lazy, and not black. I took the insert face plate off, and checked the stove, pipe, and liner for temps with the thermal imaging camera, and everything was in spec, if not lower. Opened the door, and a massive amount of smoke came out, with 0 draft. Dropped chains down the liner from the roof to make sure nothing was clogged, and all was good. I started to get a bit nosey, so i started to take out the upper bricks above the secondary tubes, and on top of the bricks, was 2 inches of rock wool insulation. The whole inside top of the insert was insulated, blocking the 6" flue. DIRTY insulation. Is this normal in older stoves? Maybe something to help with the secondaries? I took out all the insulation, put the bricks back and lit a piece of paper and threw it in the stove, and the draft seemed to be fine?