It sticks to almost everything, even when the materials are dusty. Later this flue will get a cylinder of aluminum around it to keep the insulation from ever touching it. Large rectangular chimneys are easy to seal with simple angles of sheet metal when the ceiling joists are exposed. The same approach is used to cut, fit, fasten, and then seal. But in this case, the patch will also serve to protect the insulation from concealed flaws in the flue.
Sparks jumping out of a chimney are obviously a threat to the house.
I don't ever want someone to blame me and my insulation for their attic fire. Click on image for larger view Many old houses have attic floors and they hide bypasses. We can fix most of those holes without ever seeing them by Dense Packing the floor bays. This is easy to do by removing a course of floorboards down the middle of the attic. We never dare pack cellulose into the chimney chase, so again, a sheet metal treatment is called for.