I'm installing a 6" flexible liner into my single flue masonry chimney with a 7X7 inside diameter clay flue. The clay thimble is round and has a 8 1/4 inside diameter. In the past I had a class A triple wall pipe inside the thimble (it just fit). I pushed it against the vertical flue on the chimney side and connected the stove pipe on the stove side. The interior wall was boxed in with block between the 24" studs where the thimble comes through the wall and about 8" above and below the thimble. It looks like the blocking around the thimble to the chimney was done after the sheathing was put on the house and the masonry block was put up against it. I never had a problem with this but can't say for sure how far the plywood goes. I was always glad to have the triple wall pipe inside the thimble but now with the flexile liner and the connection to it I will only have the 6" single wall pipe coming through the 8" inside diameter flue connecting the liner Tee to the stove pipe. Any advice on how to insulate inside the thimble or do I even have to. Thanks