I was reading a piece on heath.com and it said NFPA mentions something about using an insulated double wall stove pipe as a thimble. Here is the reading. Would this be up to code if I had the proper amount of air space? My masonry chimney is on an exterior wall. covered in plywood. The old thimble was clay and had a 2 inch clearance to the plywood, which I have been told is crazy and unsafe.. I cut out as my much plywood as I could and the width of the chimney is only 15 5/8'' wide. If I cut any further ill be cutting into my siding of my house. The heat fab saf-t thimble which is meant to go through a combustible wall says that the clearance needs to 16''wx17''L...however I don't have that much room. The insul flue mentioned in this writing is the same. I just don't have enough clearance and I don't know what to do https://www.hearth.com/econtent/index.php/articles/passing_a_chimney
"An alternate method is also mentioned in the NFPA guide. In this case, a section of insulated, double wall chimney is used instead of the crock…and an airspace is used around it instead of the solid brick wall patch. This involves a 9” airspace, so the total rough framing would be 26” square, and once again cement board could be used to bring the wall surface with up to the insulated pipe."
"An alternate method is also mentioned in the NFPA guide. In this case, a section of insulated, double wall chimney is used instead of the crock…and an airspace is used around it instead of the solid brick wall patch. This involves a 9” airspace, so the total rough framing would be 26” square, and once again cement board could be used to bring the wall surface with up to the insulated pipe."
