I think I know where you mind is at about the clearance to combustibles issue in regard to a chimney. If I understand the present IRC requirements, combustible framing and sheathing are allowed to abut a brick chimney provided that::::: at least 12 inches distance exists between the inner flue and the point where the combustible material abuts the exterior portions of the brick. If this distance is less the 12”, and the chimney abuts a wall which is facing the interior portion of the home, there should be a 2” air gap, and on the exterior this gap may be 1.” During a home inspection, it can in some instances be difficult to determine if this distance (12” gap) is correct. If you are not very sure, or feel uncomfortable with your gut instinct, defer it to a mason to assess and evaluate.
Chimney fire blocking must be installed in the air gap to prevent fire spread directly up into the attic or next story. This fire blocking can be non-flammable insulation, or as is standard practice here in central Texas, mortar is commonly used to fill that gap (however I have a few misgivings about that practice) Mainly because I feel that mortar can conduct heat, and transfer it to combustibles better then insulation will… I make a note, and take a photo of the material used to construct the fire block on my report if the home has a chimney.
I hope I have this figured out correctly, and I hope my info helps.