A while back on an inspection I called out that there should not be insulation packed around the B-vent pipe in the attic.  Some builders think that it is only combustible materials that need to be kept away from the pipe, but the manufacturers want not just "combustible" materials kept away but also materials that could "conduct" heat to combustible materials.  This explains why rockwool insulation (which is typically not considered combustible and is often used for fire-stopping) is not supposed to be packed against the pipe---it could conduct heat to the nearby framing.  All manufacturers of B-vent type vent pipe that I am familiar with have either:  "1 inch minimum to combustibles" or "1 inch minimum to insulation" stamped right on each length of pipe.  Some types of insulation, when wet (perhaps from roof leaks of condensate from the furnace exhaust), can become corrosive to the metal pipe.  The bottom line is that b-vent chimneys need a minimum of 1" clearance from insulation and combustible materials.

The picture on the left shows insulation up against the pipe with rusting apparent where the insulation covers the pipe.  A leaking flashing kept the insulation wet where it eventually rusted the pipe.

The picture on the right was taken two weeks later after "repairs" were made (receipt and all).

     In this next picture there wasn't room to run the B-vent through the space without touching the wood framing so it was changed to single wall pipe (which itself is touching the wood) and then back to B-vent pipe.  The problem with this is that the single wall pipe would require 6" of clearance and you are not supposed to change from one type of pipe to another along the length of the B-vent pipe.

A proper vent with baffle looks more like this:

Charles Buell