B-vents are commonly employed at natural gas or propane appliances including water heaters, furnaces, boilers and vented gas fireplaces or freestanding stoves. As often as not, attic insulation will be packed tight around B-vent(s). Fact is, there should be a minimum of 1" space between any B-vent and insulation or potentially flammable materials. This attic photo provides a really good example of a really bad installation. Recommended clearance requirements have been ignored.
B-vents can generate 300 degrees F when appliances are operating. Granted, most modern insulation is fire-resistant at normal in use temperatures, excluding perhaps extreme heat produced during house fires. But if clearances are insufficient, a metal B-vent will not cool properly and it can transfer heat to nearby materials that will burn. Also, vapor retarder backings, when present on insulation, may not be as fire-resistant as the insulation material itself.
Spacing of 1", or in some instances as much as 2", between insulation and B-vents is a standard manufacturers' requirement and the matter has been addressed in prescriptive codes. That said, many people who install attic (and sometimes crawl space) insulation are not aware of this important detail or they do not care. At least half of the time, even at new homes, the insulation will be up against the B-vent.
Good news: Every once in awhile, the inspector will find a nice installation detail -- an approved insulation shield (metal collar) around the B-vent. Here is a photo of one job where the insulation installers and/or the HVAC professional had their respective acts together.