Metal B-vents are commonly employed at natural gas or propane appliances including furnaces, water heaters and boilers. Direct vent gas fireplaces are usually vented through metal coaxial concentric vents. As often as not, attic insulation will be packed tight around metal vents. In practice, there should be a minimum of 1" space between exhaust vents and insulation or potential flammables. This attic photo is a good example of a bad job. The recommended clearance requirements have been ignored.
Some vents such as common B-vents can generate 300 degrees F when appliances are operating. Granted, modern insulation is fire-resistant at normal in-use temperatures, excluding extreme heat produced during house fires. But if clearances are insufficient, a metal vent may not cool properly and heat could be transferred to nearby combustibles. Also, vapor retarder backings, when present on insulation, may be less fire-resistant than the insulation itself.
Spacing of 1", or in some instances as much as 2", between insulation and B-vents is typically recommended by manufacturers and backed-up by prescriptive codes. (Direct vents, typically used at gas fireplaces with sealed glass panels at the front, may operate at cooler temperatures but they should be similarly separated from insulation and combustibles.) For some reason, those individuals who install attic (and sometimes crawl space) insulation are not aware of the importance of maintaining proper clearances, or else they do not care. Even at new homes, as often as not, insulation will be tight to the metal vents.
Good news: Every once in awhile, the inspector will find a nice installation detail -- an approved insulation shield (metal collar) around a metal vent. Here is a photo of one job where the insulation installers and/or the HVAC professional had their respective acts together and put this shield on a furnace B-vent.