Illustration Source: Keeping the Heat In,Fig.5-4, Pg.53

Insulation should not touch a chimney, a vent from a combustion appliance, knob and tube wiring in existing homes, or most recessed lighting.

Insulation must be a minimum of 3" away from a chimney. These areas may be framed differently; make sure to cut batts to fit properly. It may be necessary to use a layer of fire stop insulation to maintain the required separation. It is common to have an attic insulation shield installed around vent pipes that carry hot exhaust from combustion appliances.