Looking for a chimney enclosure solution before insulating attic
From the photos, the stack is class B venting.  You local big box store should be able to supply you with a part called "attic insulation shield".  It's just another pipe 2" larger in diameter than your stack &amp; the top gets sealed with a storm collar to fit.  Seal with hi-temp silicone and insulation to your hearts content.  Stacks that service a zero clearance appliances are not zero clearance as well - only the appliance is.