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Can Fiberglass Insulation Burn?
I know, it's the age-old question:  can fiberglass insulation burn?
Not really, but there is a caution! 
There are many kinds of chimney vents - masonry and metal. 
One particular type is called a "Type B vent."  A Type B vent utilizes two different tubes - an interior tube which carries the heat and a surrounding tube which does not touch the interior tube. 
The separation allows for the dissipation of heat so that what outer ring touches would not get too warm.
It is a safety feature.  This Type B vent is used for gas appliances (furnaces and fireplaces), not solid-fuel appliances like wood or coal.
However, the International Residency Code requires a separation between the outer ring and anything it might touch inside a house.
Why?  If the outer ring is cooler what could be the harm of it touching anything?  And especially something that is not considered combustible, like fiberglass insulation?
From the IRC:
G2425.4 Insulation Shield.
Where vents pass through insulated assemblies, an insulation shield constructed of not less than 26 gage sheet metal shall be installed to provide clearance between the vent and the insulation material.  The clearance shall not be less than the clearance ... more

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