Insulating around heater vent

[truckermatt]

My natural gas heater goes through the wall with cat iii pipe single wall. I have a thimble through wall. Can I put fiberglass insulation next to the thimble. I figure that would help ruduce condensation

 

[Slowgsr]

I used roxul as its fireproof.
Mine was a portion of B vent that went through a thimble, the rest of my venting was c vent.

This was for a radiant heater