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If that is a Bituminous Asphaltic type material I would be of the opinion that it is NOT SAFE. TAR is very flammable and will melt and drip if it catches, when fighting a fire that is very dangerous and will cause severe burns, possibly catching other things on fire, bad choice of material in my humble opinion. You need to look at the Manufacturers Design criteria for the exhaust vent, partcularly for through wall and floor penetrations. The vent pipe manufacturer may also be where to look for the right assembly to penetrate a combustible wall as it's connected to their pipe. When penetrating through a wall or floor, there is usually an annular space (open space) between the vent pipe and sleeve assembly, firestop ring or whatever is called for, to isolate and fireproof whatever type of wall you are penetrating from the actual vent pipe.
A typical non hot gas exhaust pipe penetrating a fire rated wall or floor, ie; domestic water supply must have a U.L. designed assembly that is equal to the rating of the floor or wall, for that floor or wall to actually be rated. Most assemblies have the annular space packed with a product like Roxul Mineral Wool, or Thermafiber, (I always preferred the first product, Roxul) which is UL rated, compressed to about 50%, then covered with at least 1/2" of Firestop material, (another UL rated material). The purpose of an assembly like this is to provide a proven and tested rating where there is an annular space around a penetration in a floor or wall, so that in the event of a fire, the entire surface of that floor is 100% rated as designed, 1 hour, 2 hour, 3 hour, which is usually called for by a particular code and occupancy classification, designed by an architect or engineer. There are also smoke seal and thousands of assemblies, reference material would be the U.L. fire resistance directory 2 volume set of orange books.
The condition you describe is unique to your building, and I would highly suggest that you speak to the manufacuturer(s) of the products used in regards to an acceptable and safe exhaust vent design through the floor and or walls you have. You should have the information available to you that either came with the unit or look it up online for the model of equipment you have, it's usually in PDF format, I have reviewed Modine's information before. There has to be specific venting information, whereas you can use the materials they call for to design what you need, including wall or floor penetrations. I just looked back at a job file and see the class B pipe called for to vent a unit heater, kind of similar application, looked into the supply house catalog, saw Firestop space rings to get that 1" clearance to combustibles where the pipe penetrates a wall or floor, the vent pipe manufacturer may also have specifications for clearances to combustibles for their pipe, it's not rocket science, take a look at what the criteria calls for and make the changes needed. I'd remove what you have and find the correct assembly to install, then you can really enjoy your heated shop in confidence, that tar is a real concern, whatever double work you may have to do, it is worth doing.
One of the reasons most of the supply houses will not sell to individuals is due to faulty installations by unqualified people, that can lead to serious consequences, apparently lawsuits are the basis of why they must approve of a contractor or other buyer, there is a lot of mark up in that business, I do buy this equipment direct for customers and either install it myself or use a qualified mechanic off hours as a side job. To start an account with these places, that was the first thing told to me, they don't want the liability anymore.
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