Proper venting of a garage heater

[x071907]

I am in the process of installing a Modine HD75 Hot Dawg heater in my attached garage and I have a question about the power exhaust and the proper vent pipe to use. I'm looking at the installation instructions for the unit and I am trying to figure out if I can use 5" Metalbestos type B double wall vent pipe with the heater. If the unit has a built-in power exhaust does this mean that I need to make the vent pipe air tight (positive vent pressure, category III)? The instructions state that that all vertically vented heaters are category I venting (negative vent pressure). I plan on venting the unit vertically through the roof of the garage. If the vent pipe has to be air tight is it possible to use the metalbestos double wall pipe and somehow seal the joints?

[Perfer no to say]

You must tape the joints with metal tape. I have a Hamilton Home unit and they say to seal the double wall joint in the pipe.