I can't quote you chapter and verse on the code in your respective area but as a Plumbing contractor, I have always gone with "no single wall pipe in unconditioned spaces".
In addition to the drafting issues, the flue gases from a gas appliance have a significant amount of moisture content. When these flue products contact cold, single wall pipe they condense. This condensate will cause damage to the metal parts of the flue system and on back to the furnace or water heater they are serving. (assuming they are properly graded) So...in the attic, under the house or exterior shed, I always call for B-vent replacement if single wall is found.