[LazyPup]

Your personal information lists your location as Hazelton. Pa, therefore you are under the IRC (International Residential Code).

Under the IRC all structures must have one "main vent" which runs undiminished in size from the building "main drain" through the roof. Once that is achieved auxiliary vents may be reduced to 1/2 the diameter of the line they serve, but not less than 1-1/4" however, in regions subject to frost all vents must be increased to 3" at least 1' inside the structure before passing through the roof. (Some local codes require 4").

The vent must rise 6" above the roof, when measured on the high side of the roof pitch, except, in regions subject to frost, the vent must rise a minimum of 6" plus the local average snowfall depth. (your local code office can tell you what their local snowfall depth figure is).

Years ago when heating fuel was cheap and attic insulation was almost non-existent attics stayed relatively warm by comparison to the outdoor temperatures and the one foot rule was effective but in today's world, with thick attic insulation even increasing the size one foot inside the structure is often not enough. I prefer to increase the size immediately after the vent enters the attic space as close to the attic floor as is practical.

Adding insulation to a vent pipe after it passes through the roof would be a total waste of time, money and effort because insulation does not generate heat, it only retards the movement of thermal energy from one space to another. Understanding that in normal operation air not only passes out through a vent, it also passes in through a vent, therefore the interior of a vent pipe is at the same temperature as the outside ambient air.

Increasing the size of the vent pipe above the roof line as you suggested would also prove to be a totally wasted effort.

Visually check the roof penetration hole in your attic space. In many cases the opening in the roof decking is already 4" or even 6" and the excess space is covered by the pipe flashing,