Welcome, I will do my best to help you with your issue
What type of vent piping do you have? Since you mention both gas water heater and furnace connected to the same vent I would assume you have double wall Type B vent piping
I believe that is what I have, it's double wall with like a small-ish 4 in outside diameter, metal.
The roof piece / flashing is like aXXXXXof metal where the piping protrudes through the roof line.
Ok, what I would suggest is wrapping the vent pipe with un-faced fiberglass insulation. Also carry the insulation up to the roof collar so you isolate the metal collar from the air in the attic.
It also seems like you have a moisture issue in your attic with too much moisture from the living space finding its way into the attic
Interesting...and that last point would be caused by not having enough regular insulation in my ceiling?
It's from having too many penetrations in the vapor barrier at the ceiling or no vapor barrier. I would seal any penetrations from the living space into the attic and be sure to run the exhaust fan when you take a shower
Would it be fair to say that on a cold day that my air, in my attic, should be much closer to the temperature of the outside than to the temperature of my house?
I don't have an attic vapor barrier, like I have in say my crawl space where the entire floor is line with black 6 mi sheeting. Are you saying the attic should be lined in the same way?
Yes if attic is adequately vented and you have sufficient insulation in the ceiling but a realtively small difference in temp can cause condensation
Ideally there should be a vapor barrier at the ceiling. But standards were lax in the past and if there was a bbarrier they typically relied on the facing of the insulation
Current standards are much tougher and need to be since current construction is much tighter
so should I purchase another 6 mil black vapor barrier and try to line my attic, along with the other things you suggested?
BTW clearance to combustibles for Type B vent is 1". Unfaced fuberglass is not combustible
It's really tough to install a vapor barrier after the fact since it should be right behind the wall board but you can plug up leaks around pipes and other penetrations including the attic access
And don't forget to run the exhaust fan. Most of the moisture comes from showering
So unfaced fiberglass, just the normal pink or white stuff, say like r-13 and wrap up that pipe with it and that flashing in the attic?
Yes but it is crucial that it is unfaced.
OK...thanks. I'll accept with an excellent. Just to be clear, you don't see any advantage in taking 6 mil sheeting and stapling it to the top of the attic joists above the insulation?
Just wouldn't do anything I guess?
You don't want to put the vapor barrier on top of the joists it could trap moisture below the barrier. The poly belongs on the bottom of the joists. Just plug up what holes you can with expanding foam and don't forget the attic access (weather stripping is advised).