Outdoor vent question

[buttaluv]

This might seem silly, but how many of you put something in your outdoor vent to block the cold air from coming in? I only use my stove when it gets really cold, and if I don't block it with something I get frost coming back in on the flue and can feel the cold air come it, I just have a standard end on it as well?

Also How clean is the flue required to be, I notice my horizontal pipe get some build up(very little) compared to my vertical pipe, I just vac'd out the horizontal and went on burning?

Thanks