Duccay has it right - that's why I posted a picture, so you wouldn't confused the make up air (thats why I referenced its name and posted a photo) with the cold air intake for furnace combustion. I obviously was too high level for your home reno level, I apologize if I caused confusion Komeskies.
Your (Komeskies) concern is likely that the pipe is allowing so much cold air in that the insulation is trapping the cold air in the main trunk in the bulkhead (ie doing too good a job, and you have wet cold air condensing in the trunk where the trunk becomes uninsulated). Closing the flap as I said, but not completely, as I also said, should help to reduce this problem. As Ducay said, it should be insulated, as that was code when your house was built.
Ducay's solution is a smart one - get an activated shutter for outside, so you only draw air when the furnace calls for it - it's easy to install and just requires 2 wires to the furnace to activate on Fan-call. Or get an HRV - they are a much better solution.
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