So where is this “condensation” everyone is talking about?
Do you require dryer vents to be insulated? Why/Why not?
Are you thinking the cold vent condenses water inside the vent?
So warm moist air passes through the vent and because the moisture is against the cold vent, it will condense? Let’s consider “Heat Transfer”;
At what temperature will the air in the vent condense? You must know this. Do you?
What is the temperature of the vent when it’s operating?
Is this temperature below the dew point of the air inside the vent? You must know this.
Hot goes to cold, so the heat and moisture (heat) heats the duct above the outdoor air temperature which it is passing through. How do we condense when we are adding heat, not removing it?
Air movement: when you blow air across your sweaty body, does it not make you cool? This is because of evaporation. How can you have condensation when evaporation is occurring?
The air movement through the vent has the ability to remove any water inside the vent at any temperature where the air is not saturated. Water vapor has a lower relative humidity (wet bulb temperature) than water, which is at 100% Rh.
Are you thinking condensation will occur on the outside of the duct?
The outdoor air in the winter (when the vent is at its coldest) always has a lower quantity of moisture per pound of air, than indoor air (even if it is at 100% Rh). When the vent is not operating, it is the same, or higher temp than the outdoor air. It can not condense at this temperature, which is above that dew point temperature we discussed earlier.
Regardless of the Building Code you’re trying to enforce, if you do not have active moisture/damage, you have nothing to report.
There are times this occurs, but is not because there is no insulation on the vent. You need to fix the cause, not cover things up with insulation.
If you have an excessive low interior building pressure (a problem), leaking outdoor air through the building envelope (another problem), condensation can occur on numerous components near the exterior of the wall, the vent being one of those. Most of the time I find leaking vent fittings blowing onto the building envelope material, which has greater mass and lower conductivity than the vent. It will then condense under the right condition. If you insulated the vent, it might leak less, but the leak is still there (still the problem). You’re insulating the building envelope, not the vent in this case.