The problem is when the Ambient Air DewPoint is higher than the temperature inside the enclosure.
Water vapor will condense on the surfaces inside, the ventilation wouldn't help in this case.
You can graph the projected 48-hour DewPoint from a weather forecast.
If your enclosure temp is ever Lower than the Ambient DewPoint, water will condense inside your enclosure if outside air is brought in.
But even without ventilation (mostly sealed enclosure), the air inside still contains a specific amount of moisture that will condense when cooled to a specific temperature.
I generally try to seal the enclosure, insulate to reduce the Maximum Enclosure Temperature, add desiccant to absorb the moisture, and use a Silicone conformal coating on the MCU and Connections for severe duty projects.
I've found that eventually the enclosure's seal will "roll" if the pressure differential gets high enough from temperature swings.
After that, the Conformal Coating is the last line of protection after the desiccant is saturated.
I haven't personally had a failure with a sealed enclosure using all 3 (insulation, desiccant, and conformal coating), but I have had failures when only 2 were used.
I'm in the SE USA and we can easily have 50 degree F temperature swings in 24 hours in late Spring.