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>Why would it be humid?

The hotter air is, the higher its capacity to hold water vapor.

https://en.wikipedia.org/wiki/Psychrometrics

https://en.wikipedia.org/wiki/File:PsychrometricChart.SeaLev...

Which actually drop relative humidity, because water does not magically appear.

Yes, but the absolute humidity is the amount of water in air, and particularly n cold climates, this water load can cause considerable trouble in structures. The correct way to design airflows and location of dew point inside the walls etc can be considerable trouble; ad hoc design tends to lead to mildew problems.

Dry cold air gets in during the winter, it expands and the RH goes down, warm humid air escapes, more dry cold air gets in, creating an extremely dry environment. Still you can get mold because of condensation when warm air meets cold rooftop.

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