Aredwood:deanos: Not quite true. If you have a positive pressure system the intake in the roof would generally be dryer during the daytime. If you have a balanced system (heat exchanger), the temperature differencial between inside and out has a dehumidifying effect at the core
How does the roof space remove moisture from the air? I get that if you heat air up, the relative humidity goes down, due to warm air being able to hold more moisture than cold air. So if there is warmth in the roof space, that can have a drying effect.
And how does the heat exchanger remove humidity? Apart from warming the incoming air?
Your roof space is generally designed to have a bit of airflow so when it heats up the moisture can evapourate. Good reason not to fill the attic up with junk
As for heat exchangers, when you heat cool moist inflow air it will condensate at the core, lowering the humidity. Many models therefore have a condensate drain.