You have attic rain. 40%RH during a cold spell is way too high I thinkSo here is a problem I'm having with this issue right now. We bought this place 4 years ago. I inspected everything and the place was solid, no serious issues. We installed hardwood floors and my wife has serious sinus problems so I installed a whole house humidifier. We keep the RH about 40%. Last winter after a long really cold spell I noticed dripping from an odd place, like the roof was leaking. I crawl in the attic and it's raining in there, sopping wet mold everywhere. I panic but discover the 2 bathroom vents just vent to the attic space. Well I correct that with proper though the roof vents and think I'm good, it'll dry in the summer heat. Well I crawled up there 2 days ago now that's cold again to make sure it's still dry and to evaluate the mold and decide if I need to look at remediation. Well it's better but I still have too much moisture. I can see a drop of water on the ends of many nails. So the only thing I can think that is different since we moved in is the humidifier. So I'm back to the drawing board to figure out what to do.
That's only when the outdoor temperature is lower than 20-below.Oh I undeserved that but if everything is properly designed and functioning then there is no issue. If you think 15% RH in your house is ever good I whole heartedly disagree.