Maybe your shop is too well insulated combined with no or poor ventilation.
Warm inside air can hold lots more moisture...mixing it with outside (colder) air
can often lower the inside humidity plus shift where the dew point is reached.
Sounds kinda silly, but try opening the doors every few days.
I live near an airport and have a
sound abated/super insulated house.
To help prevent humidity problems, the house has an automatic vent system
that recirculates the inside and outside air.
...that's my shot in the dark... 