Do you have a ceiling exhaust fan ? If so , is it venting into your
attic or out the roof or eave vent ? If there is a fan exhausting or
even leaking into the attic , your pumping warm humid air into a cold
attic causing moisture . That is a no no ! If this isn't the problem ,
you either have a roof leak ( possibly ice damming ) or your house is
so tight you have a moisture problem . Humidifiers can cause this if
the house can't breathe . Just a couple sugestions to start
investigating . Hope it helps .