What kind of heat do you have. If you are using a non-venting gas appliance then it might be the culprit since they produce water vapor as a byproduct of their combustion. Or... If you have a gas furnace with an exterior exhaust then maybe it is blocked. Maybe a racoon made a nest in your chimney and all of these gasses and water vapor are coming into your house. If you have gas heat you need to have a service call to make sure it is venting correctly. This may be a longshot, but it could be the problem.