I've got a 2 family house built in 1900. We had this same thing happen in our upstairs apartment the year that we got the house insulated with blown-in insulation. Our tenant told us that his apartment was crying. We went up to look and the walls and windows were dripping. We talked to a couple contractors and they all told us that it's fairly common in older houses when they're "over-insulated", and they all advised us to get a dehumidifier. We got our tenant a dehumidifier and our tenant says he has no more issues with it.