My old house has a layer of tar paper between the finished siding and the sheathing. It was coated with liquid tar at every joint, all of which overlap. I plan to insulate, but wonder if the tar paper already constitutes a vapor barrier, in which case (I have learned) I would not want to put another vapor barrier on the inside, even if I could. In my climate (Wisconsin) the main concern is to stay warm in all but the summer months, so I would think a vapor barrier should be on the inside; but this is on the outside. Should it be removed? Even if I cannot put a vapor barrier behind the lath and plaster on the inside?