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Tar paper vapor barrier?

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?

Re: Tar paper vapor barrier?

Your tar ( or felt ) paper is not considered a vapor barrier.

Materials having a permeable rating of 1 or lower is considered a water vapor barrier .... tar ( or felt ) paper has a permeable rating of 5 or higher which means the interior vapor within the walls will breathe outward.

It's intended purpose is provide a barrier to protect the sheathing from liquid water also to assist with being an air barrier.

You might consider using blown in cellulose which works fine for your application.

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