Adding a link to a picture of the problem.

Here is a link that might be useful: Picture of Issue

you wouldn't want to sandwich the insulation between
two vapor barriers.
finding out what the vapor barrier location is
for Oaklahoma would be the first step.
I think they put theirs to the interior
(whereas we put ours to exterior)

also the rest of the walls may be uninsulated also.
just how we built in the 70's.

as I've worked on my house I've added insulation
to the walls I've opened. does it help...but not
as much as the air sealing has.

best of luck

energy rater is correct, vapour barrier goes on the warm side. You don't want one on each side as that will trap moisture, so I would do as you mentioned but without the second vapour barrier. Heavy plastic is not necessary, just 6 mil polyethelene.

Also, tar paper is a semi permeable vapour barrier and a good air barrier, but not as good as the plastic.

Thanks, energy rater and shazapple. Really appreciate the advice. I'll advise him to go with the polyethelene on the interior side only.

Cheers!

Most of Oklahoma is a mixed-humid zone; the panhandle is in a hot humid zone, so an interior vapour barrier is not required. See BSC-Info 310.

For more on vapor barriers see link below.

Here is a link that might be useful: Lstiburek: Understanding Vapor Barriers

Sorry for the inadvertent imperialism. The map above shows North American climate zones. The linked US Department of Energy map is of the US only.