Kevin,

Code in my part of Minnesota now calls for a moisture barrier between
the insulation and the exterior wall. This is in addition to the
traditional moisture barrier between the insulation and the interior
wall. This is also the case in much of Canada.

You can just staple plastic sheeting to the studs.

It wouldn't hurt to see what your local code requires. (Even if you
aren't pulling permits.) My city has some of the more common
requirements on-line and the inspectors are usually pretty helpful.

dss