[Greenbuilding] vapor retarder/barrier?

[Bruce Donelson]

Lynn wrote:
This is our vapor profile:
I hour NW of Portland, Oregon-700' el.
6" insulated concrete mono-slab
Hardiplank siding/building paper/ext. ply/R21 fiberglass batts
Attic: vented birdboxes/ridge vent/R38 insulation
Heat: wood stove w/electric baseboard backup (code)
2-storey House

Our bldg. Inspector suggests we use 6 mil plastic on the inside exterior
walls & no retarder/barrier on the ceiling.  This doesn't sound right to
us, although we have no real knowledge.  We have been to the building
science site but are really none the wiser.  We would really appreciate
some advice.
Many thanks,
Lynn

Sounds just backwards to me. Your R-21 fiberglass walls most likely have
kraft paper facing on them, so that if you put plastic over that any trapped
or infiltrated moisture could not easily escape. This would promote rot on
wooden surfaces. Inspectors here in southern Oregon won't allow us to do
that. They do require that we put plastic on the ceilings under R-38 batts.
Doesn't seem like it would make much difference to me. The biggest thing is
to be sure that all penetrations are securely sealed where they pass through
the bottom floor or the ceiling. So foam and/or caulk around pipes, wires,
electrical boxes, etc. That will do the most to keep moisture-laden air from
getting into the attic or into wall cavities, where it can reach its dew
point and condense.
Brtuce Donelson