A.Joe Lstiburek responds: To heck with the experts — here’s my answer. Plastic vapor barriers should only be installed in vented attics in climates with more than 8,000 heating degree days. You can forego the plastic and use a vapor retarder (kraft-faced insulation or latex ceiling paint) in all other climates except hot-humid or hot-dry climates. In hot-humid climates, attics should not be vented and vapor retarders should not be installed on the interior of assemblies.
In hot-dry climates a vapor retarder should also not be installed, but attics can be vented. All attics — vented or unvented — should have an air barrier (a properly detailed airtight drywall ceiling, for example) regardless of climate.