Drying to the inside question
On homes with rigid foam on the outside of the walls, I know the house has to dry to the inside, so just latex paint or possibly a “smart” vapor barrier would be used on the interior drywall walls. My question is regarding the ceiling. I would probably be using blown-in celulose or similar (just a “normal” attic with ridge-vents and vents in soffits, etc), so would I want to poly the ceiling to keep the moisture out of the insulation? Can the house still dry to inside with poly in the ceiling?
Build will be right on the border of Zones 6 and 7 in Central MN.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
In zone 7 adding a vapor retarder might be cheap insurance, but it's not of particular value in zone 6, so you're a bit on the edge. Air tightness of the ceiling is still the most-critical aspect to keeping the insulation dry.
Using half-perm "vapor barrier latex" primer on the ceiling would be preferable to 6-mil poly. With poly vapor barriers in the ceiling in an air-conditioned house you can sometimes run into condensation and wet insulation on the bottom layers during the cooling season, but with half-perm paint it's far less likely to happen. Half-perm paint is plenty vapor retardent for managing wintertime vapor diffusion into cold insulation in a zone 7 winter with a vented attic, no need to drop that to 0.05 perm polyethylene.
Dana is correct. Pay attention to airtightness, and skip the ceiling polyethylene.
Thank you gentlemen...