Experts say that if you are putting in a second layer of insulation you should have no vapor barrier between the two layers.
No vapor barrier in attic insulation.
Not every wall does.
So for this top layer you would be purchasing unfaced insulation batts not rolls of an r value of at least 30.
The paper facing contains a layer of impermeable asphalt that prevents water vapor from passing through it.
The best approach for a vented attic in a cold climate is installing a layer of drywall with a good coat of latex paint the paint creates a semi permeable vapor barrier.
The heat being lost from the house below warmed up the attic so that moisture was lost through the vents as vapor.
Foam blocks airflow needs no vapor barrier and has a higher r value per inch than loose fill or batts so you ll get more protection with less depth.
A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter.
With spray foam unlike with loose fill or batts you must cover the soffit vents they aren t needed to keep the roof cool.
A vapor barrier in an attic assembly in a severely cold climate with the absence of an air barrier will likely be ineffective.
Then cover the foam with drywall as required for fire safety.
The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below.
Fiberglass insulation must stay dry to be effective.
Vapor barrier up or down.
This barrier is meant to keep moisture from getting to the insulation in the walls and ceilings and it is required by building codes when insulating most houses.
After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one.
Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets.