No Vapour Barrier In Attic

On the other hand an air barrier a properly detailed air tight drywall ceiling for example in the absence of a vapor barrier can be effective since it stops the flow of vapor laden air.

No vapour barrier in attic.

Because it s nearly impossible to seal penetrations in the ceiling the reality is water vapor will get into the attic. Water vapor can pass through building materials in several ways including direct transmission and by heat transfer but studies suggest that fully 98 percent of the moisture transfer through walls occurs through air gaps including cracks around electrical fixtures and outlets and gaps along baseboards. Even though we apply vapor barriers to exterior insulated walls in cold climates it s no longer recommended for ceilings floor of the attic. To stop the movement of vapor effectively through air leaks your vapor barrier should also have an air barrier such as an airtight.

Thus installing vapor barriers on wall surfaces must. Do not add a vapor barrier on top of the insulation in your attic since the paper facing on the existing insulation is the vapor barrier. A vapor barrier in an attic assembly in a severely cold climate with the absence of an air barrier will likely be ineffective. The science of moisture movement.

By the time you paint the drywall though you. The paper facing contains a layer of impermeable asphalt that prevents water vapor from passing through it. Returning to the original question about whether you need a class i vapor retarder i e a vapor barrier if you re getting almost no water vapor diffusing through something that s not a vapor retarder at all i think the answer is clear.