No Vapor Barrier In Attic

The paper facing contains a layer of impermeable asphalt that prevents water vapor from passing through it.

No vapor barrier in attic.

A vapor barrier in an attic assembly in a severely cold climate with the absence of an air barrier will likely be ineffective. Do not use poly for a vapor barrier. A third of a quart versus 30 quarts. Meanwhile air leakage through a 1 square inch hole in a sheet of drywall allows 30 quarts of water vapor to pass through the drywall under the same conditions.

Then cover the foam with drywall as required for fire safety. A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter. Improperly installed a vapor barrier may actually trap moisture inside a wall while a wall that is more porous can breathe effectively and be. 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.

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. Note that a poor effort at establishing a vapor barrier may be worse than no effort at all. The assembly must be able to dry in either direction should condensation occur. The goal of vapor barrier strategies is to prevent moisture from collecting and corrupting building materials.

After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one. 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. Vapor barrier vb insulation batt with paper facing should not be placed over any existing insulation creating a trap. With spray foam unlike with loose fill or batts you must cover the soffit vents they aren t needed to keep the roof cool.

Not every wall does. The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below. If used the vb is placed against the heated side in winter. The permeance of unpainted drywall is very high generally between 20 and 90 so it s not a vapor retarder at all.