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.

No vapor barrier in attic.

A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter. A third of a quart versus 30 quarts. A plastic vapor barrier is best installed in a vented attic within climates with over 8 000 heating degree days. Then cover the foam with drywall as required for fire safety.

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. Do not use poly for a vapor barrier. Not every wall does. Hi beth 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.

Improperly installed a vapor barrier may actually trap moisture inside a wall while a wall that is more porous can breathe effectively and be. 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. The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below. The paper facing contains a layer of impermeable asphalt that prevents water vapor from passing through it.

The permeance of unpainted drywall is very high generally between 20 and 90 so it s not a vapor retarder at all. If your climate does not meet these conditions a vapor retarder like latex ceiling paint will work well for your house if you are in all other climates except the hot dry and hot humid. 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.

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. 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. If used the vb is placed against the heated side in winter. After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one.