Do You Need A Vapor Barrier Under Attic Insulation

Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets.

Do you need a vapor barrier under attic insulation.

If you need a vapor barrier and your insulation is unfaced you must cover it with a polyethylene film. Air sealing an attic. By the time you paint the drywall though you ve brought it into the class iii vapor retarder range between 1 and 10 perms and even less water vapor will diffuse through. Seal the air leakage pathways.

However all attics vented or unvented and in all climates should have an air barrier such as an airtight drywall ceiling. What you need is an air barrier that is at every step of the way you have to pay attention to airtightness. In hot dry climates your attic can be vented though like in hot humid climates you should not install any vapor retarder or barrier. 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.

Your first job after the studs are in place is to fill those walls with insulation. 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 facing on faced insulation acts as a vapor retarder. Then cover the foam with drywall as required for fire safety.

The basics of air sealing are covered in this article. In cold climates a vapor barrier is almost always needed. After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one. The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below.

Adding a second vapor barrier could cause condensation to become trapped in the insulation between the two vapor barriers. Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days. In mixed climate areas the vapor barrier is optional depending on the total design of a building. Lucy no you don t need a vapor barrier.

Attic insulation should always be installed with the paper backing facing toward the living space the ceiling in this case. 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. 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.

You can forego the plastic and use a vapor retarder kraft faced insulation or latex ceiling paint in all other climates except hot humid or hot dry climates. This acts as a barrier to keep heated moist air from rising up into the attic during cold weather.