Do You Need A Vapor Barrier Under Attic Insulation

Seal the air leakage pathways.

Do you need a vapor barrier under attic insulation.

The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below. Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets. The facing on faced insulation acts as a vapor retarder. If you need a vapor barrier and your insulation is unfaced you must cover it with a polyethylene film.

If you are not sure if you need insulation in your walls follow along with this flow chart. 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. What you need is an air barrier that is at every step of the way you have to pay attention to airtightness. The basics of air sealing are covered in this article.

In cold climates a vapor barrier is almost always needed. 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. No you don t need a vapor retarder class i or otherwise. With spray foam unlike with loose fill or batts you must cover the soffit vents they aren t needed to keep the roof cool.

This acts as a barrier to keep heated moist air from rising up into the attic during cold weather. Your first job after the studs are in place is to fill those walls with insulation. Air sealing an attic. In mixed climate areas the vapor barrier is optional depending on the total design of a building.

After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one. Lucy no you don t need a vapor barrier. 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. In any case the vapor barrier must point to the warm side.

Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days. 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. Then cover the foam with drywall as required for fire safety. 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.

In hot dry climates your attic can be vented though like in hot humid climates you should not install any vapor retarder or barrier. However all attics vented or unvented and in all climates should have an air barrier such as an airtight drywall ceiling.