Do You Need A Vapor Barrier Under Attic Insulation

Seal the air leakage pathways.

Do you need a vapor barrier under attic insulation.

No you don t need a vapor retarder class i or otherwise. However all attics vented or unvented and in all climates should have an air barrier such as an airtight drywall ceiling. In hot dry climates your attic can be vented though like in hot humid climates you should not install any vapor retarder or barrier. If you need a vapor barrier and your insulation is unfaced you must cover it with a polyethylene film.

With spray foam unlike with loose fill or batts you must cover the soffit vents they aren t needed to keep the roof cool. The basics of air sealing are covered in this article. The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below. Then cover the foam with drywall as required for fire safety.

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. Your first job after the studs are in place is to fill those walls with insulation. 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. 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.

After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one. Adding a second vapor barrier could cause condensation to become trapped in the insulation between the two vapor barriers. Lucy no you don t need a vapor barrier. What you need is an air barrier that is at every step of the way you have to pay attention to airtightness.

Air sealing an attic. In mixed climate areas the vapor barrier is optional depending on the total design of a building. If you are not sure if you need insulation in your walls follow along with this flow chart. In cold climates a vapor barrier is almost always needed.

This acts as a barrier to keep heated moist air from rising up into the attic during cold weather. 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. The facing on faced insulation acts as a vapor retarder. In any case the vapor barrier must point to the warm side.

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