Do You Need A Vapor Barrier Under Attic Insulation

Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days.

Do you need a vapor barrier under attic insulation.

Air sealing an attic. After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one. If you need a vapor barrier and your insulation is unfaced you must cover it with a polyethylene film. The basics of air sealing are covered in this article.

Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets. Then cover the foam with drywall as required for fire safety. 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. In hot dry climates your attic can be vented though like in hot humid climates you should not install any vapor retarder or barrier.

This acts as a barrier to keep heated moist air from rising up into the attic during cold weather. However all attics vented or unvented and in all climates should have an air barrier such as an airtight drywall ceiling. The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below. 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 facing on faced insulation acts as a vapor retarder. Adding a second vapor barrier could cause condensation to become trapped in the insulation between the two vapor barriers. In mixed climate areas the vapor barrier is optional depending on the total design of a building. 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.

Lucy no you don t need a vapor barrier. 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. Seal the air leakage pathways. In any case the vapor barrier must point to the warm side.

What you need is an air barrier that is at every step of the way you have to pay attention to airtightness. In cold climates a vapor barrier is almost always needed. No you don t need a vapor retarder class i or otherwise. Attic insulation should always be installed with the paper backing facing toward the living space the ceiling in this case.

Your first job after the studs are in place is to fill those walls with insulation. Not every wall does.