Attic Vapor Retarder
A vapor barrier or vapor diffusion retarder is a material that reduces the rate at which water vapor can move through a material.
Attic vapor retarder. 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 basics of air sealing are covered in this article. 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. The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below.
In summer when humidity is high it increases permeability so that moisture can escape as air tightness is maintained. 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. A plastic vapor barrier is best installed in a vented attic within climates with over 8 000 heating degree days. Attic insulation should always be installed with the paper backing facing toward the living space the ceiling in this case.
No you don t need a vapor barrier. Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days. If this is accomplished and a similar air infiltration retarder is installed in sidewalls mechanical ventilation such as a heat recovery ventilator should be installed to prevent trapping air pollutants and moisture within the house. This acts as a barrier to keep heated moist air from rising up into the attic during cold weather.
Adding a second vapor barrier could cause condensation to become trapped in the insulation between the two vapor barriers. What you need is an air barrier that is at every step of the way you have to pay attention to airtightness. If installed in an attic a continuous vapor retarder is usually used to reduce air infiltration. Product overview for extra protection against moisture the membrain continuous air barrier and smart vapor retarder is your weapon of choice.
Air sealing an attic.