A vapor barrier is a material that stops the flow of moisture. Moisture can decrease the effectiveness of insulation, loosen plaster, blister paint and lead to structural damage of your home.
Batt or blanket insulation can be purchased with an attached vapor barrier of asphalt-coated paper or aluminum foil. Other types of insulation should have a separate vapor barrier installed.
Vapor barriers must always be on the living space side of the insulation and should not be sandwiched between new and existing insulation. When adding insulation, never put a vapor barrier next to existing insulation. For example, if the insulation you are adding to an attic has a vapor barrier, remove it or slash it thoroughly before installing over existing insulation.
Adequate ventilation in attics is also important in protecting your home from excessive moisture. The location of vents will depend on your particular home.
Experts recommend at least on square foot of inlet vent and one square foot of outlet vent for each 300 square feet of attic floor area. However, this can be reduced by one-half if you install a vapor barrier beneath the attic insulation.
In unexcavated crawl space areas, install 4 mil or thicker plastic sheeting on the ground to reduce the amount of moisture entering the living space.