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.