Vapor Barriers (also called diffusion retarders) are a way of sealing in toxic or compromising materials so that no offgassing or escaping of particulates can enter into living spaces. They also give some protection against molds and mildews.

Vapor barriers for insulation are impermeable materials that come in stainless steel, aluminum foil, reinforced aluminum or foil-coated paper in varying weights. Just remember that there are also permeable barriers, which are lightly perforated to allow moisture to pass through. These permeable barriers, generally referred to as reflective-foil or radiant barriers, serve a different purpose and are not useful as vapor barriers.
Foil backed batt insulation, kraft paper sandwiched between aluminum foil and aluminum builders foil are all good barriers. Foils should face the interior wall, extend over the studs and be taped with adhesive-backed foil tape to create continuous protection. Moisture needs a place to escape toward the exterior wall to prevent any potential mold problems. This is the side where the permeable barrier facing can be used.
Plastic reinforced aluminum is considered safe, non-toxic and effective for most chemically sensitive people.
Sealants are another choice. One type is an impermeable and relatively benign plastic which can be sprayed, rolled or brushed over any structural surface to seal in any offgassing VOCs.
Most vapor barriers contain recycled contents of paper, plastic and/or aluminum.