Vapor barriers or vapor retarders are defined by their permeance (perm). This is the number of grains of water that can pass through one square meter of material per second.
A material is a good vapor barrier if its perm rating is less than 10. There is a variety of products that have been used as vapor barriers over the years.
This is the most common type of vapor barrier used in modern construction because it serves a dual purpose as an air barrier as well. It is commonly sold in 6/1000-inch thickness (six-mil).
It is a semi-clear plastic film and is typically installed behind the drywall inside the house. It is also popular because it is inexpensive and easy to install.
The drawbacks are that it deteriorates quickly if left exposed to the sun, and it is easily punctured.
Fiberglass and mineral wool insulation batts are sold with brown Kraft paper glued to one side. This product can act as a vapor barrier in a limited fashion.
It is typically found on older homes.
Aluminum foil was intended as a radiant barrier to reflect heat, but it also acts as a vapor barrier. It is installed on batt insulation in a similar fashion as Kraft paper.
Expanded and extruded polystyrene board can act as a vapor barrier if it is thick enough. Foamed-in-place insulation can also prevent moisture penetration.
Vinyl wallpaper makes a surprisingly good vapor barrier. It is not recommended to install this type of wallpaper in hot climates for this reason.
Vapor barriers typically are installed on the exterior of the wall system in hot areas.