Permeability (perms) refers to the measure of the rate of transfer of water vapor through a material. The lower the perm, the less vapor that can pass through. Look for a vapor barrier with a perm of less than 1. Transmission rates are established by standard test methods. One common set of units is g/m²/day. or g/100in²/day. (1.0 US perm = 1.0 grain/square-foot·hour- inch of mercury ≈ 57 SI perm = 57 ng/s·m²·Pa).

A vapor barrier or moisture barrier is an essential part of the moisture control strategy for a home or building - Especially a metal or steel building. Seek a vapor barrier insulation - a product that combines the properties of a vapor barrier, radiant barrier and reflective insulation - It should reduce or eliminate condensation, add R value and eliminate radiant heat transfer (primary source of heat-flow). 

Click for a vapor barrier that doubles as cavity wall insulation and radiant barrier. This insulation vapor barrier can help you meet the DOE insulation recommendations for new homes for both the building cavity wall (R-11 to 13) and the basement interior wall (R-11). This will replace the older practice of wrapping the interior, exterior and basement walls in 4- or 6-mil polyethylene sheeting as a vapor barrier.

Through the process of convection, movement of air will always be from warmer areas to cooler areas. This means that in winter, the warm, moist air inside your house is constantly trying to escape to the colder air outside.

Warm air will hold more moisture than cold air. As this warm air expands, it will naturally move to the cooler areas of your wall cavities, crawl space and attic. When the air cools, it will contract and lose moisture causing condensation in these spaces. A vapor barrier acts as a physical shield to repel moisture. As most attics are vented, a vapor barrier is not required under the attic floor mass insulation although it may be advisable in more humid climates. As most crawl spaces are also ventilated, a vapor barrier is not necessary underneath the floor either.

The primary purpose of a vapor barrier is to keep moisture from getting inside your walls. Condensation within the walls can cause wood rot, mold, mildew and fungus growth. Signs of condensation within the walls include blistering exterior paint and mildew stains on the walls or ceiling in areas where there is no plumbing.

A vapor barrier is critically important in northern climates where there is a large difference between the indoor and outdoor temperatures.

Areas in the US where a vapor barrier is recommended. Source: EERE Consumer's Guide

Prodex Total: Vapor barrier: Class 1. perm < 0.1 Eliminates condensation 

Condensation can be a more significant problem in a steel building than temperature control. Moisture from condensation, rain and  ground can create several problems in metal buildings. First, the presence of water or ice in the insulation seriously degrades both the thermal performance and service life of fiberglass, paper, wool and other traditional insulation materials. Second, water in contact with steel contributes to corrosion and lowers the service life of the building. Third, collection of water can lead to dripping, staining, mold, mildew and odors. All of these problems can be solved with a properly installed vapor barrier insulation. A good steel building insulation will have vapor barrier component. Even in only moderately humid climates, condensation can cause serious problems in an unprotected steel building. To avoid this problem, adequately ventilate the building and install a vapor barrier even if you do not plan to insulate.

The most important place to install a vapor barrier in cold climates is between the warm interior house wall and the wall cavity insulation. If no water vapor can diffuse into the wall cavities, no condensation can occur.

The building vapor barrier must be continuous to work – no holes, leaks, un-taped seams or edges. Leakage will negate the effectiveness of the barrier. Any tears, openings, or punctures that may occur during construction must be completely sealed with aluminum tape. Ending the vapor barrier at the floor or ceiling junction has the same effect as a large hole. To create the vapor barrier, the seam between the ceiling and wall must be sealed well. Think of your house as a large balloon with the vapor barrier as the skin of the balloon. A leaky barrier is not much better than no barrier.

Electrical boxes and plumbing lines will compromise the barrier if the edges are not sealed completely with tape. To maintain the integrity of the barrier, some builders nail 2x4 or 2x6 horizontal furring strips to the studs after the barrier is in place to make a channel for wiring and drain lines. Drywall is nailed over the furring.

Reflective Insulation in the basement will keep heat and moisture in during winter. It will not absorb moisture in the event of a water leak. If you are finishing your basement, install the barrier on the basement walls before framing.

A vapor barrier is needed on the ground under the house (on top of the dirt in the crawlspace) to prevent the framing from getting wet. A vapor barrier should also be used under a slab-on-grade foundation or poured cement basement floor. Since a damp basement is one of the greatest producers of moisture (up to 50 lbs/day of vapor), the vapor barrier under concrete floor slabs and over earth floored crawl spaces are integral structural components of interior moisture control.

In the Southeast, where cooling and high humidity are the primary concerns, any vapor movement would tend to be from the outside to the inside of the building. Here, the vapor barrier should be installed on the exterior as whole house wrap.

The Natural Handyman
Insulation for your Home, Part 3 of 3, Vapor Barriers and Installation Tips
http://www.naturalhandyman.com/iip/infxtra/infinsul2.html

Building Envelope Research, Oak Ridge National Laboratory
R-Value Recommendation form
http://www.ornl.gov/sci/roofs+walls/insulation/ins_16.html

U.S. Department of Energy - Energy Efficiency and Renewable Energy
A Consumer`s Guide to Energy Efficiency and Renewable Energy
Vapor Barriers or Vapor Diffusion Retarders
http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11810

George Nash
Do-It-Yourself Housebuilding, The Complete Handbook
Sterling Publishing Company, 1995