Not Vapor Retarders	Perm Rating
3/8" Gypsum Wall Board (plain)	50
4" Unfaced Mineral Wool	30
Typical Latex Paint, ~ 0.002" thickness	5.5 to 8.6
4.4 lb./100 ft.2 Asphalt Saturated Sheathing Paper	3.3
1/4" Plywood (douglas fir, interior glue)	1.9

VI. VAPOR RETARDERS AND INSULATED WALLS
In general, the colder the climate, the greater the need for a vapor retarder. Heating climates are defined as climates with 4000 heating degree-days (HDD). Since the majority of moisture in the assembly is the result of water intrusion and air infiltration, the assembly should be designed so that excessive moisture can escape the assembly. In addition, the moisture storage capacity of the assembly is important. A wall consisting of wood studs, kraft-faced insulation, and wood sheathing is more forgiving than a steel assembly with foil-faced insulating sheathing and continuous 4 mil polyethylene sheet over unfaced insulation, since the former assembly can store more moisture when needed and release it later as conditions permit.

In climates requiring a vapor retarder on the interior surface, a kraft-faced insulation is usually sufficient. When a loose-fill product such as fiberglass or cellulose is installed, a 4 mil continuous polyethylene sheet or a vapor retarder paint on the interior drywall should be used. The poly is acceptable for heating climates and a vapor retarder paint for milder climates. In most cases, the use of a vapor retarder is not influenced by the type of cavity insulation used.

Most manufacturers of sprayed cellulose advise contractors that a vapor retarder is not necessary or desired in a wall system. If the insulation is applied with water, manufacturers generally recommend waiting between 24 to 48 hours before installing drywall. Consult manufacturer's recommendations for details.

If you are reinsulating a home with blown in insulation, installing a vapor retarder onto the sidewalls if one has not been previously installed can be quite difficult. It may be necessary to paint the interior surfaces of exterior walls and ceilings with a vapor retarder paint.