Should I use a vapor retarder in Missouri?
The US Dept of Energy also says 1/16" back drainage cavity for siding is appropriate even though capillary action occurs up to 1/4". Point being, the department of energy tends to lag significantly behind the latest in building technology. I did my undergraduate in environmental sciences at the University of Missouri and my masters in architecture at the University of Oregon (consistently #1 for sustainability in architecture). The belief that you shouldn't use a vapor barrier at all is terrible advice. You should include one and locate it as close to the center of your thermal barrier as possible (for this climate). This allows the moisture that condenses on the surface of the barrier from either side (interior in winter and exterior in summer) to dry. So, say you have a 2x6 wall with dense packed cellulose insulation, which is roughly an r-value of 20. You should place a vapor barrier to the exterior side of stud and then include 5" of eps (unfaced eps, and please use eps because xps is awful for the environment) to the exterior of the wall (equates to about r 19). This gives your wall assembly an R39 value and ensures that condensate can dry to the appropriate side. If you adjust your insulation type, thus adjusting r-values, you should adjust dimensions accordingly to ensure as close to a 50/50 split on either side of the vapor barrier as possible.