The design of the vapor management system should attempt to allow maximum drying of the wall assembly from diffusion, while limiting the amount of moisture able to be driven into the assembly. Where possible, drying to both sides of the construction assembly is encourage, however in some circumstances more stringent vapor control is required. As a general rule for standard framed construction, the vapor retarding layer should be placed to the interior of the assembly in cold climates (reducing the water vapor from the higher humidity interior air from diffusing into the assembly), while in hot humid climates, the vapor retarding layer should be placed to the exterior of the assembly (reducing the water vapor from the hot humid outside air from diffusing into the assembly).

Therefore, in a general sense, for cold climates it is preferable to use a more vapor permeable insulating sheathing (such as EPS and unfaced XPS) on the exterior and in hot humid climates, it is preferable to use a more vapor impermeable sheathing on the exterior of the assembly (such as foil faced polyisocyanurate and plastic film faced XPS).

For mixed humid climates, the system choices become more difficult as the assembly needs to be protected from wetting from both the interior as well as the exterior. The drying can be predominantly to the exterior, the interior, or in both directions in a flow through type assembly. Often these strategies need to be combined with other vapor management strategies such as building pressurization (or depressurization) and supplemental dehumidification. There is also a strategy to place the vapor control layer towards the middle of the assembly. This approach will be discussed in the next section on Condensation Resistance.

There are also some exceptions. With absorptive claddings such as brick veneer and stucco, a high inward vapor drive can occur due to solar radiation heating up the wet cladding and creating a high vapor pressure difference between the brick (or stucco) and the exterior and the brick (or stucco) and the interior. This vapor pressure differential will cause moisture to be driven into the assembly if there is not adequate vapor control on the exterior. For this reason, insulating sheathing that is installed behind a masonry veneer or stucco should be vapor semi-impermeable or it can be semi-permeable if combined with an impermeable or semi-impermeable membrane.