The control of vapor diffusion within walls is a balance between minimizing wetting and maximizing drying ability. Correctly placed vapor control layers prevent excessive moisture from diffusing into wall assemblies and potentially condensing, while vapor permeable materials allow moisture to diffuse out and are beneficial to drying performance. In the design and construction of commercial walls in cold climates, it has been common practice to install a polyethylene sheet vapor barrier at the interior of the insulation to control vapor flow (and often air flow) and therefore limit vapor diffusion wetting while using vapor permeable materials to the exterior to encourage drying. In warm climates, the opposite approach is used, and impermeable materials on the exterior such as concrete, CMU or metal claddings restrict the diffusion of vapor through the wall assembly, and permeable materials on the interior such as drywall and mineral wool insulation allow for drying to the interior.
When insulation is added to the exterior of the walls, as in the case of a split-insulated or exterior insulated wall, this insulation maintains the temperature of the stud cavity and exterior sheathing closer to interior conditions, reducing the potential for vapor diffusion and air leakage condensation to occur within the cavity. The more insulation that is installed outboard of the sheathing, the closer to interior conditions the stud cavity will be. Wherever possible, the exterior insulation ratio should be maximized, and fully exterior insulated walls work well in both cold and warm climates.
In cold climates, the type of insulation installed outboard of the sheathing (or as the sheathing) has an important impact on the vapor diffusion drying capability of the wall. Vapor permeable insulation such as mineral wool or fiberglass will allow for greater outward drying than can be achieved with vapor impermeable insulation such as foam plastics; XPS, polyiso and spray polyurethane foam insulation. This greater drying ability generally results in improved durability of the wall assembly.
In some cases, the change in temperature profile due to the addition of exterior insulation means that a vapor barrier may no longer be needed at the interior in cold climates, and alternate strategies such as a latex paint may be used instead of polyethylene. When a vapor impermeable exterior insulation is used in cold climates, an interior vapor retarder should be avoided to prevent trapping moisture within the wall assembly, or potentially an adaptive permeance smart vapor retarder material could be used. In warm climates, vapor impermeable exterior insulation will work to restrict the flow of vapor through the wall and prevent moisture accumulation within the assembly.
While walls with vapor impermeable materials on the exterior are typically not recommended for cold climates, these types of walls can provide durable performance in warm climates where the primary vapor drive is reversed. For example, exposed concrete walls insulated on the interior with vapor permeable insulation such as stone wool or fiberglass can provide highly durable performance in warm climates.
Other types of walls such as walls with moisture storing claddings create unique conditions with respect to vapor diffusion, and require careful consideration and design. When designing these walls, double vapor barrier situations are to be avoided such that drying can occur.
Overall, the correct selection and placement of vapor impermeable materials within wall assemblies is fundamental to their durability in both cold and warm climates. Failure to correctly account for the impacts of vapor diffusion can lead to damage and premature failure of wall assemblies.