While vapor retarders can be used to control vapor diffusion and consequently prevent condensation, installation of these types of materials at the wrong location within an assembly can also cause significant moisture problems. Typically, there are two conditions which can be created which are detrimental: wrong side vapor retarders, and double vapor retarders.

Wrong side vapor retarders refers to when a vapor retarder is placed on the low vapor pressure side (typically the cold side) of a wall assembly. Locating a vapor retarder in this way restricts vapor diffusion through the wall and creates a potential condensing plane within the wall assembly.

Double vapor retarders refers to when a vapor retarder is installed at two different locations in an assembly such that any moisture which manages to get between them is unable to dry effectively. When the materials between the vapor retarders are moisture sensitive, this trapped moisture can potentially lead to damage. Moisture between the vapor retarders may be the result of air leakage, rainwater ingress or built-in construction moisture. The following figure illustrates a schematic double vapor barrier situation restricting the drying of a wall assembly.