For wall assemblies in cold climates, interior vapor control is often provided using a polyethylene sheet vapor barrier, though other options such as vapor barrier paint, kraft paper and smart vapor retarder products are also used.

This interior vapor retarder limits the diffusion of moisture through the wall assembly towards the exterior. Outward vapor diffusion drying can still occur from within the wall cavity to the exterior through the sheathing, membrane and cladding.

If this vapor retarder is not provided, vapor could diffuse from the interior towards the exterior. As the temperature drops across the insulation, for the same absolute humidity level (moisture) within the air, the relative humidity (RH) will increase, and when it comes in contact with a cold surface (below the dew point temperature), water can condense within the assembly. Most commonly this occurs on the interior face of the exterior wall sheathing. Note that if the indoor vapor pressure is low (low indoor RH) then condensation may not occur at the sheathing. This explains why in older homes with high air leakage, high ventilation rates and lower RH levels, vapor diffusion may not have caused wetting, though in a more airtight new home it can. Even where condensation does not form, high RH levels are conducive to fungal growth on building materials, so it is ideal to keep the RH below 80% the vast majority of the time.

The two following figures illustrate how interior vapor control in a stud cavity insulated wall can help prevent condensation within the wall assembly in cold climates. Condensation can also occur if air leaks from the interior to the back of the sheathing. Often this can cause more significant localized damage than can vapor diffusion alone.

For comparison with the previous scenario, consider a stud insulated wall assembly in a cold climate with a vapor retarder material installed on the exterior side of the insulation under wintertime conditions. Vapor diffusion in this scenario occurs from the interior to the exterior; however, moisture is prevented from diffusing outwards and will accumulate in the form of condensation, leading to damage. This is an example of what can happen when a vapor barrier is installed on the wrong side of an assembly. The vapor barrier in this scenario could be created by the wrong type of sheathing membrane, the use of excessive self-adhered bituminous membrane application at penetrations and details or the installation of a vapor impermeable cladding such as fiber cement panel, glass cladding or metal siding. The following schematics illustrate examples of this situation and illustrate why correct vapor barrier placement is important.

A straightforward way to address vapor impermeable claddings is to provide an airspace and ventilation behind the cladding so that airflow from the outdoors will remove moisture deposited on the cladding by outward vapor diffusion. As many commercial claddings are impermeable (i.e. metal, concrete veneer, etc.), this is commonly achieved by the construction of a drained and ventilated rainscreen cavity. In some building codes, the requirement for venting behind impermeable claddings was implemented at the same time interior vapor barriers were introduced. Venting of an impermeable cladding is illustrated in the following figure.