Consider the same wall assembly as in the previous scenario but with vapor impermeable foam insulation (e.g. XPS, polyiso, medium-density spray polyurethane foam, etc.) used to the exterior of the sheathing. These insulation materials can be considered a Class I or II vapor retarder depending on type, density, thickness and facings.

Vapor impermeable foam insulation with the same R-value as in the previous scenario placed outboard of the sheathing has the effect of warming up the rest of the cavity - the more exterior insulation, the warmer the cavity and sheathing.

No interior vapor barrier has been installed in this scenario, nor would be recommended, as the inclusion of a vapor barrier (Class I or II) at the interior in conjunction with the exterior foam would create a double vapor barrier situation that would substantially restrict drying. This wall, therefore, relies on vapor diffusion drying towards the interior, as vapor diffusion drying through the exterior insulation is limited. In terms of balancing wetting sources and drying ability, this is a more sensitive wall than the previous scenario as a result of using impermeable exterior insulation.

This is particularly important if the indoor humidity is elevated within the building or the outdoor climate is wet, which can create higher indoor relative humidity levels during cold periods of the year. Even higher indoor wintertime RH levels of 40-60% (such as in coastal climates or in commercial buildings with high interior moisture generation rates such as restaurants, pools, museums, etc.) can create a challenge for this assembly due to the increased outward vapor diffusion and reduced drying potential.

The vapor pressure difference from interior to exterior within this scenario is the same as in the previous scenarios; it hasn't been affected by the presence of the exterior insulation. While vapor diffusion outwards is significantly slowed, or stopped, condensation does not occur within the wall cavity because the temperature is warm enough to keep the RH below 100%. While condensation does not form within the cavity of this assembly, moisture is prevented from travelling through the exterior insulation, which can be an issue in the event of a leak or if there is not enough insulation outboard to prevent air leakage condensation. The RH within the cavity behind the sheathing will depend on the insulation ratio and the effective vapor permeance of the foam insulation. With this assembly, it is generally safer to have more exterior insulation (or a higher insulation ratio) than with vapor permeable insulation so that the RH is kept below 80% to reduce the risk of fungal growth.

Similar to the exterior insulated wall with vapor permeable insulation, the risk of air leakage condensation is reduced for this wall due to the increased sheathing temperature. If vapor diffusion and air leakage wetting are addressed by placement of sufficient exterior insulation, the only risk of moisture damage is from an external leak. In the event of a leak, drying outwards by vapor diffusion is restricted by the foam, and drying can only occur in the inward direction. Solutions to improve outward drying include providing a small drainage layer behind the foam or using more vapor permeable insulation. The relative tightness in which the rigid foam board insulation is installed and how the joints are sealed will also affect the outward drying ability.

In a warm climate where the vapor drive is primarily inward, the use of impermeable exterior insulation can provide an effective vapor control strategy. In this case, the exterior insulation restricts the diffusion of water vapor through the wall assembly as shown in the following figure.

No interior vapor retarder would be provided in this scenario so that the wall assembly can dry effectively to the interior.