For most climate zones the management of exterior rain water is the most critical aspect of the moisture management system of the building enclosure. The fundamental principle of water management is to drain the water downwards and outwards out of the building and away from the building. In order for the building and building assemblies to drain properly, detailing of the drainage plane must be carefully designed.

There are several options for creating a drainage plane in the wall assembly. The choice of which method to use is based on weighing the risks involved.

Water Resistance

As the water penetration resistance of the assembly increases, the risk of moisture problems decreases.

Moisture Tolerance of Assembly

As the moisture tolerance of the materials that comprise the assembly increases (masonry and concrete vs. wood and steel) the risk of moisture related problems decreases.

Exposure

As the exposure to rainfall increases, the risk of moisture related problems increases.

Rainfall

As the amount of rainfall increases, the risk of moisture related problems increases.

Drying Potential

As the ability of an assembly to dry increases due to the climate, design, or both, the risk of moisture related problems decreases.

Workmanship

As the craftsmanship of the construction of the assemblies and their connection details increases, the risk of moisture related problems decreases.

There are four strategies proposed for maintaining the continuity of the drainage plane with the incorporation of insulating sheathing to the exterior.

Wall Section 1 - Insulating Sheathing and Housewrap over Plywood or OSB:

The first strategy involves installing the insulating sheathing over top of a layer of building paper or housewrap and wood sheathing. This is the most durable assembly proposed in this guide as the drainage plane material (building paper or house wrap) is supported by the plywood sheathing, and protected against wind loading and other environmental factors by the insulating sheathing. This type of assembly would be recommended in areas or high exposure and rainfall.

Wall Section 2 - Insulation Sheathing and Housewrap over Wood Studs:

The next proposed strategy is to install the insulating sheathing overtop of a housewrap that is stretched over wood studs. In this configuration, the housewrap drainage plane is protected from exterior elements (excessive wind loading, and rain exposure). The type of assembly would work effectively in most rainfall zones, though potentially not in high exposure locations. With the lack of wood sheathing support on the exterior of the framing more care is required during the installation of the housewrap and insulating sheathing.

Wall Section 3 - Housewrap installed over Insulating Sheathing and Wood Studs:

The third strategy would be to install the housewrap to the exterior of the insulating sheathing, essentially replacing the plywood or OSB in a traditional wall assembly with insulating sheathing. The housewrap is more exposed to exterior elements such as wind loading and moisture and may not be as durable as the other approaches. In addition the fasteners used to install the housewrap must be able to penetrate all the way through the insulating sheathing and into the wood studs beyond. This wall approach would function adequately in most rainfall zones.

Wall Section 4 - Insulating Sheathing as the Drainage Plane:

The final approach would be to use the insulating sheathing as the primary sheathing and drainage plane of the assembly. In order to achieve this all the vertical joints must be taped or sealed and special flashing details must be used. With some question as to the long term dimensional stability of insulating sheathing products, this approach is considered to be in the higher risk category and should only be used in areas with limited rainfall and exposure, where rain water management is not as critical.