Face seal walls are designed to achieve water tightness and air tightness at the face of the cladding. Joints in the cladding and inter-faces with other wall components are sealed to provide continuity. The exterior face of the cladding is the primary -and only-drainage path. There is no redundancy. The "face seal" must be constructed - and must be maintained - in perfect condition to effectively provide rain penetration control. However, such reliance on perfection is questionable on walls exposed to rainwater. As a rule, face seal walls should only be used where very limited amounts of water will reach the cladding surface, such as wall areas under deep overhangs or soffits or in regions where the degree of moisture hazard is not high.

Concealed barrier walls are designed with an acceptance that some water may pass beyond the face of the cladding. These walls incorporate a drainage plane within the wall assembly, as a second line of defense against rain penetration. The face of the cladding remains the primary drainage path, but secondary drainage is accomplished within the wall. An example of a concealed barrier wall is wood siding installed directly over an asphalt-saturated felt moisture barrier and plywood sheathing. The water-resistant felt constitutes the drainage plane.

Vinyl siding and drainage EIFS (exterior insulated finish system) installed over a moisture barrier should also be considered concealed barrier walls, although drainage in these cladding systems is enhanced by provision of some airspace - however discontinuous - behind the cladding. A concealed barrier strategy is appropriate for use on many exterior walls and can be expected to perform well in areas of low to moderate exposure to rain and wind. Performance in high to severe exposure conditions, however, is not assured. In all cases, the integrity of the second line of defense is highly dependent on correct detailing by the designer and proper installation by the builder. To maximize performance and service life of the assembly in high exposure conditions, consideration should be given to the use of a rain screen assembly.

Rain screen walls take water management one step further by incorporating a drainage cavity {3/8" minimum width) into the assembly, between the back of the cladding and the building paper. The drainage cavity offers enhanced protection from water intrusion by acting as a capillary break, thereby keeping most water from making contact with the moisture barrier.

The airspace also serves to ventilate the backside of the cladding, which facilitates drying of the cladding, and mitigates against potential moisture accumulation in the wall framing caused by reverse Rain screen drive. Examples of Rain screen walls include brick veneer (usually installed with a one or two inch airspace) and stucco cladding installed over vertical strapping (typically pressure-treated 1x3s at 16" Rain screen. on center). Rain screen walls are appropriate for use in all locations where high exposure to rain and wind is likely.

Pressure-equalized Rain screen represent an advancement of the basic rain screen strategy. These walls incorporate compartmentalization and increased venting of the drainage cavity to improve performance. As wind blows on a wall face, air passes through vents into the cavity behind the cladding. If this air is contained appropriately by subdividing the drainage cavity with compartment seals, an equalization of pressure occurs across the cladding, thereby eliminating one of the key driving forces behind water penetration. This strategy is most commonly applied to brick veneer walls, though conceptually it is possible to enhance any Rain screen assembly with this technology. Pressure-equalized Rain screens are appropriate for use on all exposures and offer the highest performance potential with respect to water management.

Article published and copyrighted by Canadian Wood Council 2005