Protect Reservoir Claddings From Rain Exposure and/or Specify Exterior Wall Materials to Limit Inward Vapor Diffusion

A design topic which links moisture management and sunlight is exterior “reservoir” wall materials. Reservoir cladding materials can absorb bulk water from rain which hits their surfaces and is absorbed. Cladding systems made of masonry, stucco, fiber cement, and wood can all absorb some amount of water. When these surfaces are wetted (“charged” with water) and subsequently exposed to high levels of sunlight, the cladding materials experience much higher temperatures and vapor pressures. The elevated vapor pressures drive water vapor towards the exterior but also into wall assemblies, especially if the building interior is at a much lower vapor pressure (i.e., it contains much less humidity due to air conditioning).

From a design standpoint, overhangs can be used to limit exterior wall exposure to rainfall and water absorption for these reservoir claddings. If we take away the water source, this problem is managed. See Section 4.2.2 for design guidance on sizing overhangs for moisture protection.

If overhangs are not feasible for aesthetic reasons, a need to reduce wind uplift design requirements, or other reasons, then the hygrothermal performance of the wall system must be more carefully scrutinized. In fact, even with the use of overhangs, use of reservoir cladding systems warrants more careful examination of the wall’s treatment of water vapor. Design recommendations include the following.

• If water vapor will enter the wall, make sure it can pass all the way through. In other words, if the wall sheathing, WRB, and exterior insulation (if present) collectively form a highly permeable assembly, then water vapor will be able to migrate into the wall. With this knowledge, we need to make sure the vapor can dry to the interior, so interior polyethylene vapor retarders, vinyl wall paper, and other very low perm materials should not be used on the wall interior.
• Forming the outer layer of the wall with low or moderate vapor permeance materials can limit inward vapor migration from the wetted cladding. The low/moderate vapor permeance layer might be the exterior insulation and/or the WRB. This layer will limit the diffusion of water vapor even when vapor pressure at the cladding is elevated. However, it’s critical to note that adding less vapor permeable materials towards the outside of the wall will also limit the ability for interior moisture to dry towards the exterior (as in winter conditions). Thus, this recommendation needs to be directly integrated with the design guidance on insulation levels and perm ratings found in Section 4.3.
• Walls at “high risk” from reservoir cladding / solar-driven moisture problems should be examined more closely with hygrothermal modeling (see discussion in Section 4.3). Risk factors include little/no wall protection from rainfall; high moisture storage capacity (e.g., masonry); and high solar exposure. Projects in mixed heating/cooling climates are also more challenging because the dominant vapor drive goes in both directions for significant portions of the year.

In addition to this guidance, the Additional Resources includes a reference from Building Science Corporation on reservoir cladding moisture issues.