What are water resistive barriers (WRBs) required under?

Water resistive barriers (WRBs) are essential components of building assemblies that provide protection against water infiltration while allowing for the escape of water vapor. They are specifically required under all exterior wall cladding systems. The primary function of a WRB is to prevent moisture from penetrating the building envelope, which can lead to mold growth, material degradation, and a decrease in energy efficiency.

When it comes to wall assemblies, a WRB is typically installed directly behind the cladding materials, such as siding, stucco, or brick. This placement is crucial because it acts as a first line of defense against rain or moisture that can be introduced from outside elements. The barrier must be continuous and properly integrated with adjacent components to ensure effective performance.

Given the other options, they do not typically require WRBs in the same capacity as exterior wall cladding. Interior wall structures may not necessarily be exposed to the same environmental conditions and hence do not require the same level of moisture protection. Roof underlayment does provide some moisture resistance but serves a different purpose, mainly focused on protecting the roof deck and providing a secondary line of defense against leaks. Foundation materials are typically protected through other means, such as drainage and moisture barriers, rather than a WRB.

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