What is the main purpose of a weatherproof barrier behind exterior stucco?

• A. To prevent moisture intrusion and protect the substrate
• B. To increase thermal resistance only
• C. To allow water to pass through for drying
• D. To provide a rigid backing for nails

Answer: (A)

The main idea is moisture control: the weatherproof barrier behind exterior stucco is there to keep bulk water from penetrating the wall and to protect the underlying substrate (lath, sheathing) from moisture damage. It acts as a protective layer that blocks rainwater while still allowing water vapor to escape, helping to prevent condensation and rot inside the wall. This barrier doesn’t provide meaningful insulation, it’s not intended to let liquid water pass through for drying, and it isn’t a rigid backing for nails—that role belongs to the structural sheathing or lath system. So, its primary purpose is to prevent moisture intrusion and safeguard the substrate.