To find out how water and stucco interact, we built a 20-foot-long wall assembly and installed five windows in it (see photo, below). We used two layers of Jumbo Tex building paper over open-bay framing (no sheathing), and carefully applied the metal lath with furring nails driven by hand into the studs. We then applied three-coat stucco over the course of 10 days, moisture-curing both the scratch and the brown coat. From the back, we cut holes in the building paper to expose the scratch coat.

We set up our custom spray rack to apply water to the entire surface of the wall at once. In less than two minutes we found water flowing through the holes we had cut into the building paper — convincing us of the importance of creating a watertight envelope underneath stucco.

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