It’s a drainage membrane. In this case, basically a yellow mesh — almost like a scrubbing pad. Allows water to drain behind the shingles. Highly, highly recommended. Based on past experience, we’ll never build without it. Reply
A House In A Field 
Do you know what the yellow stuff is on the zip sheathing?
It’s a drainage membrane. In this case, basically a yellow mesh — almost like a scrubbing pad. Allows water to drain behind the shingles.
Highly, highly recommended. Based on past experience, we’ll never build without it.