A rain screen is an exterior cladding system where an air gap exists between the water-resistant barrier and the back of the exterior cladding system. To code, this gap must be 10 mm, or 3/8 inch, from the water-resistant barrier on the building and the exterior cladding system. This must maintain a ratio of being 80% open with only 20% of the cladding system impeding moisture and air flow at horizontal joints.