Best Practices Manual

4. Rain Screens

Built right, rain screens are vital to modern construction in our Pacific NW climate. They are also poorly named with multiple definitions. At Hammer & Hand, when we say “rain screen” we are referring to the ventilated rain screen, with that all-important ventilated cavity between cladding and the rest of the wall assembly. That cavity serves two functions for the exterior wall assembly:

1. The cavity helps protect the wall from water intrusion. By separating the cladding from the face of the wall assembly, the rain screen cavity interrupts capillary action into the assembly and provides a drainage plane for bulk water to drain away harmlessly.

Note: we also include a WRB (water-resistive barrier) inboard of the cavity to provide a final line of defense against bulk water intrusion.

2. The cavity helps wall assemblies dry out. When ventilated properly, with a minimum 1/4” gap to the exterior at bottom and top of the rain screen and a minimum 3/8” deep vertical cavity in between, the volume of air passing through the rain screen cavity can be measured in the 10s, even 100s, of air changes per hour. This dramatically increases the assembly’s drying potential and resiliency. This is good, particularly for highly insulated walls which by design decrease airflow and thermal transfer across the assembly, reducing the assembly’s capacity to dry. The drying action of the ventilated cavity behind the rain screen helps to counteract this limitation, promoting building durability.


Continue to 4.1 Rain Screens: Top of Wall>>>