How Does a Rainscreen Cladding System Keep Your Walls Dry?

A rainscreen uses an outer cladding layer plus a vented, drained cavity behind it. Pressure equalization across the cavity stops wind-driven water penetration, and flashing with weeps at the base drains the water that enters through joints. The system works only if the cavity remains open, vented top and bottom, and terminates at flashing that directs water outward. The physics is worth understanding because it explains every detail: wind pushes rain against the cladding, and water enters through joints. If the cavity behind is vented, the air pressure inside matches the wind pressure outside, so there is no pressure difference driving water inward — it just runs down the back of the cladding to the flashing. Seal the vents and you have recreated a barrier wall with extra steps.

Large uninterrupted cavities do not equalize pressure well — wind pressure varies across a facade, so a single giant cavity has internal pressure differences that still drive water. Proper rainscreen design compartmentalizes the cavity with baffles at corners, floor lines, and large openings, creating smaller zones that equalize independently. This is the detailing step most often skipped, and it is the difference between a rainscreen that performs in a storm and one that only performs in a brochure. Flashing is the other half of the system. Every horizontal interruption — window heads, floor lines, the base of the wall — needs flashing that collects cavity water and directs it outward through weeps. Flashing must be integrated with the water-resistive barrier behind the cavity, lapped shingle-fashion, and terminated so water cannot run back into the wall. I review flashing details with the same seriousness as structural connections, because they fail just as expensively.

A rainscreen is a system of cladding, cavity, vents, compartmentalization, and flashing — the drawings have to show all five, not just the cladding material and a note saying 'rainscreen.' My rainscreen review checklist:

• Cavity depth and venting shown: minimum drainage depth, vent locations top and bottom, ember screening where required
• Compartmentalization detailed: baffles at corners, openings, and floor lines sized for the facade's wind exposure
• Flashing at every horizontal break: window heads, base of wall, and floor lines, lapped with the WRB and weeped
• Cavity protection specified: measures to keep mortar, debris, and insulation out of the drainage plane during construction
• Cladding attachment engineered: the support system carries wind and dead loads through the cavity to the structure