Rainscreen

Ideally the rainscreen prevents the wall air/water barrier from getting wet but because of cladding attachments and penetrations (such as windows and doors) water is likely to reach this point, and hence materials are selected to be moisture tolerant and integrated with flashing.

[7] Many terms have been applied to rain screen walls including basic, open, conventional, pressure-equalized, pressure-moderated rainscreen systems or assemblies.

These terms have caused confusion as to what a rain screen is but all reflect the rainscreen principle of a primary and secondary line of defense.

[9] Rainscreen cladding is a kind of double-wall construction that utilizes a surface to help keep the rain out, as well as an inner layer to offer thermal insulation, prevent excessive air leakage and carry wind loading.

The vapour-permeable air/weather barrier prevents water molecules from entering the insulated cavity but allows the passage of vapour, thus reducing the trapping of moisture within the main wall assembly.

In the pressure-equalized system the ventilation openings must be large enough to allow air-flow to equalize the pressure on both sides of the cladding.

The drainage plane must move the water out of the wall system quickly to prevent absorption and consequential rot, mold, and structural degradation.

A drainage plane is designed to shed bulk rainwater and/or condensation downward and outward in a manner that will prevent uncontrolled water penetration into the conditioned spaces of a building or structure.

In a barrier wall system, the exterior cladding also serves as the principal drainage plane and primary line of defense against bulk rainwater penetration.

[12] Air pressure difference is one of the forces for driving a rainwater into wall systems but gravity is more often the cause of practical problems.

[13] A rainscreen drainage plane that works as a predictable pressure equalization plane creates a separation (an air chamber) between the backside of a rainscreen and the exterior surface of the weather-resistant barrier that is installed on the exterior sheeting of the structural back up wall.

The air flow that exists in most wall systems is a slight draft that will not dry this condition out in a timely manner.

[24] It logically follows that the more time a part of a wall system exceeds one of these thresholds the greater chance of damage from fungal growth or rot.