A pressure-equalised rainscreen system works on a simple but clever principle. Instead of creating a perfect, sealed barrier, it strategically uses open joints in the outer cladding. This allows air to move freely into the cavity behind the panels, instantly balancing the air pressure inside the cavity with the wind pressure outside. With no pressure difference to drive it, rainwater that hits the facade isn't forced into the building. Instead, it simply runs down the face of the panels.

This system has two lines of defence. The outer HPL panels act as the primary rainscreen, shedding the vast majority of water. The air cavity and the insulated, weatherproofed backing wall form the second line. Any small amount of moisture that might get past the panel joints is managed within this ventilated space, ensuring the building's structural wall stays completely dry.

The depth of this air cavity is critical. It must be continuous and unobstructed, typically between 20mm and 50mm wide. This specific range is a functional trade-off. Anything less than 20mm risks creating surface tension that could hold water in the cavity and may not provide enough airflow. A gap larger than 50mm offers diminishing returns on ventilation and can create structural challenges for the support system.

The ventilated cavity relies on a natural phenomenon to function: the chimney effect (also known as the stack effect). Air within the cavity is warmed by heat escaping from the building and by solar radiation absorbed by the cladding panels. As this air warms, it becomes less dense and more buoyant, causing it to rise.

This upward movement of warm air draws cooler, denser air into the cavity through vents at the base of the wall. A continuous, gentle airflow is created, flowing from the bottom to the top of the facade. This constant ventilation is the system's primary mechanism for removing any moisture that enters the cavity, whether from small leaks or from interstitial condensation forming on the back of the cold cladding panels. It also helps dissipate heat during summer, reducing the building's cooling load.

For the chimney effect to work properly, the system needs carefully sized air intakes and exhausts. Without adequate openings, the airflow would be choked, rendering the ventilation ineffective. The required size of these openings is typically defined as a clear, unobstructed area per linear metre of wall.

This value ensures a sufficient volume of air can enter and exit the cavity to maintain the pressure balance and facilitate drying. For taller buildings or facades in high-exposure areas, these minimums may need to be increased. The calculation is straightforward:

• For the intake: Multiply the length of the wall base in metres by 100cm².
• For the exhaust: Multiply the length of the wall top in metres by 100cm².

These openings are typically created using perforated profiles or grilles that allow air to pass through while preventing the entry of pests or large debris. They must be maintained across the entire length of the ventilated area to ensure consistent performance.

Let's check your understanding of these core mechanics.

By combining the rainscreen principle with natural ventilation driven by the chimney effect, these facades provide a robust, multi-layered defence against the elements, protecting the building structure and ensuring its long-term durability.