Rainscreen Construction Principle basic idea is to have an exterior surface or cladding layer that can break the force of sideways, wind load, water movement, resulting in preventing water penetration into the building’s structure. For most of the buildings, from modern to traditional ones, water is the biggest enemy. However, producing a complete watertight cladding system is challenging. Thus, manufacturers started working on the rainscreen construction principle decades ago to provide better protection for the constructions in the city. Rainscreen comes in different styles and types. One of the most popular types is the ACM panels or ACP materials used widely in the modern era. These particular products offer numerous advantages along with their waterproof characteristics.
In the rainscreen construction principle part 1, we discussed the basics of these practical systems and their structure that engineers designed for better watertight characteristics. This article covers essential information about this principle that anyone in the building industry must know.
Rainscreen Construction Principle Types
The rain screen construction principle is based upon two distinct and separate barriers on the building’s cladding. The outer barrier controls most rainwater while the inner leaf performs multiple functions, including moisture and air barrier, insulation, and the structural wall. The outer section allows the water to penetrate through open joinery, and the volume of the penetration depends on the design principle. This is where two distinct types of products come into the picture: the Drained/Back-ventilated and Pressure-equalized/Compartmented systems.
Both rainscreen systems are available in ACM panels cladding materials and different siding for the facade. They use the principle of controlling water leakage without stopping it from penetrating through the exposed outer surface completely. However, the internal design of both systems functions differently based upon the approach. Moreover, the Rainscreen construction principle has been generalized and consolidated into one unified product in specifications combined with both types’ characteristics. For complicating and confusing matters, the water, and air resistance standards from the traditional building cladding systems are applied to these modern designs and specifications of the exterior material’s exposure. Placing the water and air tightness codes on the outer surface of the barrier contradicts the underlying fundamentals of the Rainscreen Construction Principle. Now let’s get to the fundamental of each system.
Pressure-Equalized/ Compartmented Rainscreen
In this article, we focus on the Pressure-equalized rainscreen principle, which is very design intensive. Please read the third part of this article to learn about Drained/Back-ventilated systems for ACM panels.
Pressure-equalized systems are susceptible to design variations and deviations from their design principle. The openings in this system are created uniquely for both static and dynamic pressure equalization allowance across the rainscreen. The essential factor that separates this system from the D/BV principle is the design and the use of the compartmented method within the cavity. The reason behind the compartment is that the pressure equalization can only happen within limited periods and in the case of controlled volume behind the Rainscreen system. This equalization is essential due to the none-uniformity of wind loads, constantly changing across a single cladding of a building.
The vent numbers and their geometry calculation depend on the cavity volume, allowing sufficient airflow in and out of the internal equalized area quickly enough to respond to the ever-changing wind loads. This promotes the pressure differences between the ACM panels or other types of panels used in the system and the internal compartment pressure resulting in equalizing the air pressure on the external cladding and the inner sides of the building facade. If the conditions exist properly, forces causing water movement will decrease; if not, they will be eliminated.
The effective area of the vent holes in this type of rainscreen construction principle depends on three factors which are:
- The airtightness of the air barrier on the inner section
- The stiffness of the cladding material and its inner leaf
- The volume of the compartments that make up the internal air space
The compartments are the essential elements of a proper pressure-equalized rain screen principle. They come with crucial properties, characteristics, and functionalities. Followings are the reasons why these elements play an indispensable role in the PE systems:
- They control lateral and vertical airflow.
- They size the volume of the space that the vent openings must be designed for facilitation.
- The most crucial role of compartments is limiting water infiltration and controlling the water drainage when air-pressure disequilibrium occurs.
The characteristics above show that rainscreens with non-compartmentalized cavities cannot be pressure equalized. However, many of this system’s construction factors and elements are almost similar to the drained/back-ventilated rain screen construction principle. Still, they have several significant and essential differences, which we have discussed in the third part of this article.