Rainscreen Construction Principle (Part III)

Rainscreen Construction Principle

Rainscreen construction principle has become an essential part of the buildings industry over the years. With the growth of technology and engineering, manufacturers managed to produce modern and efficient cladding materials such as ACM panels considered rainscreen systems for the building’s exterior. This principle’s primary purpose is to protect the facility’s structure from damaging elements such as wind loads and water infiltration.

In the first part, we provided the basics of the system, which every customer or specialist in the building industry must know. These practical systems are available in two types: Drained/Back-ventilated, which rely on the ventilation cavity, and pressure-equalized methods, which employ drainable compartmentalization for water penetration limitation. PERS method is functional during pressure disequilibrium and facilitates fast pressure equalization. It also minimizes and under certain weather conditions to eliminate water in the product’s cavity. However, you should read the second part of the article for more information about the pressure-equalized system in the rainscreen construction principle. Moreover, in this article, we will discuss mainly the D/BV and its functionality.

Drained/Back-Ventilated Rainscreen Construction Principle

Drain/Back-Ventilated rainscreen construction principle is a practical method of protecting the facility’s exterior, which is utilized in many cladding materials such as aluminum composite panels or ACM panels with various properties, advantages, and functionalities. This system employs a series of boards, sheet metals, or planks called claddings fixed to vertical support channels or rails. The supports are the outer exposed barriers or leaves.

Joints in the external claddings are open but have a unique design to obstruct water infiltration by wind force or kinetic energy. It is essential to note that engineers create these systems to minimize the infiltration, not eliminating it thoroughly. Water leakage occurred from variable air pressure due to the wind is inevitable. Thus, a considerable amount of water can infiltrate the outer leaf through its open joints. However, the system permits the water to run down the interior surface of the outer barrier with a particular control system by the ACM panels designs including vertical channels or rails combined with water head leaves in a horizontal jointing system which directs the liquid flow to the vertical components. Little wetting in the inner barrier surface can happen, which is acceptable and will cause no issues.

The series of vertical channels have a design to collect the infiltrating water and drain it through gravity action. Additionally, they cross-direct at penetrations to the lower level and external section of the outer barrier. The system penetration should have continuous flashing and detailing to collect and direct water to the external obstacles or redirect it into the vertical drainage channels.

The minimum allowable width of air space known as a cavity between the outer and inner barriers is essential to facilitate positive back-ventilation in the ACM panels. This can promote rapid evaporation of any rainwater deposited in the surfaces of the inner surface of the aluminum composite panels or any other rainscreens. However, cavity depth should be considered to be in addition to the center of cladding applied to the building’s facade not to restrict the ventilation flow.

The internal leaf is typically the structural building close wall. Thus it can accept various construction methods. However, the water-resistant insulation should be applied to the exterior side of the cladding material and is an optional term to the thermal design requirements. The use of insulation is to maximize useable building space and rule out condensation and cold bridging to the interior surface of the inner barrier. Since the Dried/Back-Ventilated rainscreen construction principle is not a pressure equalized system, the aluminum composite materials or aluminum sidings considered building claddings must have a precise design to withstand 100% of the wind load.

Difference Between the Two Types of Rainscreens

D/BV rainscreen construction principle and PERS are both designed to increase the efficiency of the building. However, like any other industrial system, they have their differences.

The most notable thing about drained and back-ventilated systems is that leakage for claddings is allowed, and there is no deliberate attempt to minimize the effects of wind loads. Instead, the cavity behind the cladding material is drained, and positive back-ventilation usage promotes the evaporation speed of any rainwater on the inner barrier.

On the other hand, the main point about the pressure-equalized rainscreen construction principle is that without relying on sealants or gaskets in the installation process, every effort is made to minimize or eliminate the leakage through the joints. However, there could be some minor leakage into the cavity, and a drainage mechanism is essential. Additionally, positive back ventilation is utilized to increase the evaporation rate of water which permeated through the inner barrier of the ACM panels.

Rainscreen Construction Principle (Part II)

Rainscreen Construction Principle

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.