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.

Rainscreen Construction Principle (Part I)

Rainscreen Principle

Rainscreen principle is an industrial product that is used widely in residential and commercial facilities for various purposes. This product utilizes the open rainscreen principle and is a subset of the screened-drained classification of wall designs. It incorporates all essential characteristics that the building industry requires. Additionally, the authentication of the open rain screen is its capability of equalizing air pressure and ventilation allowance.

What is The Rainscreen Principle?

The rainscreen product and principle, as today’s industry knows, owes much of its development to such men as Birkland and Garden. However, the terms open rain screen principle, and rain screen appeared in the 1960s. These respectful men examined five primary forces that cause rainwater infiltration in a wall. They also looked at the existing wall designs and materials that used cavities and interior drainage to control water departure. Thus, they found that if all of the five forces behind the water infiltration through a cladding can be eliminated or controlled, then the rainwater or the water present on the wall will not penetrate the building.

It is essential to note that it is not conceivable that architects or building designers can prevent the exterior section of the facility from getting wet nor guarantee that no openings will develop to permit the entrance of the water. However, researchers prove that incorporating an air chamber into the wall or joints where the air pressure is equal to that on the exterior can prevent rainwater infiltration. In essence, the external layer is an open rain screen that damages the wetting of the facility’s primary wall or air barrier.

Engineers have reasoned that the air pressure difference existing between the exterior and the interior of the wall system results in air currents that carry the moisture or water into the building’s Cladding. Unfortunately, at the first stage of rainscreen development, manufacturers could not produce products to provide a strategy to relieve air pressure differences that the Cladding was experiencing. If any building designer created a moderate pressure, it was incidental. Air pressure was the cause of water infiltration that many had not considered at the time. However, years of research and studies have shown that this force is the dominant contributor to water leakage.

Structure of a Rainscreen

As told, rainscreen provide air ventilation due to their unique structure. This product features three primary components:

  • A screen or outer layer.
  • A pressure equalization chamber with a perfect sealing system on all sides except at the vent.
  • A vent or opening that connects the air chamber to the exterior.

More specifically, for countering other forces that can cause air infiltration, a rain screen has other practical features. Therefore, manufacturers create products that have a more comprehensive design to develop the rainscreen principle fully. The structure of a fully developed main screen contains:

  • Screen or Cladding- the screen is a durable material that is exceptionally non-porous and is generally in the form of a panel. This section of a rain screen allows the water to run off both sides of the product.
  • Open Joints or Air Vents- this section of a rain screen includes gaps for ventilation. The product features a labyrinth type of interlocking dry joints for controlling momentum, surface tension, gravity, and capillary action forces. For better sealing and more efficient protection, it is possible to use wet seals such as caulk behind the Cladding.
  • Air Flow Toward and Out of PECs (Pressure Equalized Chamber)- the primary wall of the rainscreen products is divided into much smaller self-contained chambers. Each PEC size is determined precisely and as airtight as possible. The more profound and more extensive the section is, the more air intake is required. Thus, bigger vents will be needed.
  • Chamber Baffles- these are the sidewalls of the pressure equalized chambers that should resist the pressure and wind loads inside the section. Thus, they need to be relatively airtight. Chamber baffles can be any divider such as a structural connector or clips that support the Cladding.
  • Air and Moisture Barrier- these barriers create an airtight blockade between the chambers and the inside of the facility. They must be strong adequate to handle the exterior loads from wind and pressure. Some materials can double the air and water barrier for better sealing characteristics.
  • Drip Edges, Flashing, and Drainage Channels- drip edges and flashes funnel water out of the pressure equalized chambers and run away from the facility. In addition, rainscreen panels may have feature channels that evacuate liquids that drain along the surface or the back sides of the cladding material such as ACM panels.
  • Waterproof insulation- This particular section is optional, and many manufacturers do not utilize them for creating and installing buildings rainscreen panels.
  • Inner Wall Structure

If you want to know more about Rain screen principle, you should read the Rainscreen Construction Principle (Part II) article.