What is a Rainscreen in Building Construction?

Watertight facades are a paramount feature for achieving building durability. These facades are essential features to improve the moisture resistance of the building envelope thereby protecting its structural integrity and preventing deterioration of exterior surfaces.

Rainscreen is one such multi-layered façade system to achieve watertight facades that are becoming popular in protecting buildings.  Rainscreen is found effective in controlling water or moisture penetration into the building and suits perfectly to the current sustainable architecture standards.

This article comprehends the features and working of rainscreen façade systems implemented for various buildings and their types available.

What is a Rainscreen?

Rainscreen is an exterior cladding system that is installed away from the building’s exterior wall to protect the structure from moisture penetration and thereby protect the structural integrity of the building.

Rainscreen can be picturized as a raincoat for the building whose primary objective is to act as a barrier to block water penetration.

Image Credits: Advanced Building Products

Water can enter the building through holes, joints, gaps, micro-cracks, or even through porous building materials. The four forces that play a role in letting water penetrate into the building include:

➤Kinetic forces (Wind driven rain)
➤Surface Tension (Water enters due to pressure difference between the outer and inner walls)

➤Capillary Action (Combination of surface tension and the adhesive force between the water and building materials, when they come in contact)

Rainscreens can effectively fight against the water that has a chance to enter the building through all the above forces. Among these, the water penetration through capillary action is found to be more severe.

Hence, a properly designed rainscreen cladding or wall system serves the purpose of controlling,

1. Water infiltration
2. Ultraviolet radiation
3. Negative wind pressures
4. Heat transfer into and out of the building
5. Air infiltration
6. Vapour Transmission

What is the Principle Behind Rainscreen?

The rainscreen cladding system undergoes double wall construction, where the outer layer keeps the rainwater out. A cavity is created between the outer wall and the inner wall. This drained and ventilated cavity eliminates any moisture penetrated by natural drainage and evaporation.

Rain Screen System
Image Credits: SFS USA

Components of Rainscreen

A better understanding of rainscreen systems can be achieved by studying their basic components. 

Working and Components of Rainscreens

These include:

1. Cladding: The outermost layer (outer wall) of a rainscreen system forms the wall panel cladding as shown below. It not only protects the building against the wind but also protects from negative wind pressure and ultraviolet radiation. The cladding can be made using aluminium, steel, terracotta, timber, and slate. 
2. Vented Air Space: This is an air cavity just behind the outer wall cladding that promotes residual water drainage and airflow behind the wall panel. This facility helps dry out any excess water that gets through and ventilates moisten-laden air within the cavity.
3. Insulation: The insulation or the weather barrier is assembled after a fixed air space as shown in the figure below. This barrier helps to reduce the heat flow into and out of the building. An effective and energy-efficient design would prefer the application of insulation as a continuous layer throughout the exterior side of the air barrier. 
4. Air Barrier: Any residual amounts of water that bypass all these layers will be stopped by the air barrier. When the water reaches the air barrier, it is either drained or will be evaporated out of the wall.
5. Structural Wall: This wall is the main structural surface of the building to which all these above elements are attached. This is the structural wall that needs to be protected from these external weathering factors. 

Working of Rainscreen

As mentioned before, a rainscreen cladding system consists of main structural wall, an insulation layer, a mounting track for fixing cladding. The whole arrangement also involves air cavity in between for proper ventilation. This is called as rainscreen drainage plan.

A rainscreen drainage plane is essentially a gap and a water-resistant layer that helps water drain out of a wall. It creates a clear path for water to travel from where it enters the wall (usually at the top) to where it exits (usually at the bottom). This system moves water out quickly to prevent problems like rot, mold, and damage to the wall's structure. The drainage plane is designed to direct rainwater or condensation downward and outward, stopping water from getting inside the building's living or working spaces.

In barrier wall systems, the outer layer of the wall (cladding) acts as the main barrier against rain. In cavity wall construction, the main drainage layer is inside the wall cavity. This layer is usually on the inside surface of the air space, and if the wall has insulation, the drainage layer is on the outside surface of the insulation. 

In summary, a rain-screen drainage plane ensures water drains out of a wall efficiently, keeping the inside dry and preventing damage.

Types of Rainscreen

There are three main types of rainscreen systems: vented, drained and ventilated, and pressure-equalized (PE). Each serves a specific purpose and is suitable for different building heights and conditions.

1. Vented Systems are cavity open at the bottom only, providing drainage but no convective ventilation. This system uses a warm cavity, preventing ventilation and ensuring a low risk of moisture penetration. Although commonly referred to as "vented," this system is essentially a "drained" system since it does not allow for ventilation.                                                                     
• Vented rainscreens are typically used with a render and should not be used on buildings over four stories high due to the risk of water penetration and high wind loads, which can cause condensation build-up that the system cannot dry out.                   
2. Drained and Vented Systems are not watertight and require the cavity to be open at both the top and bottom. This allows for both drainage and convective ventilation. The cavity must be protected with an overhang, and the backing structure should have a moisture barrier.              
•  Insulation should be non-hygroscopic or closed cells, like PIR or Rockwool. This system operates with a cold cavity structural build-up and is best used for buildings less than 12 meters in height. 
• Similar to vented systems, drained and vented systems should not be used on buildings over four stories to prevent condensation build-up and potential panel removal by high wind loads.                                                                                                                                      
3. Pressure-equalized (PE) Systems have compartmentalized cavities and openings calculated to allow rapid air movement. This design equalizes the internal and external pressures, reducing water ingress.

•  PE systems are drained above compartment levels and are suited for taller buildings. This system allows wind to blow in and behind the rainscreen panels, equalizing the pressure and resulting in low wind load on the panels themselves, with the remaining load transferred to the structure.
•  PE systems are commonly used on buildings over four stories and require precise design according to standards like BS6399-2-1997 to handle correct wind loadings.

Difference Between a Curtain wall and Rainscreen

Aspect	Curtain Wall	Rainscreen
Definition	Outer covering of a building with non-structural outer walls.	Cladding system with a ventilated air gap behind the cladding.
Construction	Made of lightweight materials like glass, aluminum, or metal panels.	Consists of outer cladding, a ventilated air gap, and an inner moisture barrier.
Function	Provides weather barrier, structural support, and aesthetic appearance.	Manages moisture through drainage and ventilation, offers pressure equalization.
Weather Barrier	Acts as a barrier against wind, rain, and other weather elements.	Outer layer deflects rain, air gap allows for drainage and evaporation.
Moisture Management	Relies on seals and barriers to prevent water ingress.	Uses dual-layer approach for moisture management with drainage and ventilation.
Air Gap	No ventilated air gap.	Features a ventilated air gap behind the cladding.
Breathability	Sealed system, not designed to be breathable.	Breathable system allowing air circulation to dry out moisture.
Sealing	Joints between panels are sealed with weatherstripping, gaskets, and sealants.	Utilizes the air gap and inner barrier to handle residual moisture.
Application	Commonly used for large, glazed facades on commercial buildings.	Used for residential and commercial buildings requiring robust moisture management.
Structural Role	Does not bear any structural load except its own weight and environmental forces.	Cladding does not bear structural loads, focus is on moisture management.
Appearance	Often features large glass panels for natural light and views.	Can use various cladding materials, less focus on transparency.

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