Rainscreen Facades

At first glance, the concept of a rainscreen façade is as simple as its name; it’s an exterior wall with an additional ‘screen’ of protection from wind driven rain.  Numerous wall assemblies and cladding materials are commonly classified as rainscreen facades.  Within this broad category there are important distinctions that characterize different types of rainscreens.  Here is a more detailed look at the make-up of such systems.

Context

To put a rainscreen into context, there are four conceptual approaches to keeping water out of a building.

1. Mass:  A traditional solid assembly sheds most water and absorbs the remainder.  (i.e. solid concrete or masonry)
2. Barrier:  A surface that completely sheds water.  (i.e. EIFS or Glass)
3. Internally drained:  An assembly in which a moisture barrier is placed between the structure and the cladding.  (i.e. siding with housewrap)
4. Rain screen:  An assembly consisting of cladding, air cavity, drainage plane and an airtight supporting wall.  A basic example is a brick cavity wall.  Other examples include thinner cladding systems such as metal or terra cotta panels.

Rainscreen Basics

In the broadest sense, a rainscreen is an exterior wall that pulls the building’s cladding away from the water-tight layer leaving a drained and vented cavity in between.  The cladding provides an extra line of defense that prevents most rain from reaching the air cavity and moisture barrier.  However, wind driven rain can work its way beyond the rainscreen cladding.  To address this issue, the cavity is ventilated and drained.  The air cavity mitigates some of the forces that drive water into the façade such as capillary action and surface tension.  By the broadest definition, any cavity wall system is a rainscreen.  However, the following two categories of rainscreens are defined by the design of the air cavity.

Two Types of Rainscreens

Both of the following two types of rainscreens employ open joinery, vents or weeps that can allow some moisture to penetrate beyond the outer cladding. Basic rainscreens are called Drained / Back Ventilated Rainscreens.  The system provides the basic benefit of a rainscreen – the deflection of most water from the moisture barrier.  These assemblies have a continuous drained and vented cavity to drain or dry-out any water that penetrates into the cavity.  One key concern in drain / back ventilated systems is the continuous air space.  As wind enters the continuous cavity, air pressure differentials can vary significantly between the exterior and the cavity.  If air is allowed to flow from high pressure areas to low pressure areas, it can potentially bring water with it.  Continued air pressure differentials can force water into joints in the moisture barrier.  To address this pressure differential, Pressure Equalized Rainscreens attempt to quickly equalize air pressure within the cavity.  This is achieved by creating smaller compartments within the cavity.  With this approach, air pressure equalization can be achieved in each compartment, even if air pressures vary across the entire building.

Pressure Equalized Rainscreens: More Information

A key component to achieving pressure equalization is careful joint design at the rainscreen layer.  Joints should be small enough and of such geometry that little or no water gets past the rainscreen; however, they should also be large enough to allow air to flow.  Another critical component in the assembly is the air & vapor barrier system.  To reduce air pressure in the cavity, a proper air & vapor barrier at the back-up wall is required.

Pressure Equalized Rainscreens: Compartmentalization

The smaller compartments that allow pressure equalization are critical to the performance of pressure equalized rainscreens.  There is debate as to the proper size of compartments, but in general compartments should be smaller at locations susceptible to varied wind pressures such as corners and parapets.  Compartment seals must be robust and strong enough to resist the pressure differences from one cavity to another.  The following details depict typical horizontal and vertical connections that create compartmentalization in pressure equalized systems.

Maintenance

It is important to acknowledge that in the event of a leak, repairs will require dismantling a portion of the façade.  A rainscreen system cannot be repaired by applying sealant outermost layer.

Benefits of Rainscreen

• Can reduce heating and cooling losses
• Can Minimize condensation
• Proper airflow in the cavities minimizes potential mold growth
• Properly designed and installed rainscreen systems can be very low maintenance

Air & Vapor Barriers

A critical component of the rainscreen principle is the air and moisture proof membrane located inboard of the ventilation cavity.  This ‘last line of defense’ against weather and moisture must perform as designed.  Air barrier materials are identified by the level of permeance.  To be an air barrier the air permeance must be equal to or less than 0.02L/(s•m2) @ 75Pa.  Two common types of combined air & vapor barriers for rainscreen applications are surface adhered and liquid applied.

• Surface Adhered
  • These systems are self-adhering membranes consisting of rubberized asphalt adhesive.  For consistent substrate surfaces, these simple peel & stick systems provide continuous air/vapor barrier in above grade wall assemblies.
• Liquid Applied
  • These systems are applied by spray or roller on CMU walls with masonry ties or other areas where using a sheet membrane is difficult.  The assembly is a water-borne, rubberized asphalt

These two types of air & vapor barriers may be combined when the air barrier must adapt to various substrates or geometries along the same façade.  An excellent summary of the critical building science related to air & vapor barriers is available at the Whole Building Design Guide: http://www.wbdg.org/resources/airbarriers.php

Sources

Architectural Record: Rain-Screen Facades Are More Than Skin Deep

http://archrecord.construction.com/resources/conteduc/archives/0702edit-1.asp

Understanding ‘The Rainscreen Principle’

http://www.edcmag.com/EDC/Home/Files/PDFs/mca07_Rainscreen.pdf

Toolbase.org: Rain Screen Exterior Walls

http://www.toolbase.org/TechInventory/TechDetails.aspx?ContentDetailID=1020

Carlisle Coatings and Waterproofing

http://www.carlisle-ccw.com/findsolution/system.aspx?pl=3

Air Barrier Association of America

http://www.airbarrier.org/views/design_e.php

Comments

[Peter]

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