The Benefits and design of Rain Screen Wall Systems Improving Drainage, Image, and Quality of Living

The Need for Better Drainage Traditional Water Resistant Barriers (WRBs) are not fail- proof. They run the risk of not providing sufficient moisture drainage in some cases, especially when not properly installed. Traditional WRBs also deteriorate over time with prolonged exposure to moisture causing an increase in water migration and a deterioration of underlying components.

Water Intrusion 3 Factors must be present for water intrusion to occur in a building enclosure: Water An Opening A Force Drainage Planes minimize the intrusion of water into building enclosure The Drainage Plane is defined as a point at which water is diverted and either drained out or drained downward, away from the remainder of the wall assembly.

Rainwater Control Strategies 3 types of rain water control strategies: Face Sealed Barrier wall Concealed Barrier Wall Rain Screen Wall All three strategies use “drainage planes” in different locations to control moisture penetration.

Face Sealed Barrier Wall Face sealed barriers elude water at the outermost face. No provisions are made to collect or redirect water should it penetrate the exterior wall. Water Entrapment behind exterior face can cause serious problems. Barrier-type EIFS (Exterior Insulation Finishing System) is a common example of face- sealed barrier wall construction

Concealed Barrier Wall Concealed Barriers resist penetration in two ways: Exterior cladding sheds most of the water at the outer surface. It protects against water that manages to penetrate behind the exterior surface with the use of a WRB (water resistive barrier) If water manages to penetrate the WRB, it can trap and hold the water behind the absorptive cladding causing decay.

Rain Screen Walls A Rain Screen resists penetration in 3 ways: Like “face sealed” and “concealed” barrier walls, it sheds most of the water at the outer surface. It intercepts any water that crossed the outer surface with a dedicated air space/capillary break. The space is then vented to the outside to not only assist drainage but to encourage drying as well The addition of an air break provides the wall with more free drainage and drying than the “concealed” barrier, diverting water downward by gravity. If water were to cross the air break, the rain screen system also employs a layer of WRB on the back side of the air space… just in case.

The Rain Screen Wall System Three Types of Rain Wall Systems Factors Affecting Rain Screen Design Advantages and Disadvantages Summery of Critical Features Product Examples

Types of Rain Screen Systems The Drained Cavity Wall System In this system, water penetrating the outer wall must be collected and directed out of the cavity through the use of flashing and weep holes The drained cavity wall system has some aspects of a rain screen but can not be properly called a rain screen wall because it does not address air pressure gradients. If the outer wall is more airtight, then pressure will build in the lower air pressure within the cavity causing rainwater to infiltrate through the interior wall surface and cause decay.

Types of Rain Screen Systems Simple Cladding Rain Screen Wall System on Wood Studs Simple Brick Veneer Rain Wall System on Metal Studs The Open (or Simple) Rain Screen System The open rain screen system differs from the drained cavity wall system in the placement of the air barrier. The outer screen layer is intentionally vented to the exterior, while the air barrier is located at the inner layer or backup wall The inner surface bear the brunt of the wind pressure loads since it is the most airtight. This relieves the pressure gradient across the outer wall, which would otherwise tend to draw moisture inward.

Types of Rain Screen Systems Pressure-equalized Rain Screen Wall Pressure-equalized rain screen walls work with the existing external wind loads to promote pressure equalization within the cavity Pressure-equalization minimizes the force which drives moisture through the outer wall. The external cladding must be able to perform structurally against wind loads until pressure- equalization occurs.

Pressure Equalization Concept When outside air pressure is transferred to an air space behind exterior cladding, the cladding is exposed to a near-zero pressure differential. The air chamber compartments must be small enough, the air barrier system must be airtight enough, and the area of the venting through the rain screen must be large enough to allow sufficient air to move in and out of the compartments under the applied air pressure. The strategy lies in the control of airflow within and through the wall assembly. The higher cavity pressure brought about during gusting wind conditions creates a negative (outward) load on the rain screen. This tends to force water out of openings in the cladding, providing further defense against rain penetration. Perfect pressure equalization across the rain screen at all times is neither achievable nor necessary for adequate rain penetration control. Pressure equalization strategies do not eliminate the need for sufficient drainage and drying capacity. More information and specific guidelines regarding rain screen wall components can be found at : www.cmhc-schl.gc.ca

Advantages and Disadvantages Most reliable choice for applications exposed to significant rain and wind Rain shedding by the use of cladding Drainage and venting to speed the removal of water that passes through the rain screen Air pressure equalization in the cavity to minimize forces drawing water through the cladding, with the cladding also providing a capillary break A second layer of moisture protection at the backup wall Requires less maintenance over its service life compared to traditional rainwater control strategies The detailing and construction to achieve a pressure-equalized rain screen wall or joint may be complex and expensive compared to other wall systems Rain screen wall systems are not always the best solution in every instance – In some climates and some applications, face-sealed walls perform well, and rain screen construction might be unsuitable due to expense, space, structural considerations, etc.

Summery of Critical Features Provision for water shedding at the outer cladding, away from joints, with drips under any projections to prevent water collecting at the building face. A cavity of appropriate width to allow pressure equalization across the cladding system and prevent capillary movement (allow for construction tolerances!). A continuous and effective (for example, airtight to a maximum air leakage of 0.1 L/s/m2) air barrier within the backup wall. Drainage of the cavity through continuous flashings and weep openings, and proper management of drained water. Adequate venting of the cavity provided through properly located openings in the cladding (appropriate ratio achieved between vent area and leakage of air barrier and seals). Additional provision for drainage at the backup wall (located on the “warm” side of the insulation to avoid condensation problems). Effective compartmentalization of the cavity at each building face with airtight seals, and additionally across the width of the façade as required (refer to calculations). Sufficient rigidity and/or structural support of the air barrier to resist wind loads and limit deflection. Sufficient rigidity of the cladding to limit deflection and resist wind loads as required. Special attention paid to water-resistance and drainage at building edges and parapets (areas subject to heaviest rain-wetting and wind pressure differences, where pressure equalization not be achievable).

Dri-Design® Dri-Design is a dry joint, pressure equalized rain-screen, metal wall panel system. Panel manufacturing is highly automated and the installation is fast and simple, which makes Dri-Design an economical choice. More information at: http://www.dri-design.com/company.asp

PaperStone® PaperStone rain screen is made with a natural phenolic resin and 100% recycled paper that is virtually impervious to water. As a result it sheds it quickly on the outside reducing infiltration. For water transported by mechanical forces behind it, the venting and draining holes in the mounting brackets quickly expel or evaporate the moisture. The PaperStone rain screen is a pressure equalized rain screen. This is a system that prevents migration into the inner wall by removing the pressure differential behind the outer wall, making it equal the pressure on the outside surfaces. More information at: http://www.paperstoneproducts.com/ps_rainscreen.php

NorthClad® Exposed Fastener Panel System Tested for air, water and structural performance (ASTM 283, 330 331) Choose painted material or natural finishes including wood grain Cost savings are generated by minimizing routing and forming time Panels can be installed out of sequence and are easily replaced if damaged Complete flashing systems available in matching colors Field modification of most panel materials is possible and simple Vented rain screen design for the health of your building More information at: http://northclad.com/

Copper.org Copper Rain Screen Wall System More information at: www.copper.org

VaproShield® VaproShield Membranes, Rain Screen Design Components and 3D Window Flashing Elements provide an affordable and innovative solution to build a breathable building envelope. Rain Screen Design Components include VaproBatten and VaproVentStrip. Combined with WallShield or WrapShield, these components create a very effective, easy to install rain screen design. They are engineered to ensure trapped moisture moves quickly through the building envelope ensuring maximum ventilation within drying cavity.  More infromation at: http://www.vaproshield.com/media/A new standard for rain screen design FINAL.pdf

The End. Project Name: Emergency Operations Center Location: Springfield, Illinois Architect: DeStefano and Partners, Ltd. Contractor: River City Construction, LLC Copper Applications: Exterior rain screen Recourses: http://www.greenhomebuilding.com/pdf/RainScreen.pdf http://www.toolbase.org/Design-Construction-Guides/Exterior-Walls/rain-screen-designs http://www.aaa.ab.ca/pages/members/media/RainScreenWallQAwa.pdf