Seven Ps for successful rainscreen design and execution

by Katie Daniel | February 6, 2018 3:07 pm

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by Stephen J. Scharr, Esq.
Employment of metal rainscreen exterior envelopes in order to keep buildings tight and dry surpassed many barrier wall construction methods in the United States during the last two decades. In spite of the industry’s increased focus on metal rainscreens, one can still see design flaws in project detailing.

This article’s purpose is to not only highlight the design flexibility and positive attributes of solid metal rainscreen systems, but also help the design team focus on recurring quality issues. Focusing on the subtle nuances of detailing during the management of high-risk areas of a wall system that permit moisture to enter the wall cavity is critical for ensuring a dry and comfortable interior. Controlling moisture in the cavity and mitigating the risk of penetration is a paramount consideration for any successful project.

Advantages with metal rainscreen design
There are many good reasons to choose a metal rainscreen system over a more traditional barrier wall design. Open venting of the space between the primary air barrier and the outer leaf of the façade promotes drying, avoids trapped moisture in the wall cavity, and prevents mold growth. Further, pressure equalization from the wall’s exterior minimizes the negative pressures that promote ‘sucking,’ or capillary action inside the cavity. Metal rainscreen systems prevent the pull of moisture—through defects or tears in the air barrier—into the building’s interior.

These systems are also advantageous because installing any water-resistant insulation products outboard of the air barrier increases the continuity of the insulation layer. It also improves thermal performance by limiting heat transfer from the secondary furring system at the thermal layer.

Metal rainscreens can also eliminate the need for exposed exterior sealants and gaskets, which are subject to ultraviolet (UV) degradation, create dirt streaks on exterior façades, and can be costly to maintain.

Design flexibility
For projects of any scale, design visions can be carried out with aluminum, zinc, or stainless steel plates, available in panel sizes exceeding 4.7 m² (50 sf) per unit or in plank styles as small as 0.18 m² (2 sf) per unit. (The panels can be 1524 x 6096 mm [60 x 240 in.] or 1823 x 6069 mm [72 x 240 in.]. The plank-styled plates are available from 25 x 610 mm[6 x 24 in.] to 914 x 4572 mm [36 x 180 in.].) Unlike masonry, exterior insulation and finish systems (EIFS), and other materials, solid metal panels do not absorb moisture.

The embossing of textures on rainscreen panels also affords designers an endless finish palette to choose from. The panels can be colored and finished using polyvinylidene fluoride (PVDF) coatings or through anodizing processes and forming techniques to create bold or subtle shadow patterns.

Additionally, a uniform, high-quality air/water barrier system at the drain plane permits the designer to mix and match exterior rainscreen cladding products (e.g. brick, metal, terra cotta, ultra-high performance concrete [UHPC], and fiber cement panels) to achieve a variety of façade textures.

Forming/assembly options
Solid metal rainscreens can be assembled using large-format flat plates installed on an engineered extruded aluminum frame platform. This method enables designers to provide a ‘floating plate’ appearance with crisp edges, clean lines, and bold reveals. The most popular panel joint options are:

• spline-reveal perimeter;
• hook and pin; and
• stacking male/female joinery.

The panels can also be curved or segmented over vertical walls or undulating surfaces (Figures 1 and 2).

Metal is flexible and can be formed to an array of edge conditions and radii to create softer edges and projecting features. Formed metal panels are usually produced with welded corner folds and miters. They can be polished smooth prior to finishing, thereby presenting a seamless edge appearance. This method of forming is often employed in projects where occupants and pedestrians could come into direct contact with the panels (Figure 3).

Prefinished aluminum, copper, stainless steel, and zinc are all highly economical choices for repetitive, smaller-format panel profiles. In this ‘plank-style’ configuration, edges are folded into interlocking profiles to reduce the number of components, which in turn brings down installation costs. Additionally, prefinished colors can be mixed and matched to create random or repeating color schemes in a highly efficient manner (Figure 4).

Solid metal plate
Metal plate offers a high degree of impact/scratch resistance when subjected to the wear and tear typical for building façades. This option also provides a highly durable alternative for entry portals and other surfaces in close proximity with pedestrians and occupants.

If damaged beyond repair, individual panel sections can be easily removed and replaced. Figure 5 provides a good reason to select solid metal as a substrate for rainscreen designs. The photo shows metal composite material (ACM) panels—thin sheet metal outer facings bonded to plastic cores—are prone to denting.

Solid metal plate is noncombustible and nontoxic when exposed to flame. It does not contain volatile organic compounds (VOCs), and no plastics or petrochemical feedstocks are employed in constituent components. Produced from homogenous naturally occurring ores, the material is recyclable at the end of its life cycle.

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Design and execution: The seven Ps
The positive attributes of solid metal rainscreens can be rendered ineffective by insufficient attention to the proper relationship, product selection, detailing, and installation of all system components. Often, the level of detailing to properly install the exterior façade is left to the contractor.

Many design firms prepare outline specifications with aesthetic attributes, performance criteria, and a requirement for delegated design from the contractor who is responsible for the exterior building envelope. Whether the recommended disciplines are established in the contract documents or at shop drawing review, the same principles will apply.

When one is cognizant of the following Ps, a successful project is more likely to occur.

Product selection
Selecting the right system for the type of construction is vital to a project’s success. When selecting a large-format panel—large stainless steel or aluminum plate—attention must be paid to the attachment points for the exterior metal panel assembly.

When a large surface area panel is employed in cold-formed steel (CFS) framing or concrete masonry unit (CMU) backup wall systems, light-gauge metal framing may not be sufficient to resist the negative loads imparted by the fasteners.

Light-gauge studs or Z-furring sections are perfectly adequate to resist the loads for exterior-grade sheathing and siding when they are attached in a 406 x 406-mm (16 x 16-in.) grid. Large metal rainscreen panels, however, are often fastened through the sheathing into the underlying substrate (stud or CMU). If a panel is 1524 x 4572 mm (60 x 180 in.), the large tributary surface area creates significant negative pressure on the fasteners at the panel’s perimeter. Therefore, it requires higher than normal pull-out values for the fastener to stud/CMU connections.

Manufacturers should be consulted to estimate the point loads that can be applied to exterior substrates when selecting a large-format exterior wall design. The same is true when the design lends itself to offsite assembly or unitization, where larger surface areas are pre-assembled onto a secondary support frame and hung on building-mounted anchor clips. The wall supports should
be configured to structurally anchor the walls at slab edges or to providea reinforced backup wall at unit connection points.

For both 3D and flat walls, the primary building enclosure air barrier is inside the outer leaf. As a result, using metal rainscreens permits the designer to be more creative when planning non-flat, articulated exterior façades. Whether the exterior façade is flat or undulating, the backup wall should be vertically oriented to not interfere with the function of the drain plane. The metal rainscreen panels must be properly stiffened and reinforced to withstand positive and negative loads within each unit.

Figure 6 offers an example of a 3D articulated feature located on an internally reinforced rainscreen backup wall system. Although the panel face is projecting outward, the drain plane at the face of the sheathing is vertically oriented. In this way, the moisture entering the panel faces drains vertically without any interruptions. Any larger projecting features (e.g. sunshades and projecting window brow assemblies) should be similarly treated with a reinforced structural sub-assembly while still maintaining a relatively vertical drain plane.

Maintaining a vertical and continuous drain plane should always be a priority in metal rainscreen designs. Figure 6 also provides a comparison of a supplemental support with an internally reinforced projecting assembly, which permits the drain plane to be vertical and continuous.

A projecting panel or feature such as sunshade, slab-edge projection, cornice features, or projecting brows can be large enough to turn into a snow or ice-collecting shelf, thereby creating a live load condition. These conditions may need extra reinforcement or strengthened connection points embedded within the wall structure to permit additional loads without interrupting the drain plane. The projecting feature should be structurally analyzed for all design loads and also reinforced by adding concealed stiffeners, gussets, or assembly methods, and thereby creating a stiff and structurally adequate unit. The addition of stiffeners or gussets is far superior to employing structural concrete or miscellaneous steel projections that can penetrate or interrupt the drain plane.

Penetrations in the air vapor barrier
Any rainscreen wall cladding design is only as good as the existing condition of the air vapor barrier system covering the drain plane. Best practice is to minimize and manage the number and type of air barrier penetrations to avoid leaks into the interior cavities of the wall system. The most successful outcomes can be achieved when the design professional identifies, details, and assigns those penetrations with the correct sealant and repair information. This should be done for each type of penetration, either through performance specifications and detailing or both. The rainscreen system should at least be specified so the exterior façade installer either controls the attachment for the secondary supports and thermal layer connections through the air barrier or the exterior façade contractor supplies/installs the project’s air barrier system in an integrated and coordinated approach with the exterior leaf rainscreen systems
(Figure 7).

Perimeters
Flashing and sealants at the perimeter conditions of the metal rainscreen assemblies, which are interrupted with the fenestration (i.e. windows, louvers, doors, and curtain walls) or termination of other adjacent wall systems, is a critical weak point. The edges of the assembly are most exposed to the elements, particularly wind-driven rain. Perimeter details must be designed to:

• protect from the battering effects of wind-driven rain and UV exposure to maintain longevity (Figure 8);
• divert drain plane surface drainage from the areas above the opening or penetration, around the penetration, or toward the outer surface of the system (flash openings from the bottom up—sills and jambs should be shingled under the head conditions [Figure 9]);
• place sealants and flashing interface properly with the air barrier and
  the seals for the fenestration that are independent of the rainscreen leaf elements (formed metal flashing should be ‘tied into’ or ‘stripped into’ the air barrier system with the appropriate flexible flashing or applied sealant material to maintain continuity of the air barrier and seal any fastener penetrations from the flashings [Figure 10]);
• avoid sealant details applied to dissimilar materials to ensure the sealant joints are not stressed by differing adhesion properties or thermal expansion and contraction coefficients; and
• allow access to exterior sealants for future inspection and replacement (blind seals are difficult to locate, diagnose, and test in the event of a wall failure).

Parapets
The proper treatment of skyward-facing parapet copings or projections is a common oversight. When a designer selects a rainscreen façade system, there seems to be a temptation to design a ‘dry-joint’ parapet or brow for appearance’s sake. This practice should be avoided due to the amount of water that can enter the wall system, which must then be drained. The best technique is to continuously seal the top of the wall—preventing a buildup of water into open spaces of the system. Exposed skyward-facing joints must be sealed with an exposed sealant joint. For extra longevity and protection, a continuous substrate and flexible roof membrane can be installed under all horizontal surfaces large enough to collect rain or snow. Eventually, all sealant joints fail and render any horizontal surface vulnerable to moisture intrusion (Figure 11).

Pre-installation preparation and testing
In addition to attentive detailing and notations, performance-quality procedures must be ensured by designating a mockup area on the building, or offsite for third-party lab testing. Every project is different. Testing actual job-specific conditions is the best way to catch interferences, sequencing problems, and coordination conflicts. One can conduct iterative field-testing of in-place sample areas of work (e.g. rainscreen walls and fenestration with perimeter flashings and sealants) or lab mockups to identify trouble spots.

For a wide variety of field quality performance tests and range of project designs and budgets, one can refer to Architectural Testing Labs’ array of American Architectural Manufacturers Association (AAMA) and ASTM performance testing protocols. (Visit www.archtest.com[13] for more information on the testing protocols.) Even a relatively inexpensive AAMA 501.2, Field Hose Test, can help identify trouble spots prior to installation. (This test method is often used on rainscreen wall projects to test the air barrier, flashing, substrates, and fenestrations as well as their perimeter conditions. It is also commonly employed in rainscreen wall assemblies, including the wall substrates, air/water barrier combination as well as any fenestration assemblies [windows, doors, and louvers] within the designated wall area. It is particularly useful to analyze the perimeter conditions as they interface with the wall assembly.) Diagnosing problems and fixes after the project is underway can cost owners a lot of time and money.

Iterative testing is important to ensure subsequent steps during the installation process do not impair or impact the performance of previously installed sub-assemblies. The author found the following iterative testing to be useful without being cost prohibitive:

• test the quality of the specified air barrier after application and cure
  on a flat wall surface without any supplemental furring, anchor cleats, or perimeter flashings;
• test the air barrier and penetration conditions of the assembly after installation of any components requiring attachment through the air barrier field and subsequent repairs such as furring, anchor cleats, or perimeter flashings;
• test air barrier, perimeter flashings, and sealants at one or more window areas after all field repairs of penetrations and perimeter flashings are installed and cured, also using the air barrier manufacturer’s recommended patch methods for fastener and clip penetrations; and
• test the final assembly of the rainscreen components after the installation of the insulation, decorative rainscreen trims, and rainscreen-facing panels.

Positive drainage
One of the most common errors seen in metal rainscreen design is the unnecessary interruption of the drain plane by supplemental framing for projecting elements or slab-edge projections. Protecting the air barrier from penetrations and changes in direction decreases the risk of costly maintenance. Whenever possible, arrange attachment clips and fastener penetrations in the vertical surfaces of the wall substrate to promote good drainage and migration to the exit points of the wall system. Use intermittent anchor clips in lieu of continuous furring sections for metal rainscreen panel attachment. This way, moisture can traverse around the interruptions and not collect at or near fastener penetrations (Figure 12).

To ensure positive drainage along the drain plane, it is important to maintain an air space within the system for venting.

Proper ventilation is important for all types of rainscreens. Façade design must have good weeps and channeling to allow moisture to collect, drain vertically, and exit the system at the designated flashing points. These can occur at the head of any fenestration, the base of the wall system at grade, or projecting floors/roofs (Figure 13). (For a discussion on the wonders of venting, read the article “The Ins and Outs of Metal Rainscreen Wall Systems,” written by JosephW. Lstiburek, PhD, P.Eng., an American Society of Heating, Refrigerating and Air-conditioning Engineers [ASHRAE] Fellow, republished from an ASHRAE journal in May of 2008 through www.buildingscience.com[18]. You can read it at www.smacna.org/resources/resource/2016/03/10/the-ins-and-outs-of-metal-rainscreen-wall-systems-simpson-gumpertz-heger[19].)

On high-rise buildings, supplemental exit points may be necessary. Accumulated moisture needs a variety of exit points to avoid the possibility of overwhelming the drainage space. Flashing this moisture to exit at every other floor is a good practice on large shear walls (Figure 14).

Performance
Perhaps the most important of all considerations during the design development of a façade is to ask what performance characteristics are essential for the client and project, set in a particular community. Some of the important performance considerations include:

• thermal performance;
• longevity of materials and resilience;
• sustainability of the design elements, as well as any constituent components;
• impact resistance/durability;
• UV resistance;
• combustibility; and
• toxicity.

A solid metal façade is highly engineered and allows the project team to meet all these criteria. Metal rainscreens also adapt well to continuous insulation (ci) and air/water barrier techniques, which are continuously evolving.

Overall performance of a rainscreen wall system is usually defined in the specification documents by the levels of moisture and air intrusion permitted to enter the cavity behind the outer metal layer, into the thermal cavity, and back to the drain plane. Open-joint rainscreens or drained and back vented are very popular because designers are given more freedom to use open-joint reveals and articulations. These designs are based on the premise the primary air and water barrier is at the drain plane.

This style of performance is defined in testing under AAMA 509, Voluntary Test and Classification Method for Drained and Back Ventilated Rainscreen Wall Cladding Systems, test protocols, where water and air intrusion past the rainscreen is permissible up to a point. This testing assists in a third-party monitored measurement for a comparison of one system to another. There is no pass or fail criteria—the testing is merely a measurement regime involving qualitative analysis of air and water intrusion past the outer layers of the rainscreen components and back to the air and water barrier.

A more conservative approach is to follow the pass/fail protocols that are found in AAMA 508, Voluntary Test Method and Specification for Pressure Equalized Rainscreen Wall Cladding Systems, which dictates prescriptive permissible limits for air and water intrusion and pressure equalization intervals, accompanied by dynamic structural testing. Systems passing this stricter testing are considered PERS-certified and are generally more costly to produce and install. The extra components necessary to baffle wind-driven rain, vent, and provide guttering for moisture management behind the outer leaf lead to this increased cost. For the same reasons, these systems may also require additional wall thickness space. Selecting the appropriate level of performance for the project is important. (For an in-depth review of these two test protocols—pressure equalized AAMA 508 vs drained and back vented AAMA 509—there is a handy comparison bulletin published by Architectural Testing, a high-quality third-party independent testing lab. Read it at buildingscience.com/documents/insights/bsi-004-drainage-holes-and-moderation[20].)

Conclusion
The design and installation of solid metal rainscreen systems requires a high level of technical attention and experience, and there are many critical details to consider. The seven Ps discussed in this article can help ensure a successful design and consistent installations. Offering high performance, longevity, sustainability, and design options, solid metal rainscreens can help solve the common façade issues of the past. Architects and designers seeking a product for moisture mitigation, pressure equalization, air and water barrier, thermal performance, and low maintenance can consider metal rainscreens.

Stephen J. Scharr, Esq. is a business development professional with Metalwërks, a manufacturer of architectural metal façades and features. He was president of Metalwërks for more than 20 years. Scharr is also a licensed attorney specializing in construction-related issues. He has provided legal liability and contract consulting services for manufacturers and specialty contractors. In 2014, Scharr co-authored a section of the book, Construction Subcontracting: A Comprehensive Practical and Legal Guide, published by the American Bar Association (ABA). Scharr is a member of ABA’s construction forum and the Pennsylvania Bar Association. He can be reached via e-mail at stevescharr@metalwerksusa.com[21].

Source URL: https://www.constructionspecifier.com/seven-ps-for-successful-rainscreen-design-and-execution/

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