More than one way to skin a building

April 9, 2018

All images courtesy Simpson Gumpertz & Heger

by A. Judson Taylor, RA, and Christopher W. Norton, PE
Prior to the mid-20th century, building walls relied on their thickness and density to resist water penetration. Moisture would mainly deflect from the wall face or be absorbed and later evaporate from the mass wall. Air leakage was only managed to prevent uncomfortable drafts at doors and windows. Walls relied on thermal mass rather than discrete insulation and were not, therefore, vulnerable to steep temperature gradients resulting in planes for condensation.

Development of lightweight “contemporary curtain walls” in the mid-20th century, and subsequent energy conservation concerns, resulted in the need to provide discrete insulation (i.e. thermal barrier) which cannot be left exposed and requires protection (or at least architectural cover in the case of mineral wool). (Contemporary curtain walls are supported by, rather than part of the building primary structure. They comprise opaque, concealed barrier with cladding and face barrier cladding as well as glass and metal fenestration also referred to as “curtain wall.”) Prior to the advent of modern high-performing sealants, lightweight cladding was not up to the task of protecting insulation from water intrusion. The industry, therefore, evolved to develop dedicated water-resistive membranes, starting with roofing felt and expanding over time to a myriad of sheet and fluid-applied products serving as air- and water-resistive barriers (A/WRBs). Insulation created more drastic temperature gradients within walls, which drove the need to use air and vapor barriers for preventing moist air from reaching the condensation planes.

In short, building exterior walls progressed from general purpose, low-performing, but predictable mass walls to a variety of wall assemblies comprising four barriers—water, air, thermal, and vapor. (For more, read the article “The Four Barriers: Concepts for proper wall design and construction,” by Brent Gabby, PE, and Vince Cammalleri, AIA, in the August 2016 issue of The Construction Specifier.) Mass walls generally worked in all temperate climates with little customization. Contemporary concealed-barrier walls (as well as insulated face-barrier walls, which resist weather at their exposed face), however, must be carefully designed for the local climate and interior conditions. (According to Architectural Graphic Standards, Element B: Exterior Enclosure, “Face-sealed Barrier Walls rely on a perfect continuous seal at the exterior face.” “Drainage Walls” [also known as concealed-barrier walls] “resist air and water penetration with an outer layer to block bulk precipitation and an inner water barrier.”)

Initially guided by trial, error, and tradition, designers tended to employ relatively simple concealed- or face-barrier walls systems. However, from the late 20th century, buildings have increasingly employed multiple systems including face- and concealed-barrier assemblies.

Building with adjacent concealed- and face-barrier wall systems
It is has become increasingly common for high-end buildings (i.e. prominent institutional and commercial structures) to incorporate face-barrier elements (e.g. precast concrete) into mostly concealed A/WRB and rainscreen façades and vice versa. By definition, the A/WRB in face- and concealed-barrier walls is in different plane; their thermal and vapor barriers may be as well. The interruption of the continuity of the concealed A/WRB as well as thermal and/or vapor barriers of adjacent face- and concealed-barrier elements presents unique challenges for designers.

Air and water-resistive barrier requirements
Section 1403.2, “Water-resistive barrier,” of the 2018 International Building Code (IBC) establishes exterior walls must protect the building interior from weather by “providing a water-resistive barrier behind the exterior veneer,” as well as provide protection “against condensation in the exterior wall assembly.” WRB may be any approved system but must, at a minimum, be equivalent to a single layer of No.15 asphalt felt.

Section C402.5, “Air leakage–thermal envelope,” of the International Energy Conservation Code (IECC) requires the exterior walls of the building have an assembly of materials and components to limit air passage. Compliance may be achieved through different ways—by following prescriptive requirements of 402.5.1 through 402.5.8 for each material and system or by whole building air leakage testing per ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, or by a method approved by the building official. In general, the intent is to restrict bulk air movement between the interior and exterior environments.

Acceptable wall coverings are established in Table 1405.2, “Minimum Thickness of Weather Coverings” of IBC and include both adhered and anchored masonry veneer, panels, stone, a variety of metal panels and fiber cement siding, and conventional stucco/cement plaster. The concealed A/WRB materials behind these coverings can include any number of fluid-applied, mechanically attached, or self-adhered materials over a back-up wall (e.g. sheathed steel stud framing, concrete, or concrete masonry units [CMUs]).

While the default Code definition (1403.2, “Water-resistive barrier”) of exterior walls is a drainage wall with a concealed-barrier system, IBC allows exceptions including concrete and masonry walls, exterior insulation and finish systems (EIFS), and other assemblies passing ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. Typical systems able to meet the requirements
of this last category include:

To prevent air and water intrusion, these face-barrier walls rely on inherent density of the finish material or an applied continuous coating. With the exception of masonry and architectural cast-in-place concrete walls, these systems typically rely on single- or dual-stage sealant joints at penetrations and boundaries to maintain air and water resistance.

Face- and concealed-barrier wall systems use different strategies to meet the Code performance requirements for exterior walls. The Code does not, however, prohibit using both systems in the same area. At transitions, designers must reconcile the differing air, water, thermal, and vapor management strategies of adjacent systems.

Insulation and condensation control requirements
To protect the structure against condensation, Section 1405.3, “Vapor retarders,” of IBC requires the installation of vapor retarders as described by the Code or as part of an approved design using a hygrothermal analysis (the analysis may determine a vapor retarder is not necessary. The placement of the vapor barrier in both wall types requires coordination with the location and amount of insulation, dictated by Code in either a prescriptive or a performance-based approach. Similar to air and water management strategies, face- and concealed-barrier systems differ in the location and type of materials employed to achieve thermal and condensation performance requirements.

In a concealed-barrier assembly, the insulation may be placed inboard and/or outboard of the A/WRB. Conversely, insulation must be inboard of the air and water barrier in a face-barrier cladding system.

Where insulation aligns in adjacent face- and concealed-barrier walls, locating the vapor barrier (if required) is relatively straightforward. Yet, if the insulation is on opposite sides of the A/WRB, different vapor barrier locations in the wall assembly will be required.

Transitions of the A/WRB between face- and concealed-barrier walls complicate and may prevent insulation and vapor barrier continuity. In the absence of insulation or vapor barrier continuity, cold bridging or vapor bypass may occur. Combining these systems, therefore, requires evaluation at the transitions to negotiate the impact of thermal bridging or possible gaps in the vapor barrier.

Offsets and discontinuity in the A/WRB plane
A building—for aesthetics, structural attachment, or constructability reasons—may not rely on a single wall type across the entire façade. In transitions between a face-barrier system to one with cladding outboard of a concealed barrier, the A/WRBs will be at different planes in each assembly. The detailing of these transitions can be challenging, but is crucial for maintaining the building’s overall performance. A barrier system passing ASTM E331 must include transitions in the test specimen, and the assembly’s manufacturer often has typical details to illustrate these transitions. These are a good place to start in designing these transitions. However, each system is different, and the designer must take the differing geometry and materials for each assembly into account. As face-barrier systems typically rely upon sealant joints at the perimeters, the transitions can usually be achieved by integrating flashing elements with the air barrier as substrate for the sealant joint (Figure 1).

The designer must also consider transitions to other enclosure systems, including roofing, below grade, and plaza waterproofing. Figure 2 illustrates a transition from a face to concealed-barrier wall at a roof parapet.

Combined fenestration termination to face and concealed A/WRB
With fenestration, the best air- and water-resisting performance is achieved by aligning the plane of the A/WRB with the inner face of the fenestration element’s “wet zone,” which is the glazing pocket or frame element collecting and draining exterior water. The inner plane of the wet zone is just inboard of glazing pocket for curtain walls (i.e. pressure bar systems) and at the back edge of the frame for storefront and sill can fenestration; prefabricated windows with block frames are similar to storefronts unless they have a nail fin.

If the fenestration is surrounded solely by face- or concealed-barrier façade, the sealant joint can be set in one continuous plane. Combining the face and barrier façade elements at one fenestration creates the need to coordinate both the planes with the fenestration. This can result in the need to create an offset in the sealant joints and to accommodate the geometries of all three (Figure 3).

Structural attachment and boundary conditions
Face barrier precast concrete and GFRC must be attached to primary or relatively robust secondary structural members. It is impractical to have continuous back-up A/WRB on sheathing. While not problematic for continuous-barrier façades, transitions to the concealed ones create difficulty in coordination and tolerances between the differing systems.

Generally if the precast/GFRC elements are installed first, the concealed A/WRB can run to and be sealed to the sides of the panels. If delays in procuring precast/GFRC occur, it will be necessary to leave the adjacent edges of concealed A/WRB incomplete so they can be installed with proper clearances to the precast. Attachment locations may, however, preclude installation of face-barrier elements following concealed barrier cladding. See Figure 4 for a jamb transition for precast to concealed A/WRB detailing with steel stud backup wall. In this example, the structural mount of the precast panels interferes with the A/WRB and possible flashing transition illustrated in Figure 1.

For concrete backup wall, it is necessary for some of the precast/GFRC attachments to protrude beyond the edges of the panel (typically the top or side “swinging” in to make the final connections). In this case, the concealed A/WRB and flashings must be custom fit around the attachments requiring field fabrication of sheet metal and flexible flashing for an airtight as well as watertight transition.

Structural movement and boundary conditions
While most concealed-barrier façade systems rely on a sheathed backup wall, many face-barrier systems can attach directly to the main building’s structural system. This difference in structural attachment (and therefore, differential movement between the face- and concealed-barrier elements) can cause problems at these boundary condition details. To remain airtight and watertight, face-barrier façades must act as rigid bodies, as racking or deformation risks cracking or tearing of the face seal. Wet-sealed metal panels have a greater ability to accommodate deformation than precast concrete or GFRC
but generally less than concealed-barrier façades. With the exception of cement plaster, concealed-barrier claddings can be separated and decoupled from the underlying A/WRB, allowing the wall’s concealed portion and barrier systems to move and deform as required. The ability of the A/WRB to accommodate racking and deflection of the underlying wall depends on the performance of the material at the joints. Some systems include materials capable of spanning and taking compression or tension across these joints (e.g. preformed silicone joints) while others may require a mechanical means (such as a bellows of material). The overlying cladding can be allowed to move in a way best-suited for its properties and performance requirements, either through slip joints in the attachments to the wall or in the underlying attachment subframe.

Under interstory drift (i.e. lateral movement caused by seismic and wind), precast/GFRC panels, due to their rigidity, must rotate as “solid bodies.” Typically movement is allowed by pinning one corner of the attachment and letting the other three rotate in a plane (Figure 5). Conversely, most concealed A/WRB walls are designed to rack (i.e. go out of square) and/or translate (stay square and move side-to side) to accommodate drift. Under low to moderate drifts caused by wind, or in rigid structures, joints can usually be sized to accommodate the differing movements. Under seismic drift in flexible moment frame structures, joints would need to be excessively large to accommodate the movements. In Figure 6 (page 56), a portion of a wall is 3.6-m (12-ft) tall with 2 percent interstory drift, which equals 73 mm (2.88 in.) between the floor-lines. Two 1.5 x 2-m (5 x 6-ft) precast panels are adjacent to a 2 x 2.7-m (5 x 9-ft) window above a 1.5 x 0.9-m (5 x 3-ft) metal panel over the concealed A/WRB. Since the precast panels rotate (approximately 3 degrees) and the window and metal panel rack (54.8 and
18.2 mm [2.16 and 0.72 in.], respectively), collisions between the precast panel and window of approximately 31 and 508 mm (1 ¼ and 2 ¼ in.) occur. The 15 x 305-mm (5 x 12-ft) metal panel to the right of the window racks but does not collide with the window or lower metal panel.

The ability to use both face- and concealed-barrier systems on a project affords designers and contractors flexibility in meeting overall project goals. However, the details of the transitions between these systems must be closely examined to maintain continuity of the air, water, thermal, and vapor barriers. Considering these performance factors of the exterior walls in coordination with the attachment, movement, and sequencing of work are critical to the design of a successful enclosure.

Judson “Jud” Taylor, RA, is principal of Simpson Gumpertz & Heger Inc. (SGH), which he joined in 2000. He has been in SGH’s Southern California building technology division since 2003. An architect registered by the States of California, Arizona, and Washington, Taylor is a specialist in the design and construction of building-envelope materials and systems—in all aspects of construction including wood-framed and high-rise structures, masonry, and with all forms of exterior cladding including stucco, exterior insulation and finish systems (EIFS), prefabricated panels, windows, window walls, curtain walls, plaza/terrace decks over occupied and unoccupied spaces, balconies, decks, roofs, and above- and below-grade waterproofing. He can be reached at[8].

Christopher W. Norton, PE, has been with SGH since 2008 in the Southern California building technology division. He has a broad range of experience with building enclosure systems, including new design, existing building repairs and retrofits, heritage conservation projects, and daylighting design concepts. A licensed professional engineer in California, Norton is a member of the Association for Preservation Technology (APT). He can be reached at[9].

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