Tag Archives: Walls

Continuing Education on Continuous Insulation

continuous - Tom's article 2015 - 5th-&-Alton Shopping-Center

All images courtesy Sto Corp.

by Tom Remmele, CSI
Continuous insulation (ci) has been a component of exterior wall assemblies for more than 40 years in North America and even longer in Europe. It has always been the smart way to design wall assemblies from the standpoint of energy conservation and water management. By minimizing energy loss caused by thermal bridging and the risk of condensation caused by water vapor diffusion, exterior ci can improve building durability and benefit the environment.

Standards-writing and regulatory bodies, government agencies, and the building science community are in alignment in viewing exterior ci as a sensible strategy to conserve energy in buildings. The American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) has steadily driven energy conservation standards—and hence, the International Energy Conservation Code (IECC)—to ci prescriptive R-value requirements (in combination with stud cavity insulation) as a pathway to greater energy conservation in buildings.

In sponsoring the 2012 IECC code changes, the Department of Energy (DOE) helped achieve the “largest one-step energy efficiency increase in the history of our energy code.” Building Science Corporation identifies the ‘perfect wall’ (i.e. one working in any climate zone), as having ci outbound of the structure. Thus, for the foreseeable future ci is likely to be a fixture in most exterior wall assemblies.

Types of foam plastic ci
Figure 1 summarizes properties of rigid cellular polystyrene and polyisocyanurate (polyiso) thermal insulations as published in ASTM C578-14, Standard Specification for Rigid, Cellular Polystyrene Insulation, and ASTM C1289-14, Standard Specification for Faced, Rigid Cellular Polyisocyanurate Thermal Insulation Board, respectively. Commonly used insulating materials conforming to these property requirements are Type I expanded polystyrene (EPS), Type IV extruded polystyrene (XPS), and Type I, Class 1 or 2 polyiso.

ContinuousInsulation_Figure1

Common types of rigid foam plastic continuous insulation (ci) used in exterior wall assemblies.

Each insulating material has its own benefits and limitations influencing which one should be used for a given project. For example, EPS is the insulation commonly used in exterior insulation and finish systems (EIFS) because of its dimensional stability, water vapor permeability, and adhesion compatibility with EIFS adhesives and base coats. Due to their higher R-values, XPS and polyiso boards are more commonly used behind brick veneer to allow for thinner wall sections. This becomes important when considering the total thickness of a brick veneer cavity wall with ci and the implications on size and thermal bridging of supporting shelf angles and lintels.

Water vapor permeability of the insulating material can be an advantage or disadvantage. For example, in hot, humid climate zones where vapor drive is predominantly inward, exterior polyiso or XPS insulation can retard inward vapor drive and reduce the potential for condensation on the relatively cold conditioned surface of interior drywall over metal studs. In mixed climates, where vapor drive is both inward and outward for long periods during the course of a year, EPS ci is advantageous because its higher vapor permeability allows water vapor to diffuse, which aids in drying of the wall assembly in the event of condensation.

A wall analysis during design is a valuable tool for selecting the best type of ci material in this regard. Dynamic computer models are a good approach, since they characterize wall assembly hygric performance through seasonal change, but even a simplified steady-state analysis for worst case winter and summer months can be a helpful tool to assist in making material choices for a given wall assembly.

Other things to consider beyond physical properties are jobsite handling, storage, and compatibility with other materials, as well as the construction Type, whether Types I−IV or Type V, and design wind pressure requirements. EPS and XPS have limited ultraviolet (UV) resistance and should not be left exposed to sun for extended periods as the surface will degrade (chalk).

While this degradation has no significant effect on R-value, it can interfere with adhesion of joint treatments, tapes, EIFS base coats, and membrane materials that rely on adhesion to the surface, unless the surface is rasped or sanded to remove the chalked material. Chalking is not an issue when the insulation is ‘faced’ with glass mat facing or aluminum foil facing as with most polyiso boards, although adhesion to the facing materials still has to be evaluated.

Equally important to consider (if not more so) on jobsites is the combustibility of foam plastics. They should be protected from sparks, flame, or any other source of ignition. All foam plastic insulation boards are produced with flame retardant, but they behave differently in the presence of flame; while EPS and XPS melt, polyiso chars.

Despite their combustibility, all these foam plastic insulating materials can be used on buildings required to be of noncombustible construction (Types I−IV) with proper material and ‘end use’ testing to support the proposed assembly. Likewise, they can all be used in wind-resistant assemblies provided they are constrained (i.e. sandwiched) in the negative and positive direction by another material (e.g. sheathing/cladding) capable of resisting design wind pressures. Alternatively, appropriate tests can be performed to determine wind load resistance of the insulation relative to project and/or building code requirements.

Fire safety considerations
Fire safety in the design of foam plastic-based wall assemblies is an important factor when considering their use. Since such materials are combustible, building codes strictly regulate the use of foam plastics. Chapter 26 of the 2015 International Building Code (IBC) has seven requirements that must be met for foam plastics to be approved for use in walls (Figure 2).

CS Feb FIgure 2 (2)

Summary of the 2015 International Building Code (IBC) Chapter 26 requirements for use of foam plastic insulation in exterior wall assemblies.

For the design professional, listing and labeling of the insulation by an approved independent third party is the first step in verifying code compliance. Most insulation board manufacturers hold International Code Council Evaluation Service (ICC-ES) evaluation reports (ESRs), or Underwriters Laboratory (UL) or other listings, simplifying verification.

These listings also demonstrate other aspects of code compliance, for example, compliance with flame spread and smoke development criteria to qualify as a Class A building material, or special uses such as below-grade or attic insulation. Other code compliance requirements are more difficult to verify because they involve wall assembly tests that may exist with the insulation board manufacturer, the cladding manufacturer, or, in some cases, with the air barrier/water-resistive barrier (WRB) manufacturer.

Potential heat, a measure of the foam plastic’s stored heat energy, is a function of the type of foam plastic insulation, its thickness, and density. IBC effectively limits potential heat for construction Types I−IV to the insulation thickness and density successfully tested in the National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components.

NFPA 285 is a qualifying wall assembly test for the use of foam plastic in wall assemblies of Types I, II, II, or IV construction. As a ‘worst-case’ surrogate for exterior wall fires, the test addresses the effects of a simulated fire in an interior room and vertical flame propagation from floor-to-floor and room-to-room vertically and laterally. An example of an assembly that meets NFPA 285 acceptance criteria is shown in Figure 3, and the actual test is depicted in Figure 4.

continuous_61s01

Figure 3: Exterior brick veneer wall assembly with ci that complies with National Fire Protection Association (NFPA) 285 acceptance criteria (refer to International Code Council Evaluation Service Reports (ICC-ESRs) 1233 [6] 2141[7]).

continuous_NFPA 285 Test

Figure 4: As shown above, NFPA 285 test exposes the wall assembly to fire from an interior compartment and evaluates vertical and lateral flame propagation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

While the test enables approval of the assembly in Types I−IV construction, it also establishes limits—maximum allowable thickness and density of insulation and detailing around the opening that must conform to (or be more conservative, from a fire protection standpoint) what was tested. Test results can sometimes be extended to other claddings or backup wall construction when evaluated by a qualified fire protection engineer.

For example, in Figure 3, the results of the fire tests with masonry veneer over steel stud wall construction were extended to backup wall construction of concrete or concrete masonry unit (CMU) in lieu of steel stud with gypsum sheathing. Once again, ICC-ES evaluation reports can be a valuable resource for the design professional to know what assemblies have been tested and meet acceptance criteria, or where results of tests have been extended, evaluated, and recognized by ICC-ES.

NFPA 285 is not the only assembly test to be considered. ASTM E119-12a, Standard Test Methods for Fire Tests of Building Construction and Materials, is necessary when walls are required to have an hourly fire-resistance rating—a common requirement for commercial office, institutional, and some retail and multi-family type construction. The test evaluates the ability of the assembly to resist temperature rise, collapse, flaming, or ignition on the unexposed side of the assembly.

If the assembly is asymmetrical, it must be tested from both sides—in other words, with the fire originating from the interior or exterior. Further, the effects of a hose stream (used to provide additional structural evaluation) are evaluated to ensure the unexposed side remains intact and there is no breach or collapse of the assembly as a result of the hose stream.

Engineering analysis or modeling by a qualified fire protection engineer can be done to qualify substitute materials or to make minor revisions to what was tested to provide the design professional with a wider range of material options for the wall assembly. For example, if an hourly rating is achieved with a frame wall assembly with gypsum sheathing on the exterior and gypsum wall board on the interior, it is readily assumed a ‘mass wall’ fire-resistive wall construction (e.g. 152-mm [6-in.] cast-in-place concrete or 203-mm [8-in.] CMU) would provide equal or better resistance than the frame wall with the same exterior ci and cladding assembly. ICC-ES evaluation reports, UL listings, and Gypsum Association’s (GA’s) Fire Resistance Design Manual are valuable resources for the design professional to identify tested fire-resistance rated wall assemblies.

The last of the assembly tests is NFPA 268, Standard Test Method for Determining Ignitability of Exterior Wall Assemblies Using a Radiant Heat Energy Source. The test evaluates a wall assembly’s susceptibility to ignite from the radiant heat produced by a fire in an adjacent building. It is an important test for EIFS and other foam plastic-based wall assemblies that do not conform to one of the six wall covering exceptions listed in Section 2603.5.7 of the 2015 IBC:

  • 
minimum 15-minute thermal barrier;
  • 
minimum 25-mm (1-in.) of concrete or masonry;
  • 
at least 9.5-mm (38-in.) glass-fiber-reinforced concrete (GFRC);
  • 
metal-faced panels meeting the prescribed composition and thickness;
  • 
minimum 22.2-mm (78-in.) stucco; and
  • 
minimum 6-mm (14-in.) fiber cement lap, panel, or shingle siding.

A final requirement of the code for all types of construction is separation of the combustible foam plastic insulation from interior space with a 15-minute thermal barrier, typically 13-mm (12-in.) interior drywall or exterior gypsum sheathing. Commercial attic space or the interior wall area above suspended ceilings must have this 15-minute thermal barrier in place on the interior if it does not exist on the exterior side of the wall to separate the foam plastic insulation (with some exceptions permitted in the 2015 IBC’s Section 2603.4.1). Between-the-stud fiberglass batt insulation does not count as a thermal barrier since it is discontinuous.

Thus, building codes strictly regulate the use of foam plastics in wall assemblies. Manufacturers of wall assembly components—cladding, ci, air barrier, and sheathing—must demonstrate compliance with these requirements. ICC ESRs are an excellent resource to facilitate verification of wall assembly compliance.

Moisture-related durability considerations
One of the ways exterior ci can help in managing water is by changing the location of the dewpoint in cold climate zones so water vapor diffusion condensation potential is minimized or eliminated. Continuous insulation can also aid in controlling moisture in hot humid climate zones as demonstrated in recent research conducted by the US DOE and the EIFS Industry Members Association (EIMA). The research compared the hygrothermal performance of various wall assemblies—EIFS, stucco, brick, and fiber cement siding (15 assemblies in total)—installed on a test hut (Figure 5) exposed to natural weather in Hollywood, South Carolina (Climate Zone 3A).

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Figure 5: The image to the left shows an EIFS Industry Members Association/Department of Energy (EIMA/DOE) test hut in Hollywood, South Carolina with wall panels monitored for a two-year period for hygrothermal performance. On the right, the test hut rainwater collection device ‘delivers’ rain into the panel at the plane of the water-resistive barrier (WRB) during the second year of exposure. The device was intended to simulate a flaw (breach) in the panel exterior wallcovering.

Temperature, heat flux, relative humidity (RH), and moisture content measurements were taken 24 hours a day with sensors placed in the wall panels. After a little more than a year of exposure, a flaw (i.e. opening) was created in some of the wall panels to introduce rainwater onto the plane of the WRB behind the cladding. The cladding with ci performed the best from the standpoint of temperature and moisture control as measured by the heat flux sensor on the inside face of interior gypsum wallboard and the relative humidity sensor on the face of the wall sheathing directly behind the WRB (Figure 6).

continuous_EIMA Test Hut Data Profiles

Figure 6: Assembly with exterior ci shows improved moisture control in comparison to assemblies without ci as indicated by relative humidity (RH) measurements at the exterior face of the sheathing. The assemblies were installed over nominal 2×4 wood framing. In order, they are (a) 102-mm (4-in.) brick veneer cavity wall over paper water-resistive barrier (WRB) on 11-mm (7/16-in.) oriented strandboard (OSB) with unfaced R-11 batt insulation, (b) 102-mm exterior insulation finish system (EIFS) with fluid-applied air barrier/WRB on 13-mm (1/2-in.) plywood with no batt insulation, and (c) 22-mm (7/8-in.) portland cement stucco over two layers of paper WRB on 11-mm OSB with unfaced R-11 batt insulation.

The closer the heat flux sensor stayed to the zero base line (which would represent constant interior temperature), the better the assembly’s thermal performance. An average monthly relative humidity of below 80 percent was considered acceptable based on ASHRAE STP 160, Criteria for Moisture-control Design Analysis in Buildings. The ci assembly proved not only to be best from a thermal standpoint, but also kept wall components dry, even when rain was deliberately directed into the assembly during the second year of exposure. This is important not only from the standpoint of durability, but also because insulation, if it stays moist, loses some of its insulating value.

Key factors in the moisture control success of the ci assembly were:

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exterior ci kept wall sheathing above the dewpoint during winter;
  • 
combination of low water absorption exterior finish materials and relatively low water vapor permeability of the insulation prevented high exterior RH in summer from significantly increasing the sheathing’s relative humidity;
  • 
seamless fluid-applied air barrier/WRB behind the cladding was effective in resisting air leakage (and condensation potential) and was unaffected by the rain introduced into the wall during the second year of exposure (the other claddings had paper WRBs); and
  • 
drainage feature of the ci assembly prevented excess amounts of rain from accumulating in the assembly.

Conclusion
As building codes have evolved to the point where ci is now mandatory for many wall assemblies, rigid foam plastic ci wall assemblies have become more prevalent than in the past. They have special design considerations that need to be addressed at the design stage with an awareness of what the building code requires in relation to the use of foam plastics and their effects on the physics of the wall construction, as well as design details.

While this feature looked at the basic types of materials available, and focused on fire safety and moisture-related durability, this author is also developing another technical article that explores the added complexity of design details, along with structural considerations, environmental impacts, and cost control for a future issue of The Construction Specifier.

Tom Remmele, CSI, is the director technical services/R&D for Sto Corp., a manufacturer of air barriers, coatings, exterior insulation and finish systems (EIFS), and stucco products. He has held technical management positions in the construction industry for more than 25 years. Remmele is a past Technical Committee chair of the EIFS Industry Members Association (EIMA). He can be reached at tremmele@stocorp.com.

Cold-formed Steel Framing Gets Complicated

CFS_02 BP Hall by Alex Pitt - Courtesy The Music Center

Photo © Alex Pitt, The Music Center

by Chuck Mears, FAIA, Ryan Rademacher, AIA, 
Sheri Carter, AIA, and Michael Chusid, RA, FCSI, CCS
During the medieval period, complex Gothic structures were built from drawings that communicated a designer’s overall vision without detailing specific means of construction. Master craftsmen translated designs into buildable structures using simple tools available at the time. Now, in some respects, the construction industry has come full circle.

Complex concepts envisioned by contemporary designers are being translated into buildable structures by a new generation of master builders. The differences, however, are today’s building materials can be considerably lighter weight than stone masonry of yore, and the craftsman’s tool kit includes building information modeling (BIM) capabilities.

Recent advances in cold-formed steel (CFS) framing illustrate this transition. The American Iron and Steel Institute (AISI) defines cold-formed steel as:

shapes manufactured by press-braking blanks sheared from sheets, cut lengths of coils or plates, or by roll-forming cold-rolled or hot-rolled coils or sheets; both forming operations being performed at ambient room temperature, that is, without manifest addition of heat such as would be required from hot forming.

CFS uses thinner materials with different structural characteristics than hot-rolled sections. Thanks to the efforts of AISI, Cold-formed Steel Engineers Institute (CFSEI), and other industry organizations, there are well-established engineering and fabrication guidelines for orthogonal CFS structures. However, using CFS for complex curved or faceted surfaces still relies on master crafters—now called subject matter experts (SME)—with specialized skills and knowledge.

The relationship between an SME and the project’s architect/engineer (A/E) and contractor has to be defined within terms of the project’s contract documents. For example, a specialist could be a:

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vendor assisting the A/E or contractor on a promotional basis;
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professional consultant hired by the A/E or contractor to advise on, or take responsibility for, engineering;
  • 
properly licensed design professional hired by the contractor; or
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supplier providing framing for a project. (Any recommendations to improve or simplify framing would require appropriate change orders or construction change directives prior to deviating from construction documents.)

The BIM boom
Walt Disney Concert Hall in Los Angeles is a poster child for complex architectural surfaces. Many of the curvilinear finishes inside the Frank Gehry-designed building are shaped and supported by armatures of CFS members. The project’s contractor hired an SME to engineer framing solutions and create BIM files to drive computer numerical controlled (CNC) fabrication, communicate with other trades, coordinate dimensions within tolerances of primary structure, and detect clashes with other building elements.

CFS_01 Villard_de_Honnecourt_-_Sketchbook_-_29

Dating back to 13th century France, these reflected ceiling plans by Villard de Honnecourt required subject matter experts (SMEs) to translate design intent into stone. Complex structures still need similar expertise to translate building information models (BIMs) into cold-formed steel. Image courtesy Holst Architecture

While BIM may someday be as well-established as hammer and chisel are to stone masonry, the construction industry is still grappling with the best way to use its digital toolkit. Architects that design complex surfaces tend to be with the same firms gravitating toward BIM. Their models assist in visualizing spaces or establishing overall geometry, yet often lack information necessary to construct a project.

Even when BIM is available for a project, the construction contracts are usually based on sets of drawings and models are issued to builders solely for reference. Despite this, many construction contracts stipulate contractors provide digital data that can be added to the BIM file to show framing and facilitate clash detection. Amidst all this data, someone still has to figure out the best way to install framing and make the translation from virtual to physical. As one experienced installer explains it, “the model still doesn’t tell me where I need to put the stud.”

In addition to growing complexity of architectural shapes and digital practices, the steel framing industry has also evolved.

“In just the past five years, the steel stud industry has undergone a fundamental change,” said Steven A. Etkin, executive vice president/CEO of Association of the Wall and Ceiling Industry (AWCI). “Where once the ‘generic’ steel stud reigned as king, it is rapidly being dethroned by the expanding use of proprietary products with unique profiles, varying stud thickness, and even specialized coatings.”

New tools simplify fabrication of complex shapes. For example, curving studs or tracks used to require a time-consuming process of making multiple cuts in members and then securing them into shape with straps, screws, or welds. New tools bend framing members by making origami-like plications (folds or pleats), and computer numerically controlled (CNC) lasers cut intricate shapes from light-gage steel such as tabs or entire CFS shapes that simplify assembly of components.

Further, codes and standards affecting CFS have been recently revised. There are now three industry associations with competing certification standards. Increased attention also has to be given to sustainability. A subject matter expert has to stay abreast of advances like these.

Alternative to other materials
One North is an urban infill office and retail development currently being constructed in Portland, Oregon. Intended to achieve Platinum certification under the Leadership in Energy and Environmental Design (LEED) program, the Holst Architecture design calls for deep apertures—pods—at windows to funnel daylight into offices yet block direct sun that could create glare and contribute to excess heat gain in the building. The building has continuous insulation with high thermal resistance. To minimize thermal bridging through insulation, each aperture will attach only by its four corners to the building.

The project’s structural engineer proposed welded hot-rolled rectangular steel tubes to create a cage at each aperture; trusses would hang from attachment points at sides of apertures and beams would span 
6 to 9 m (20 to 36 ft) between trusses. Hot-rolled steel is frequently used for structures like this because the engineering and detailing are well understood.

The architect, however, was willing to push the envelope and consider alternatives. Working with an SME, they were able to reduce weight of apertures by about 60 percent. LEED v4 uses ‘dematerialization’ to describe reduction in materials required for construction. This has the direct benefit of reducing environmental impact of extracting, fabricating, and transporting building products. It achieves additional benefits by reducing structural loads, thereby reducing material requirements throughout superstructure and foundations. Since raw steel is a significant part of a steel structure’s cost, switching to lightweight CFS helped reduce estimated in-place cost of apertures by about 30 percent.

Had hot-rolled steel been used, cages would have required infill framing to support finishes. In a CFS system, structural members also serve as substrate for finishes, simplifying construction and contributing to the material’s economy.

The expansive gallery housing the Anderson Collection is defined by a convex ceiling. It is finished with acoustical plaster to create a seamless surface and control reverberation, and is suspended from a cold-formed steel framework. Daylight infiltrates through perimeter clearstories with frosted glazing to diffuse light. Photo © Tim Griffith

Simplified framing logic
The Anderson Collection of 20th Century Art goes on display this fall in a new building on Stanford University’s campus in Palo Alto, California. The building, designed by Ennead Architects, has an interior capped with an expansive 57 x 24.5 m (187 x 81 ft) ceiling. Referred to as ‘the Belly,’ it is both convex and complex—no two portions of its doubly curved surface have the same shape. Rising from a height of 8.6 m (28 ft) near the building’s center, it reaches 11.7 m (38 ft) around the perimeter where it meets a continuous clerestory that introduces diffused daylight into the hall.

Given the prestige of collection and importance of ceilings to the gallery’s interior design, the project was the antithesis of the ‘beat-to-fit, paint-to-match’ attitude that can lead to forming complex surfaces by brute force. While the architect built a digital model defining ceiling contours, means and methods of construction were not detailed. The contractor initially considered a conventional tee-bar ceiling suspension grid, but decided it would be too difficult to maintain dimensional tolerances working with straight framing elements. The SME was asked for assistance in determining framing logic for the ceiling and to develop a method of installing it to exacting dimensions.

The SME began by building a more precise model of the space for better control of the ceiling’s geometry. Its recommendation to use a system of curved light-gage steel ribs located 1.2 m (4 ft) on center (o.c.), was accepted; the firm was hired to detail and fabricate a system to meet California’s rigorous seismic-resistance criteria and state-approval process.

Cold-formed channels were used to suspend ribs from the roof deck to obtain more stability than would have been practical with wire hangers usually employed for ceiling suspension. The channels penetrated the roof deck and were attached to horizontal member on top of the deck so loads on fasteners were in shear, not tension—this meant greater seismic reliability. Holes in the deck were drilled based on dimensions taken from the SME’s BIM.

Each rib has a unique profile, and the SME established control points on each one to continuously fit the desired shape. Especially close tolerances were required, since deviations in ceiling surface would have been visually exaggerated by glancing light from clerestories. The ribs were factory-curved and color-coded to match installation drawings that, along with sh

op drawings, were generated with information extracted from BIMs. Pre-curved hat channels span between ribs at 400 mm (16 in.) o.c. to provide transverse resistance to seismic forces and more precise control of geometry for finishes.

According to ceiling installer, J&J Acoustics, a conventional ceiling suspension grid would have required scaffolding for a work platform, but the CFS suspension system could be installed from telescoping boom lifts. While scaffolding was eventually required to apply an acoustical plaster finish, use of lifts to install the suspension system gave the general contractor more time with unrestricted access to work floor.

The curved exterior walls of the National Center for Civil and Human Rights (NCCHR) in Atlanta symbolize arms linked in unity, and variation in cladding panels represent diversity of humans. Photo © Gene Phillips Photography

Ruled surfaces or curved framing?
While the Anderson Collection building’s ceiling geometry was complex, it had few interfaces with building elements other than perimeter. The same cannot be said for exterior wall framing of the recently completed National Center for Civil and Human Rights (NCCHR) in Atlanta, designed by The Freelon Group (now part of Perkins+Will) in collaboration with HOK as architect of record. The museum’s exterior walls have compound curvatures, lean inward from bottom to top, and interface with fenestration and curving floor and roof decks.

The A/E designed the walls as ‘ruled’ surfaces—that is, one that can be generated by straight lines. In theory, this should have made it simple to build with linear studs. However, to conform to the complex geometry, studs had to be installed out of plumb, and the degree and direction of inclination varied from stud to stud. Combined with walls’ inward tilt, this meant gravity and wind loads, along with deflection of superstructure, had to be resolved as forces acting both axially and perpendicularly to double-leaning studs. For example, each connection at intermediate floor levels required a unique hot-rolled steel bracket for studs to lean against in addition to normal stand-off clips.

Faced with the complexity, framing contractor Principle Partners retained the SME to model structure, establish X-Y-Z coordinates, and assist with installation.

For the NCCHR in Atlanta, CFS studs lean inward from bottom to top, and are inclined to various degrees left and right. While the resulting ruled surface is intuitively elegant, curved framing members might have simplified installation, connections, and window openings. Photo courtesy Principle Partners

While the project was executed with the straight framing members the architect envisioned, SME determined installation could have been simplified by using curved CFS framing members instead of straight studs. Curved studs could be installed plumb to resist gravity loads without the customized brackets. Eliminating the brackets and simplifying labor would more than offset the cost of curving the studs.

Window openings would also be simplified because the hot-rolled steel used to frame openings could be replaced by conventional CFS headers. The project had advanced to the point, however, where it was impractical to accept the proposed redesign.

This demonstrates why subject matter experts are best brought onto project teams early in the design process. According to the American Institute of Architects’ (AIA’s) 2007 Integrated Project Delivery: A Guide, this:

allows the designer to benefit from the early contribution of constructors’ expertise during the design phase, such as accurate budget estimates to inform design decisions and the pre-construction resolution of design-related issues resulting in improved project quality and financial performance.

Panelization
The SME was also able to expedite construction by panelizing a multi-faceted suspended ceiling that zigzagged above the NCCHR’s 230-m2 (2500-sf) events space. The installer initially hired the consultant just to prepare shop drawings for the complex framing. However, as the deadline for the museum’s opening drew nearer, the general contractor determined either scaffolding to assemble framing in-place or fabricating the ceiling on the floor would have interfered with other activities to be performed in the building.

The light weight of light-gage framing made it simple to transport and handle panelized elements. Panels as long as 9 m (30 ft) were light enough to be carried into the building as needed, and then quickly lifted into place with two or three scissor lifts (depending on configuration).

Conclusion
The success of each of these projects was a team effort that included an A/E to establish vision, a contractor to execute that vision, and an SME to provide specialized expertise. This type of three-way relationship is relatively new in the light-gage steel industry, but well-established in many other aspects of construction. With precast concrete, for example, the architect or engineer of record will specify loads and performance criteria for structure, but design of actual members is delegated to specialist consultants and fabricators.

As mentioned, the SME could be one of a number of parties, ranging from vendors and consultants to suppliers, as defined in the project documents. However the team is put together, SMEs can be said to fill the role of the guilds that built the Gothic structures referenced at beginning of article—applying the art and science of their trade toward making great architecture.

Chuck Mears, FAIA, is CEO and chief design officer of Radius Track, a firm specializing in engineering and fabrication of curved and complex cold-formed steel (CFS) framing tools and systems. He can be reached at chuck@radiustrack.com.

Ryan Rademacher, AIA, is design director at Radius Track. He uses building information modeling (BIM) and parametric design to integrate digital fabrication with artisanal craft. He can be reached at ryan@radiustrack.com.

Sheri Carter, AIA, is the marketing manager at Radius Track. She has a master’s degree in architecture from the University of Buffalo and was in architectural practice before moving into building product sales and marketing. Carter leads the firm’s efforts in product development, sustainable design, and continuing education. She can be reached at sheri@radiustrack.com.

Michael Chusid, RA, FCSI, CCS, started his career working for a cold-formed steel framing manufacturer in 1978. He has been a marketing and technical consultant to many firms in the industry since then. He can be reached at michael@chusid.com.

 

Metal Wall Panels on the Roof: How to achieve durability and reliability using sheet metal

All images courtesy Simpson Gumpertz & Heger

All images courtesy Simpson Gumpertz & Heger

by Scott A. Tomlinson, PE, and Matthew M. Copeland, PE

Sheet metal has been used as a roofing material for centuries on all types of structures. Copper, stainless and galvanized steel, aluminum, and zinc are commonly employed in various low- and steep-slope assemblies. Designers and contractors know from centuries of accumulated knowledge how to achieve durable and reliable roofing assemblies using these types of sheet metal. But what about newer materials?

Modern architectural designs often use newer materials and metal panel assemblies for roofing applications, some of which are not intended to be used as roofing and, therefore, may not be well-suited for it. One such trend is the use of architectural metal wall panels (e.g. metal composite material [MCM]) as roofing to create a visually seamless transition between building walls and roof surfaces, such as low-slope setbacks in the façade. Whether in low- or steep-slope assemblies, using metal wall panels in roof applications presents unique challenges for the designer and contractor.

This article focuses on use of architectural metal wall panels in low-slope ‘brow’ roofing (Figure 1). It also includes a brief discussion of contractor coordination essential to success, particularly since metal wall panel contractors typically have a different set of skills and training than roofing contractors. The installation of metal wall panels on larger low-slope roof areas includes additional complications and risks not discussed in this article.

Generally, architectural metal wall panels are not well-suited for roofing applications. Even in vertical applications, these systems are not typically intended to be watertight. The systems incorporate water management strategies and a water-resistive barrier (WRB) behind the panels (i.e. a rainscreen) to accommodate water that penetrates the panels. It is unreasonable to expect these same panel systems and water-resistive barriers to be watertight when installed nearly horizontal and skyward-facing.

Therefore, the panels must be considered a water-shedding layer, much like roofing shingles, and roofing underlayment is required to accommodate water that penetrates the panel system. The underlayment must be designed and constructed to perform as ‘waterproofing.’ Self-adhering membrane roofing underlayment (referred to as a ‘membrane’ in this article) is well-suited for this application.

Panel skyward-facing surface water management
The panels should be considered the water-shedding layer and the roofing underlayment the primary waterproofing. That said, the panels should be designed and installed to shed as much water as practical to drainage locations and minimize water penetration to the waterproofing membrane. Allowing an excessive volume of water to penetrate the panel system to the membrane increases the risk of leakage through panel attachments, vulnerable details, and construction imperfections in the membrane system. Providing the maximum slope permitted by the design generally directs water away from vulnerable flashings and joints, and decreases water penetration through the panel system.CS_September_2014.indd

Providing a gutter system underneath skyward-facing panel joints (e.g. using splines in panel joints to create a gutter) and sealing the joints (e.g. sealant and backer rod if aesthetically acceptable) minimizes water penetration to the membrane (Figure 2); gutters should be used when possible. Depending on the roof geometry, some assemblies may not accommodate gutters underneath the joints because there is no place to drain water in the gutters; sealant should still be installed in these joints and should be maintained over time as the joints deteriorate. Returns at panel joints should be used to form gutters and receive sealant.

Panel internal vs. eave edge drainage
Sloping the panels to drain over edges is preferred to internally draining the roof. Systems that drain over edges more readily accommodate steep slopes, and minimize water penetration to the membrane (i.e. water does not have to flow to a drain in the membrane). Further, sloping of the panel planes and valleys is simpler because they do not need to conduct water to drain holes. Properly designed snow guards can mitigate the risk of falling snow and ice in colder climates.

Where draining to the exterior wall is unacceptable, the design must incorporate internal drainage or a perimeter gutter. Roof drains must provide drainage at both panel and membrane levels. One option is to provide a drain at the membrane level and holes in the panels directly over the drains (covered with perforated metal—additional ultraviolet [UV] protection of the membrane below may be required). Bi-level drains can also be used. The drains must be installed at the low points in the roof system. Slope and valleys in the panel system must direct water to the drain holes. Designers should consider heat tracing the drains where water can freeze and impede drainage (i.e. at canopy conditions where the drains do not enter heated space).

When a perimeter gutter system is used, the membrane and panels must integrate with the gutters. However, this detailing has intricacies beyond this article’s scope.

Panel impact on membrane level drainage
Unimpeded water drainage is critical at both the panel and membrane surfaces. The risk of water leakage through the self-adhering membrane (e.g. through weak seams, holes, and other imperfections) rises as the water’s volume, depth, and dwell time on the membrane increases.

Panel edges returned to form joints and gutters, and to receive sealant joints, should not extend all the way to the membrane surface. There should be a gap provided between the bottom edge of the return and self-adhering membrane to allow for drainage and reduce the risk of damage to the self-adhering membrane. The gap should be as large as practical (e.g. 25 mm [1 in.]), but not less than 6 mm (1/4 in.).

Panel attachment
The panels must be anchored to the roof structure to meet wind uplift and other loading requirements while maintaining the self-adhering membrane’s waterproofing integrity. Panel attachment clips are typically fastened with screws through the membrane to the structural sheathing or framing underneath the sheathing.

Panel attachment clips should be designed and positioned so they do not impede membrane level drainage. Using discrete clips (e.g. 100-mm [4-in.] long) in lieu of longer or continuous ones reduces the risk of impeding drainage. It is also important to install clips as far as practical away from drains and designed drainage paths such as valleys. This reduces the risk of impeding drainage and water leakage through the fastener penetrations, which are the weak point in the membrane system.

The clips should be fastened with wood screws to increase the likelihood of the membrane self-sealing to the fastener. The drilling action of self-drilling screws can damage the membrane and reduce the self-sealability.

Installing the clips on multiple pieces of membrane squares (e.g. 100-mm [4-in.] square, as shown in Figure 2) each, adhered to the layer below, increases the membrane thickness and self-sealability, and raises the elevation of the fastener penetrations above the membrane surface on which water flows. Setting the attachment clips in mastic atop the squares, before fastening, further increases the self-sealability of the fastener penetration.

A more reliable solution to attaching the panels with clips directly through the membrane waterproofing is to construct stanchions to which the panels are mounted. Stanchions provide penetrations that can be reliably waterproofed, but is a more custom assembly that increases the roof assembly’s overall thickness and cost.

Waterproofing materials
The waterproofing membrane is the critical component in a roof assembly using metal wall panels, and a self-adhering membrane is well suited for this application. Like most products, not all self-adhering membranes are equal. Some membranes form more reliable laps, adhere better, seal better around fasteners, are easier to handle and install, or can tolerate higher service temperatures.CS_September_2014.indd

Additionally, some membrane manufacturers have more ancillary products, technical support, and a longer track record of success. The designer must determine the appropriate membrane products, with the understanding it will be used as waterproofing.

Use self-adhering membrane designed for use as a roofing underlayment.
The membrane should meet ASTM D1970, Standard Specification for Self-adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection; the requirements for self-sealability and lap integrity under a head of water are particularly important. More flexible membranes are typically better for detailing, but can be more difficult to install.

The designer should consider specifying flood testing of the installed membrane. Also, it is important to note these products function as vapor retarders—therefore, the system as a whole must be evaluated for condensation resistance.

Specify rubberized asphalt-based adhesive instead of butyl-based adhesive where service temperature and compatibility permits.
It is the authors’ experience rubberized asphalt adhesive achieves higher initial and long-term adhesion. Substrates should be primed to improve membrane bond, even when not required by the membrane manufacturer.

Specify mastic and fluid-applied membrane at seams and details.
The membrane must be installed watertight on its own, but mastic installed over the membrane at seams and details can provide an additional layer of protection. Fluid-applied membrane can be installed underneath the membrane to form smooth transitions (e.g. cants) at inside corner details and to make three-dimensional details watertight.

Waterproofing installation
The self-adhering membrane must be properly installed to perform as waterproofing. The membrane system (and panel system flashing) must turn up building walls and integrate with flashings and the air/water/vapor barrier on the walls. The membrane must be uninterrupted by metal flashings so it is fully waterproof.

Slope
The membrane must be sloped to the drains (internally drained) or eave edges (edge drained). For an edge-drained system, the design must incorporate a gap at the eave edges to allow water on the membrane to drain out of the roofing assembly. The 2012 International Building Code (IBC) requires a minimum of 1/4 in. per ft for roofs—more slope is generally better. This minimum slope must be provided at the membrane level, which serves as the primary waterproofing (Figure 3), but should also be provided at the skyward-facing panel surface to help reduce water infiltration down to the membrane.

Substrate preparation
The substrate must be properly supported and attached and have smooth transitions at joints to prevent damage to the membrane during service. Uneven transitions and gaps should be bridged with sheet metal well-fastened to limit thermal movement that can cut the membrane. A fluid-applied cant improves seam reliability by easing the membrane transition and allowing the installer to apply the required pressure to form a reliable seal at seams. CS_September_2014.indd

Proper adhesion and reliable seams
Self-adhering membrane must be ‘fully’ supported and adhered to provide effective waterproofing and reliable seams. The substrate must be clean and dry, and the membrane must be firmly ‘pressed’ onto the substrate; hard rollers can help.

The pieces of membrane must be ‘fully’ adhered to each other at seams with no pathways for water (e.g. wrinkles).

Serviceability
Roof drains require maintenance. The hole in the panels at drains must be covered with a perforated material; this cover must be removable to allow maintenance workers to access and maintain the drain.

Metal wall panels are not designed to accommodate live loads imposed by maintenance workers in a horizontal application. If sealant joints and drains cannot be serviced without walking or kneeling on the panels, the designer should consider methods of providing supplemental panel support and/or maintenance worker load distribution.

The authors have considered using moisture-tolerant high-load capacity rigid insulation over drainage mat between the top of the self-adhering membrane and the underside of the panels (fit snugly) to provide additional support. However, such a system has yet to be installed or tested.

Contractor co-ordination
Critical to the successful performance of a metal wall panel low-slope roof is both the designer and contractor believing the panels must minimize water penetration to the membrane, and the membrane must perform as ‘waterproofing.’

A pre-construction meeting is beneficial to clarify design intent and project requirements. The pre-construction meeting should include the:

  • designer;
  • general contractor;
  • metal panel subcontractor;
  • subcontractor responsible for installing the membrane; and
  • other involved contractors.

The meeting should include discussion of the following:

  1. The designer and contractor must understand the panels are a first line of defense intended to shed water and minimize the volume of water that reaches the membrane. The panels themselves are impermeable, but the joints and flashing, even if protected with gutters and sealants, will permit water penetration through the panel system. Water penetration will increase as sealants deteriorate.
  2. The construction schedule should allow adequate time for inspection of the membrane installation before installation of any overlying materials, including any mastics/sealants applied over seams in the membrane (i.e. membrane must be inspected prior to applying mastics/sealants). The membrane installation must be watertight on its own; flood testing requirements need to be coordinated with the project team at the pre-construction meeting. The designer and contractor should expect that water will pond on the membrane under certain conditions.
  3. Panel shop drawings must be coordinated with the roofing design and installation so water drainage is directed to the appropriate location. For example, panel valleys should conduct water directly to the drains, not simply to their general vicinity. Shop drawings should indicate location and size of metal panel anchors as well as joints between panels. Perimeter flashing and other transitions to adjacent construction should be coordinated on the shop drawings, with clear indication of responsibility. Adequate coordination may require additional effort on behalf of the general contractor.

Conclusion
Architectural metal wall panels can be used with success in a low-slope roofing application with proper design and construction. Success requires the designer, contractor, and end user to understand the panels are a first line of defense intended to shed as much water as practical, while the self-adhering membrane level must be designed and installed as waterproofing. Additionally, the system must be maintained. Sealant joints require regular inspection and replacement, and drains demand cleaning.

Before designing and constructing a roofing system using architectural metal wall panels, the owner and designer must be aware of the risks. These include leakage, warranty implications (i.e. will leakage be covered by a long-term warranty?), and cost of repair. In some instances, the risks may be more than the designer and/or owner can tolerate.

Scott A. Tomlinson, PE, is a senior project manager at Simpson Gumpertz & Heger Inc. (SGH), with more than 15 years of experience designing, constructing, investigating, and repairing building-envelope systems of all types. He can be reached at satomlinson@sgh.com.

Matthew M. Copeland, PE, is a senior staff I at SGH, with more than eight years of experience. He specializes in the design, investigation, and rehabilitation of building envelope systems for both historic and contemporary structures, with a focus on materials science issues. Copeland can be contacted via e-mail at mmcopeland@sgh.com.

 

Abst

Standards and Terminologies

In the May 2014 issue of The Construction Specifier, we published the article, “Passive Fire Protection and Interior Wall Assemblies,” by Gregg Stahl. Soon after, a reader contacted us regarding what he considered inaccuracies. We reached out to the author and, in the interest of continuing the discourse about this important topic, excerpts from both sides are included below.

Reader: The first issue is the reference to ASTM E603. The author mentions this is one of two standards that rates assemblies. Actually, ASTM E603 is a “guide” standard, and is used to explain the various types of fire tests, whether they are ASTM, NFPA, UL, or FM, and how they can be compared and contrasted. This standard is not a test method.
Author: The reader brings up several good points in regard to the article on passive fire protection. It should be noted, however, this piece was intended to provide a general overview on the basic principles of passive fire protection. As to the first point, the reader is technically correct. E603 is in fact an ASTM “Guide,” not an ASTM “Standard.” In the “Scope” section of this guide, it does state one of the purposes is to “allow(s) users to obtain fire-test-response characteristics of materials, products, or assemblies, which are useful data for describing or appraising their fire performance under actual fire conditions.” In the subsequent paragraphs, I go on to describe how A603 is used as well as differentiating it from the E119 fire test, which is testing the effectiveness of a particular assembly.

Reader: The second issue is the article states ASTM E119 tests the effectiveness of an assembly as a “fire barrier.” Although not untrue, the use of “fire barrier” seems to limit the type of fire-rated assembly that is tested, since a “fire barrier” is a specific type of fire-rated assembly used by the IBC and NFPA. ASTM E119 is used to test any type of assembly for fire-resistance, whether it is a wall, roof system, floor system, column, beam, etc.
Author: I should have been more precise in the selection of the terminology used. The intent of the term was to use a dictionary meaning, not a fire test assembly meaning. A Google search for the term will produce numerous definitions, such as the one below:

fire barrier: a continuous vertical or horizontal assembly, such as a wall or floor, that is designed and constructed with a specified fire resistance rating to limit the spread of fire and that also will restrict the movement of smoke. Such barriers might have protected openings.

Reader: The third issue is mentioning the hose stream test is used to “measure an assembly’s resistance to water pressure.” This is misleading. The hose stream test is not really a measure of an assembly’s resistance to water pressure, but to test the system’s integrity. As the commentary to the standard states, the hose stream tests the “ability of the construction to resist disintegration under adverse conditions.” In other words, it is a way of testing, from a distance (it is very hot) the assembly’s integrity from falling debris.
Author: The reader references “the standard,” but I do not know to which standard he is referring. ASTM E2226, Standard Practice for Application of Hose Stream, states:

1.3 – The result derived from this practice is one factor in assessing the integrity of building elements after fire exposure. The practice prescribes a standard hose stream exposure for comparing performance of building elements after fire exposure and evaluates various materials and construction techniques under common conditions.

The application of the hose stream does exert pressure on the assembly after it has completed either the full cycle of an E119 fire test or 50 percent of the time of the rated wall assembly. I agree the single word “pressure” does not go far enough to explain—the intent was to determine the integrity of the remaining assembly.

Reader: The fourth and final issue is the use of “area separation firewalls” in the article, and its associated endnote. The use of “area separation” walls was dropped when the IBC was published in 2000, and is not a term used by NFPA’s standards. The correct term used by both the IBC and NFPA is “fire wall” (not a single word). The endnote (no. 3) gives the impression these “area separation firewalls” are used to separate residential units or commercial tenants. This is incorrect. A fire wall divides a building—residential or commercial—into separate buildings so they can be considered independently when applying the code. “Fire partitions” are used for residential unit and commercial tenant separations within a single building and do not require the type of requirements described in the article.
Author: I respectfully disagree with the reader, who seems to be making the reference to area separation walls fit his use without recognizing the term can have more than one use or intent. It was employed here with no reference to NFPA or IBC, and was not intended as the reader interpreted it.
The term “area separation wall”—or “ASW” as it is commonly abbreviated—is used for a particular type of fire-rated wall assembly with a two-hour fire resistance rating, which is typically intended to permit controlled collapse of one unit in a multifamily residence, while still remaining intact and able to protect the adjacent unit in a fire situation. This is a common term in the construction industry. The reader can check the literature of various manufacturers and find this type of assembly. There are also various UL assemblies for this type of construction.

Clarification on wall systems article

The April 2013 issue of The Construction Specifier included a technical feature by J.W. Mollohan, CSI, CCPR, CEP, LEED GA, entitled, “Exterior Wall Assemblies: Are You Getting What You Specified?”  We received the following letter from Cliff Black, a CSI member and a building envelope product manager for Firestone Building Products.

I am writing in regard to the article on exterior wall assemblies. I agree with the author the issue is certainly a challenging one for the design and specifying community. I would like to cite the bracketed statement at the top of page 57, which states, “buildings of two stories or more.” This appears to be taken in the context of the design of National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components, addressing multi-story fire propagation.

However, the International Building Code (IBC) 2603.5 states NFPA 285 is required for buildings of any height for Types I through IV construction incorporating combustible plastic insulation in the exterior wall assembly. IBC Chapter 14 (“Exterior Walls”) calls for differing requirements for water-resistant barriers (WRBs) and various combustible claddings, qualified by height.

In this case, I believe the statement should read “buildings of any height,” rather than “buildings of two stories or more.”

 

Mr. Mollohan replied to Mr. Black, and has allowed us to share it with other readers of the magazine:

 

Good catch, Clint! You are absolutely correct that one must be familiar with multiple chapters of the IBC to determine whether an NFPA 285 test is required. My error, and your correction, illustrates the difficulty of this provision. I am attaching an adaptation of a flow chart originally created by Barbara Horwitz-Bennett of DuPont Building Innovations for guidance to interested readers:

Diagram_edited-1