The last and final element of all rainscreen assemblies is the back-up wall which is the wall where the rainscreen is applied. This back-up wall, often referred to as the substrate, can be a steel or wood stud wall with sheathing, concrete masonry units (CMU), simple concrete, or cross-laminated timber (CLT). Yet the back-up wall provides more than just something to attach everything to. It must be stiff enough to not cause damage to other components and be structurally adequate to withstand the loads transferred to it.

Accessories and other minor components exist and will serve important, vital, roles. These include flashing, trim pieces, and fasteners, to name a few.

SELECTING FOR RESILIENCE: CLADDING ATTACHMENT AND SUPPORT SYSTEMS

It is often said–and demonstrated–that humans are creatures of habit. As a species, we tend to gravitate towards the familiar, especially in times of duress or when the costs of failure may be high. There is comfort in the known, being able to predict an outcome or do something quickly, accurately, and with confidence. Change is not bad and in fact, it is often warranted and needed, however, one must be vigilant and take great care when enacting change. In fact, a person can gravitate towards the familiar while still accomplishing a new goal or objective, especially if the consequences of failure are great.

A material choice can be an easy door to open for change, however, the choice, when made, can have a large impact on many factors of the design such as durability, resilience, fire safety, and strength. The building industry has extreme familiarity with many materials due to their long histories, proven track records, and even past failures. One of the most and arguably best-known materials in construction is steel. Mankind has been using steel for thousands of years in one form or another. It is absolutely noncombustible, very resilient to changing conditions, one of the strongest industrial materials available, and has been proven to be quite durable when done properly.

Photo courtesy of Knight Wall Systems

Opting for steel as a material in cladding attachment and support systems means selecting a strong, durable, conscious material.

The material chosen for the cladding support system can be critical to the performance of the wall assembly. The most common materials used are aluminum, steel, stainless steel, or Fiberglass Reinforced Polymer (FRP). Each of these materials have their strengths and weaknesses, but steel stands tall with its broad range of attributes making it the perfect, familiar, material for the job.

Meeting the Thermal Needs

A major factor in specifying a rainscreen attachment system is its impact on energy usage and performance. Namely the impact on thermal transfer at opaque wall assemblies.

Two primary, baseline building energy codes may be adopted by states and local jurisdictions to regulate the design and construction of new buildings: the International Energy Conservation Code® (IECC) and the ANSI/ASHRAE/IESNA Standard 90.1 Energy Standard for Buildings except Low-Rise Residential Buildings.

Driven by a desire to decrease CO2 emissions, escalating energy costs, and resultant increases in building operating expenses, energy codes are becoming increasingly stringent. The overall goal of these policies is to increase the performance of the wall assembly in resisting the transfer of thermal energy so that the conditioned space requires less work from the HVAC system in order to maintain desirable conditions, thereby reducing the amount of energy needed to maintain a conditioned space. This drive has been the key factor in the proliferation of rainscreen assemblies utilizing cladding support systems.

Over the past 20 years, we have increased the opaque wall insulation requirements substantially across all climate zones and for all opaque wall element types. In fact, DOE analysis indicates buildings meeting the 2021 IECC vs the 2018 IECC would result in over 9 percent energy savings overall.

Designing effective insulation is then the necessary strategy for achieving energy efficiency and meeting code, rather than simply increasing insulation amounts. One of the most effective strategies to achieve this is the use of exterior insulation. How exterior insulation is applied to a building can have a dramatic effect on the thermal performance of the wall assembly. But how do we attach cladding to a wall assembly when we have insulation on the outside? Enter the cladding support system…

Thermal Bridging in Walls

It has long been understood that typical metal stud wall assemblies with fluffy, batt, insulation in between has horrendous insulating values. Why is this? Because the solid metal framing members interrupt the insulation and create a pathway for heat to transfer. Heat, much like water, finds the path of least resistance and travels along. The insulation offers much resistance to the heat, but the metal studs offer a highway for the heat to travel along and bypass the insulation. This thereby reduces the actual insulating effects of the wall by 50 percent. Adding exterior insulation can solve the problem if done correctly. The exterior insulation will cover the studs, and building as a whole, much like a coat covers a person’s body. This solid wrap of insulation has dramatic effects on the building’s ability to retain, or repel, heat-reducing HVAC usage and boosting energy savings.

But first, we must understand what actually contributes to conductive thermal transfer in an assembly. Factors include the contact area of the connected materials, the cross-sectional area of the material penetrating the insulation, and the conductivity of the material penetrating the insulation and between adjoining materials. Therefore, simply adding more insulation may not increase the effective R-value without addressing the root factors.

Applying these factors to a cladding support system selection, or design, can generate a great result for thermal performance. One could simply choose the lowest conductive material, but this would result in a loss of other crucial performance criteria, such as structural and fire, both of which are generally viewed as higher priorities in design. And remember, thermal conductivity is only one piece of a complex equation. In fact, thermally isolated cladding support systems primarily comprised of steel have been proven to be one of the most thermally efficient designs, including surpassing systems using clips made entirely of low conductive FRP. How is this?

• EXAMPLE: a wall assembly of Interior Gypsum with Steel Studs 16” OC; Exterior Gypsum, and R-16.8 nominal exterior insulation. That assembly using FRP clips with 16” x 26” spacing has an effective R-value of 15.7, a 26 percent loss. Thermally isolated steel brackets, placed 16” x 24”, result in an effective R-value of 17.2, or a loss of insulating ability of 17 percent.

The thermally isolated system deploys a multi-prong approach to combat thermal bridging. First, the brackets are smaller with a special shape to reduce the cross section penetrating the insulation. This is feasible due to the strength of the steel. Secondly, the steel brackets use specially designed plastic pieces between the metal components and even the back-up wall. This plastic is there to slow the transfer of thermal energy between the conductive materials. Lastly, the design of the plastic pieces is specific to reduce the contact area between materials; if it does not touch, it cannot transfer. When all this is put together, we witness a dramatic increase in performance.

Another example of deploying this strategy is a thermally isolated steel Z-furring. Here, the web of the steel Z-furring is punched to reduce the cross section of steel penetrating the insulation by up to 75 percent. Again, this is feasible due to the strength of the steel. Then a specially designed plastic piece is placed between the Z and the back-up wall to slow heat transfer. Lastly, the plastic piece is hollow with only ribs connecting the front and back again, if it does not touch it cannot transfer. This thermally isolated Z-Furring results in an effective R-value of 17.2 when spaced at 24” OC with a steel stud wall and R-16.8 nominal exterior insulation.

Can’t Be Too Strong

Arguably the most important aspect of any building design is strength. Is it structurally sound and is it safe? Two of the most important questions we must always ask of everything we build. If an assembly cannot remain intact and in good service condition, the fire performance, thermal performance or even sustainability measurements of a material little matter. And with regards to life safety, this is of utmost importance.

The most strenuous work the cladding support system must do is all strength-based. As mentioned, the key role of the cladding support system is to support the cladding and connect it to the building. In order for the system to do this, it must be able to resist the loads imposed upon it without deforming or catastrophically failing. The loads it must always resist are the weight of the cladding itself and the wind pressures acting upon the cladding. Other loads it must resist in certain circumstances include the ability to remain intact under seismic activity, ice loads, and/or snow loads. The latter is highly dependent upon the cladding type and details of the building.

With the system constantly being loaded in a repeated fashion, choosing steel for the material is a natural choice, once again due to its familiarity–and rightfully so. Compared to all the common materials used in cladding support systems discussed, it is by far the strongest and most resilient material of the bunch. It is also easy to design with as we have been designing buildings with steel for over a century.

Steel offers dimensional stability naturally. Changes in moisture content and changes in relative humidity do not impact it. Expansion and contraction are minimal. In fact, according to the American Society of Mechanical Engineers (ASME) B31.3 standard, FRP expansion is 2.5 times more than carbon steel, on average. And aluminum is over 50 percent greater than steel. This one attribute can have catastrophic consequences if not accounted for in the design and installation of the cladding support system.

Other attributes of steel include not warping, splitting, cracking, or undergoing long-term creep when simple, industry-wide, design standards are followed. Steel’s strength-to-weight ratio far surpasses those of FRP and aluminum. It attains a higher strength versus FRP – ultimate flexural strength well in excess of 50 ksi vs 30 ksi on average for FRP. To that end, steel has a ductile failure mechanism versus brittle failure which avoids sudden catastrophic failure in a system. Brittle failure modes of FRP and aluminum alloys require higher safety factors, reducing the allowed load on a component versus a ductile material such as steel. Overall, steel is a safer product for structural support, such is the case with a cladding support system. Steel provides long-term, consistent performance, as it is isotropic, providing equal strength and dimensional properties in any direction. In contrast, FRP products are anisotropic, meaning that their strength is different in different directions. A product, or shape, may be strong in one particular direction of loading but is unlikely to have much strength in the perpendicular direction.

Will It Last?

The cavity of a rainscreen is exposed to the elements. It is a moist environment. And the cladding support system resides in this space, so durability and longevity must be considered.

Contrary to perception, steel offers resistance to pests and mold and can certainly offer corrosion resistance. Building codes and industry standards require that steel used in buildings be designed to tolerate corrosion or be protected against corrosion where corrosion may impair strength or serviceability. Technological advances in coating are dramatically increasing steel’s defense against corrosion.

Galvanized steel is the most common material that comes to mind for most when thinking of steel. This form of steel comes in varying degrees of corrosion resistance by varying the coating thickness. G60 and G90 are the most common thicknesses commercially used. This approach to corrosion protection is outdated, but unfortunately, it is familiar and where most minds go when thinking about the longevity and durability of steel. But things have been changing.

One better option is 55 Al-Zn coated steel (ASTM A792), commonly referenced in the industry as Galvalume, with a grade of AZ55. This is not a new coating by any stretch of the imagination, but not the most common either. AZ55 coated steel has displayed up to three times the life span of G90 galvanized in the harshest marine atmosphere and over five times in moderate atmospheres. The coating composition of 55 percent aluminum and 45 percent zinc provides corrosion resistance via the presence of microscopic aluminum-rich areas within the coating, which corrode very slowly, and zinc-rich areas, which provide galvanic protection.

Arguably the best coating gaining popularity is Zinc-Aluminum-Magnesium, commonly referred to as ZAM produced to ASTM A1046. ZAM has outperformed all current steel coatings in a variety of salt-spray tests and has even gained a reputation as being the bridge between stainless and heavy galvanized coatings. ZAM is a highly corrosion-resistant hot-dip-coated steel that has a coating layer comprised of zinc, 6 percent aluminum, and 3 percent magnesium. A Zn-Al-Mg (ZAM) coating of equal thickness lasts up to 14 times that of typical galvanizing (G90) treatment based on salt-spray testing (ASTM B117) and long-term, real-world, exposure testing. ZAM has superior corrosion resistance on bend-processed parts, cut edge, scratches, and in ammonia environments. ZAM also requires a thinner coating than other metallic coatings for the same life expectancy, reducing cost and benefiting the environment. This steel is by far the most versatile, longest lasting, commercially available on the market, ensuring a quality cladding support system installation for decades to come.

By contrast, FRP products are widely seen to only have a 20- to 25-year lifespan, although conditions of application can shorten or lengthen given weather and exposure. Numerous studies have examined the effects of weathering on FRP products and without the use of proper resins, coatings, and engineered design, the effects of weathering can dramatically reduce the strength of the FRP over time. Thus it is important to thoroughly understand the make-up, and manufacturing quality control, of any FRP product when used in a sensitive manner such as a cladding support element.