Conforming to Code
Other Important Considerations of RainScreen Assemblies Effecting Performance
There are several considerations important to realizing the full potential of rainscreen performance.
The decision to specify aluminum, steel or other material brackets should be carefully considered. Thermal conductivity is directly related to thermal bridging. As has been mentioned, if something is very conductive penetrating a layer of insulation, it provides a path with very little resistance for the heat energy to transfer right through. Metal with lower conductivity has less thermal bridging and better overall performance. Aluminum is five times more conductive of heat than steel, and 17 times more than stainless steel. While extremely long lasting, manufactured aluminum parts are also more expensive than steel.
With changes in temperature, aluminum's expansion and contraction is over 50% greater than steel, which is generally considered the more stable material. Although the amount of expansion is usually a small percentage, the effects of that expansion must be accommodated in the rainscreen design to avoid structural problems. The more complex the design, the more difficult the installation, increasing costs and possibly creating quality control issues.
That is not to say that any type of steel will make for the ideal bracket. Galvanized steel is the most common, but typically has a far shorter lifespan when compared to other steel coatings. Carbon steel coated with anti-corrosive properties ensure metal components have maximum service lives. One option is 55 Al-Zn coated steel (ASTM A792, commonly referenced in the industry as Galvalume, with a grade of AZ55. AZ55 coated steel has up to 3 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. Another 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 the reputation as being the bridge between stainless and heavy galvanized coatings.
Another option is 304 stainless steel, one of the most versatile and most widely used stainless steels. Composed of 18 percent chromium and 8 percent nickel (otherwise known as 18/8), this grade of stainless steel resists most oxidizing acids and is very corrosion resistant, providing a virtually indestructible surface, and at a surprisingly competitive rate compared to extruded aluminum attachment systems. Remember that stainless steel is 17 times less conductive than aluminum as well.
A good installation entails more than meets the eye. In order to achieve predicted performance, the rainscreen insulation installation must be a tight fit, without the gaps and cracks that lead to energy (heat) leakage, a leading cause of energy waste, and potential condensation problems. Even small gaps can reduce efficiency as much as 25 percent, compromising the building's energy performance, driving up operating costs and requiring costly and disruptive callbacks. Considering the way in which the installer must place the insulation on the wall and around the brackets or girts can have a large impact on costs. For example, if brackets are ¾ inches thick penetrating through the insulation, the insulation would have to be carved out by hand around each bracket in order to fit snuggly. If the brackets were only a 1/16 inch thick, such a metal, the insulation would fit tightly, eliminating the need for trimming.
Another installation consideration involves the NFPA (National Fire Protection Agency) 285 fire-test requirement for multi-story, non-combustible construction. While this has been a part of the building code for more than two decades (both pre- and post-merger of the building codes into IBC), for many years designers rarely looked at this requirement or even knew it existed. Recent developments in ASHRAE, IECC and LEED have increased the interest in high performance building envelope designs with continuous insulation, which commonly involve combustible plastic foam.
Note how the insulation is falling away from the wall with the brick tie. Not held back properly leading to condensation points on the wall potentially. Note on the right how using rigid insulation with a bracket system is difficult because it’s very hard to carve out and make it fit tight. If it doesn’t fit tight it will not perform correctly.
Photos courtesy of Knight Wall Systems
With the increased use of thick foam insulation in sustainable, energy efficient designs, architects should be aware that all foam plastic insulations used in exterior wall assemblies are required to pass all seven elements of Chapter 26 of the IBC, including Section 2603.5.5, which requires compliance with NFPA 285. Foam plastic insulation manufacturers should provide verification that products are part of an approved NFPA 285 assembly. It is important to note that the NFPA 285-2006 standard fire test is an assembly test, not a component test. The details of the test assembly and application materials should be strictly followed in practice.
It is also important to note that the NFPA 285 fire-test does not pertain to all plastics used in exterior, non-combustible construction. A thermoset plastic isolator, such as ones described earlier, does not fall into any category within the IBC that references the requirement for the NFPA 285 fire-test (however another component within the assembly could require NFPA 285). However if the plastic isolator, or plastic bracket, is not simply a thermoset plastic but fiber-reinforced plastic (FRP), then NFPA 285 fire-test requirements will apply.
Conclusion and Wrap-Up
With ever more stringent energy codes, architects and designers must fully understand how to achieve energy efficient buildings. By reducing the thermal bridging, the overall performance of the wall is dramatically increased, and operational costs reduced. The manner in which exterior insulation has been typically added to wall assemblies dramatically decreases thermal performance. Exterior continuous insulation systems and systems with highly engineered isolated steel brackets are one proven way to significantly reduce thermal bridging and lead to optimum performance in a wall assembly with minimal risk.
Award-winning Knight Wall Systems manufactures versatile rainscreen attachment systems that accommodate a wide array of cladding options with reduced thermal bridging. www.knightwallsystems.com