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The Doctor Has Called in a Prescription for You

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Prescriptive Requirements vs. Performance-Based Testing in the Construction Market

image001Sometimes your doctor writes a prescription for your ailment, and sometimes your doctor wants to run some tests first. The first example is a prescriptive approach (do this and you will be okay), whereas the second example is a performance-based approach to solving your problem (take some meaningful measurements and then determine what to do). The same two options are also used in the metal building industry.

In the construction market, states’ code bodies often wrestle with determining whether a prescriptive approach or a performance-based approach will be most effective to minimize the energy demand for air conditioning, especially for those states where the weather is generally warm. Often these code bodies give the building developer the option to choose between:

1) a prescribed set of specifications, or

2) a process where the developer can demonstrate—often using computer modeling—that their performance-based approach will achieve the energy-saving requirements that the code body wishes to achieve.

image002In most cases, it is straightforward: prepainted metal roofing must meet the prescribed requirements for solar reflectance and thermal emittance. Of course, there is more to a building than roofs, and the developer needs to consider windows, insulation, orientation, etc., when planning to build.

If we look into our own backyard, we see that the Metal Construction Association (MCA) created a Metal Roofing Certification Program several years ago. It is an interesting combination of prescriptive and performance-based requirements. Because MCA focuses on metal used in construction, and because NCCA focuses on the creation of prepainted metal—mostly used in construction—the MCA program can help us better understand the various prescriptive or performance-based requirements established by various code bodies.

The MCA Roofing Certification has many details, but here we will discuss the program from our own perspective. Most all prepainted construction products are composed of a metal substrate (either aluminum or metallic-coated steel), followed by a pretreatment and primer, and—on the top side—a high-performance topcoat that provides aesthetics coupled with durability, and—on the back side—a backer that provides damage resistance during fabrication and transportation. The MCA program aims to provide consumers with reliable prepainted products. How do they do this?

Let’s start with the substrate. The MCA program prescribes—specifies—the substrate. The prescribed prepainted substrates in the MCA program are:

  • Aluminum, ASTM B209, Alloy 3003, 3004, or 3105
  • Galvanized steel, ASTM A653/A924, G-90 coating weight
  • Galvalume or Zincalume, ASTM A792, 55% aluminum-zinc alloy-coated steel, AZ 50 coating weight
  • Galfan, ASTM A875, 5% aluminum-zinc alloy-coated steel, GF 90

Why not make the substrate recommendations performance-based? Over the last 50+ years, many different substrates have been used in the U.S. coil coating industry, but only a handful of them provided the necessary long-term performance properties needed in the construction industry. This was not realized through extensive testing, but rather by studying the actual performance of in-service prepainted products in the field and in many environments for decades. Yep, trial and error triumphed! Because of this study, and because there were materials that were proved to provide the necessary performance, it made sense for the MCA program to state, “These are the substrates that you must use.” A sensible prescription!

Pretreatments, primers, and topcoats are quite a different story. Much has been done to improve pretreatments and primers and to make them more sustainable. This evolution is constant and desirable. The same is true regarding topcoat technology. To avoid discouraging technological advancements, MCA provides a performance-based approach to its approval process for these three materials. Of course, this can only be done if a test method is available and acceptable to researchers that correctly predicts long-term performance in the field. When considering pretreatments and primers, corrosion prediction is important. For topcoats, durability against the elements is considered. In the latter case, MCA has wisely chosen real-time weathering testing—not accelerated testing—in South Florida as the standard test protocol (45° S, open-rack exposure).

Still, why not prescribe pretreatments, primers, and topcoats? Unlike metal substrates—which are made by many mills but produced to the same industry standards—pretreatments, primers, and topcoats are elegant blends of chemical materials produced by several manufacturers, and none of them are identical.

A prescriptive approach is recommended when, as an industry, we can say, “Yes, we do know best.” A performance-based approach is recommended when an appropriate test method is available to elucidate the performance of a new technology. One approach is not necessarily better than the other, and the MCA program is an excellent example of the suitability of a blended prescriptive and performance-based approach.

When you are sick, a doctor must take one of two paths: feeling secure that you have been properly diagnosed and, therefore, that prescribing medication is the best approach, or feeling that tests are needed to diagnose your problem through “performance” results, which can show the best path to take.

Code bodies face the same situation, so you will likely see either one or a combination of these two approaches, depending on your state and its climate and typical construction types. After all, the “doctor” knows best!

 

David Cocuzzi

NCCA Technical Director

 

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One thought on “The Doctor Has Called in a Prescription for You

  1. Pingback: MCA’S ROOFING CERTIFICATION PROGRAM | Metal Construction Association Blog

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