For instance, mineral wool has a nominal R-value of about 4 per inch of material, and the type of foam plastic insulation recommended for exterior walls, namely extruded polystyrene, has an R-value of 5. For an even higher R-value, polyisocyanurate insulation is rated at 5.6 per inch.

The benefit of mineral wool insulation is that it is noncombustible and will not prompt a NFPA 285 test. In addition to noncombustibility, mineral wool differs from foam plastics due to its high vapor permeability. This directly increases the wall's ability to dry out if water gets into the wall assembly through a penetration leak or condensation from air transported moisture.

Essentially, the increasing use of exterior continuous insulation contributes to the growing complexity of designing exterior wall systems. Thickness, permeability, and location of the insulation in the wall assembly have an effect on how it responds to thermal air and moisture loads. There is no one single perfect wall design. The building science aspects of material properties, their locations in the wall assembly, the attachment methods, assembly performance requirements, constructability, and climate conditions must all be considered in a high performance building envelope.

Designing NFPA 285-Compliant Building Envelope Systems

Another wrinkle in the NFPA 285 testing process is the fact that only three laboratories are currently equipped to do the testing. These include Intertek and Southwest Research Institute, both in San Antonio, Texas, and Architectural Testing in York, Pennsylvania. So not only does the cost of $50,000 to $60,000 per test (Journal of Building Enclosure Design, Summer 2012) quickly add up, but the standard lead time of 8 to 10 weeks can delay a project's design and construction schedule.

Fortunately, a fourth testing facility should be coming on line within the year, which may speed things up, although concrete and masonry systems will still require a 4- to 6-week curing time.

At one point, NFPA looked at trying to reduce the scale of the test to reduce the cost and lead time, but such an undertaking would have required a significant investment of time, money and resources, so that work was set aside, according to Beitel.

As mentioned, every configuration requires a separate test. So even if a specific WRB, cladding and insulation type were to pass the test, if the designer decides to swap out even one of the components, an entirely new test must be conducted.

For example, there are many types of foam plastics and performance varies, explains Beitel. “Based upon the wall assembly you put them into, some will pass and some will fail. That is the problem that the architects perceive they have. They want to use a given foam with a given veneer, but it has to pass the test.

“None of the foams have a perfect track record and that's the confusing part here,” he adds.

In some cases, manufacturers who have tested various combinations of their products in wall assemblies, can be very helpful. So specifying a specific wall assembly which has already been tested by the manufacturer will obviously spare the architect the time and money for testing, which can amount to a savings of thousands of dollars, according to Vecchiarelli.

However, this means that every element of the wall assembly must be exactly the same as when it was tested and this, in turn, forces wall assemblies into a proprietary status, rendering them un-biddable. Consequently, to get around this, architects must now design their walls with multiple 285-compliant assemblies in mind.

Making the situation even more complicated, NFPA 285 test results are owned exclusively by the manufacturer and at present, there is no catalog of compliant assemblies that architects can reference.

“In fact, some manufacturers are not publishing their information in this regard for fear of their competitors copying them,” notes Keleher. “This makes if exceeding difficult to find compliant assemblies that suit individual projects.”