Fire Testing Exterior Walls
Robert Grupe / July 2016
Q: I’m working on a project where the exterior envelope is part of the bid package. In the construction documents there is a requirement for the exterior wall to meet NFPA 285 testing. What is that?
A: This is a fire test that is unique to the exterior walls of that envelope. Contractors should make sure that they are not required to provide for proposed design solutions. The cost of such a test can be prohibitive, and the time element for running the investigation can be problematic, given the time constraints of a normal construction project.
The National Fire Protection Association, NFPA 285 focuses on the fire performance of an exterior wall. The title of the test, Standard Fire Test for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components, accurately defines its scope. It is a fairly new standard when compared to the more common fire resistance testing, which is defined in ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials. ASTM E119 is well over 100 years old. NFPA 285 can be traced back to the energy crisis of the 1970s. NFPA 285 was originally developed to address the concern of using foam plastic insulation in exterior walls that are code required to be non-combustible. In recent years it has gained increased attention because of the need for continuous insulation, air barriers and water-resistive barriers as mandated in the current energy codes such as the International Energy Conservation Code and standards such as ASHRAE 90.1, Energy Standard for Buildings Except Low-Rise Residential. The requirement for testing of these types of materials is then regulated in the International Building Code.
The exterior wall provides unique challenges to fire resistance. Heat transfer through the wall, the passage of flame and hot gases, and the structural adequacy of the wall against impact loads are evaluated in the ASTM E119 procedure. The design challenge is increased when assessing an exterior wall. In this condition the wall must potentially resist fire exposure on both sides of the wall, and fire advancing vertically as well as horizontally across the exterior face of the wall. Additionally, the test covers the condition where fire may advance within the combustible material that may be part of the wall assembly.
The system to be evaluated is constructed as a two story assembly with an opening in the wall that simulates a window on the first floor. The wall is exposed to fire on the first floor on the interior side of the wall as well as on the exterior side of the wall at the window head opening. This second exposure assumes that the window will fail during a fire event and that fire will spread to the outside of the structure. Fire exposure is limited to 35 minutes, and there are four requirements to pass the test:
- Temperatures within the assembly must remain below 1,000 degrees Fahrenheit.
- The fire cannot spread to the second floor through that floor.
- On the outside of the wall the flame front cannot spread 10 feet above the window.
- The flame front cannot spread horizontally 5 feet from the centerline of the window.
The IBC requires this testing for very specific conditions. NFPA 285 is required when the exterior wall must be non-combustible as in buildings of Type I, Type II, Type III and Type IV. NFPA 285 is required where there is foam plastics in an otherwise non-combustible wall. NFPA is required if the exterior cladding is considered combustible in the above building types, and the intended building height is 40 feet or more above grade. Of special interest for this condition is the use of metal composite materials or high pressure laminate panels. These are typically used in rain screen systems and therefore fall into the need for the NFPA testing. NFPA 285 is required where a combustible water resistive barrier is used on a structure that is 40 feet in height over grade.
Most importantly, the results are based on the performance of the total assembly and all the materials contained in the wall. Any deviations from what was tested is seen as negating the overall performance of the wall. Herein lies the problem for the contractor. Most proprietary system manufacturers have run this test for their respective products and assemblies. Consequently, the contactor or the design team who modify the assembly may, in fact, trigger the NFPA 285 requirement.
The best advice for the contractor is to do some research before the bid. Understand how the IBC is being interpreted in the local jurisdiction. Take a hard look at the assembly that is being specified. Confirm that documentation is available to support code compliance. Work closely with your vendor partner to understand the needed requirements not only for the specified assembly, but also any alternates that are being considered.
Robert Grupe is AWCI’s director of technical services. Send your questions to firstname.lastname@example.org, or call him directly at (703) 538.1611.