FM Approvals recently introduced a new standard designed to address the hidden risks associated with cavity walls, an exterior wall construction system in which two wall layers are separated by an air gap. Approval Standard 4411, Cavity Walls and Rainscreens, is the first standard in the property protection industry to provide third-party certification for internal fire safety and other product performance factors for this important product category.
The outer wall layer of cavity wall construction may be made of a noncombustible material such as brick, block or precast concrete, while the inner wall can be combustible or noncombustible material. Cavity walls are primarily used to allow moisture that has penetrated the outer wall to drain down inside the cavity space where flashing directs the water out of the wall base through weep holes.
In many cases, insulation is attached to the inner wall to improve energy efficiency. Depending on the combustibility of insulation and other wall components (e.g., vapor barriers, drainage mats, sealants, anchors), cavity walls and rainscreen walls can pose a fire risk. In fact, during a 15-year period (through 2007), worldwide clients of a major property insurer experienced more than US$156 million in total losses due to fires in concealed wall spaces.
A 2014 report by the Fire Protection Research Foundation, entitled "Fire Hazards of Exterior Wall Assemblies Containing Combustible Components", noted that for the period of 2007 through 2011, a total of 5,346 fires were reported in the United States in which the area of origin, the item first ignited or the item contributing most to flame spread was an exterior wall. These fires resulted in approximately US$295 million in property damages.
The same report included fire statistics from Australia and New Zealand. From 2004 through 2007, the New South Wales Fire Brigade of Australia reported that 129 fires originated in wall assemblies and/or concealed wall spaces. In New Zealand, from 2005 through 2010, 501 fires were reported to have originated in wall assemblies and/or concealed wall spaces.
According to Richard Davis, assistant vice president, senior engineering technical specialist, with FM Global’s engineering standards team, potential sources of fires inside cavity walls and rainscreen-clad walls can include hot work such as welding or soldering, electrical shorts and exposure from fires originating inside a building. “Once a fire gets into a wall cavity, if the insulation is combustible, the fire can burn undetected for a long while and may be difficult for fire departments to locate and extinguish,” notes Davis. “Since these types of walls are often used in large facilities such as school dormitories, hospitals and office buildings, there can be extensive smoke damage that can render the building uninhabitable for long periods.”
Davis says that cavity walls that use plastic foam insulations such as expanded or extruded polystyrene are particularly vulnerable. “Manufacturers and builders like these types of plastic insulation because they are moisture-resistant, lightweight and provide a high level of insulation,” he notes. “However, polystyrene has a lot of fuel content and is highly combustible. Buildings with cavity walls and rainscreen cladding, built with combustible insulation and other materials, are especially susceptible when they are under construction or undergoing renovations or repair. That’s when there’s often hot work and wiring taking place.”
The foundational work to develop a consistent and cost-effective fire performance test for the evaluation of cavity walls began as a demonstration by FM Global for a group of health care and educational clients in 2007. “To our knowledge, there has been no standard fire performance test available anywhere that addressed what happens if there is a fire source inside a wall assembly such as a cavity wall or rainscreen construction,” notes Kristin Jamison, a senior research engineer for FM Global. “Wall assemblies for testing are meticulously constructed and tested against external fire conditions, but that testing is not addressing the full risk potential for cavity walls that combine internal insulation and other potentially combustible components.”
One challenge is that certain industry fire tests, particularly ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials, has been used for many years to evaluate the flame spread index of foam plastic insulation. However, according to Jamison, this test is inadequate because the test specimen is mounted horizontally and the material can shrink away from the test fire, indicating artificially favorable fire characteristics.
Jamison and her research colleagues developed a new fire test apparatus (Figure 1) that more accurately simulates the characteristics and configuration of cavity walls and rainscreen constructions. This new fire test, which is the foundation for Approval Standard 4411, enables insulation and other wall components to be evaluated in the same vertical orientation in which they would be used on a building.
The test apparatus was evaluated in the Fire Technology Laboratory at the FM Global Research Campus in West Glocester, Rhode Island, USA, with a series of tests using rigid extruded polystyrene (EPS) and sprayed polyurethane foam insulation. A total of 11 test samples were constructed and tested under a 5-MW calorimeter. Samples were mounted between two noncombustible panels and subjected to a propane burner ignition source designed to simulate a real-world exposure from hot work or electrical equipment.
Tests were conducted using 2-inch (51-mm) and 4-inch (102-mm) air gaps in the cavity wall assemblies. Not surprisingly, the rigid polystyrene insulation performed much worse (Figure 2) than the sprayed polyurethane, with flame spread extending beyond the 8-foot (2.4-m) test apparatus for both air gap sizes with charring or complete consumption of the sample over its full length. By contrast, flame spread and charring of the sprayed polyurethane samples (Figure 3) was limited to 5.9 feet (1.8 meters). The peak chemical heat release rates measured during the EPS insulation tests ranged from 835 to 1,630 kW, while the chemical heat release rates for the sprayed polyurethane remained below 100 kW throughout.
Following this successful series of tests, a team led by FM Approvals senior engineer Jill Norcott incorporated the new test methodology into Approval Standard 4411. Along with the new fire spread test described above, the standard includes tests for:
- Static and Cyclic Wind Loading. Cavity wall and rainscreen constructions that have an exterior surface made of concrete, masonry or similar materials will not be subjected to these tests.
- Corrosion Resistance. Corrosion resistance testing is conducted on fasteners and fastening components that are located inside cavity wall or rainscreen construction using ASTM D6294, Standard Test Method for Corrosion Resistance of Ferrous Metal Fastener Assemblies Used in Roofing and Waterproofing, or EN 871, Building Hardware—Corrosion Resistance—Requirements and Test Methods.
- Hail Resistance. Cavity wall and rainscreen constructions that have an exterior surface made of concrete, masonry or similar materials will not be subjected to this test.
- pH Value Determination. Insulation materials used within cavity walls and rainscreen construction must demonstrate a limited contribution to the corrosion of metallic components.
- Noncombustible Core Rated Insulation (optional).
- Manufacturing Quality Control Tests. Data from a series of small-scale quality control tests will be maintained on file by FM Approvals and periodically retested to ensure manufacturing consistency.
- Demonstrated Quality Control Program, Surveillance Audits and Installation Inspections.
“Since the majority of cavity walls use some type of masonry or concrete exterior, we require only calculations from a registered design professional to confirm the wind load ratings,” notes Norcott. “Of course, if the wall has a brick or concrete exterior surface we’ll grant that manufacturer a severe hail rating as well. For other materials such as metal, we require the hail resistance test.”
Norcott adds, “Until now, there have been no industry standards that fully address the interior fire risks of cavity wall and rainscreen systems. Even tests required by the current building codes, such as NFPA 285, do not fully evaluate the internal fire risk and fire spread inside these wall systems. Approval Standard 4411 provides a new way for manufacturers to differentiate their cavity wall and rainscreen systems and provides designers, buildings owners and others with a third-party certified solution that can help reduce the real-world fire risk posed by these types of wall systems.”