The cavity wall conundrum

An NPFA 285 fire propagation assembly test in progress.
Photo courtesy ATI

Making it airtight
Air barriers have been part of the National Building Code of Canada (NBC) since 1985. In the United States, air barriers were first adopted in the State of Massachusetts Building Code in 2000. For many other states, air barriers began to gain recognition with designers when the Code of Record became the 2012 IBC, which requires a continuous air barrier inclusive of the roof.

Part of specifying a proper air barrier involves selecting one with the right vapor permeance. Proper vapor permeance is determined by several parameters, including climate zone, interior relative humidity (RH), and the mechanical system (and whether it is designed to provide a positive or negative pressure), but primarily where the insulation is in the wall.

There are a few different ways to evaluate an air barrier, and these also happen to be the same compliance paths in IBC and IECC. To be compliant with both codes, an air barrier needs to pass one of the following evaluation methods, listed in order of magnitude:

Material test
ASTM E2178, Standard Test Method for Air Permeance of Building Materials, is a pass/fail test at the threshold of 0.02 L/(s m2) @ 75 Pa (0.004 cfm/sf @ 0.3” w.c.). This test is based on the air permeance of 13-mm (½-in.) gypsum. While it is fairly easy for materials to pass, as with all tests, it is important the air barrier manufacturer has the evaluation performed by an accredited third-party testing facility.

Assembly test
ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, is more rigorous than ASTM E2178, as it evaluates an entire assembly rather than just the AWB material. Since it is performed in a lab, manufacturers can use fastener cap washers, tapes, and sealants not typically employed in the field to pass the test. This is a pass/fail test in which an opaque wall is evaluated against one with a mock window buck, penetrations, and an outlet. The air barrier system is also terminated at what would be the foundation and the roof.

The sample walls are put under sustained cyclic and gust loads replicating worst-case conditions. If the wall with the penetrations leaks more than 10 percent at 75 Pa versus the opaque wall, it fails. When the Air Barrier Association of America (ABAA) evaluates air barrier products, part of the assessment includes ASTM E2357. The association uses 0.20 L/(s m2) @ 75 pa (0.04 cfm/sf @ 1.56 lb/sf) as its pass criteria.

Whole building airtightness
ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, is the gold standard in air barrier performance testing. The U.S. Army Corps of Engineers (USACE), having proven airtight buildings offer profound energy savings, has required ASTM E779 for several years. The national standard requires testing the building @ 75 Pa. However, while a material can leak at 0.02 L/(s m2) @ 75 Pa (0.004 cfm/sf @ 0.3 in. w.c.), an entire building can only leak at 2 L/(s m2) @ 75 Pa (0.4 cfm/sf @ 0.3 in. w.c.). USACE lowers the national standard to 1.25 L/(s m2) @ 75 Pa (0.25 cfm/sf @ 0.3 in. w.c.).

Thermal resistance and ci
The 2012 IECC requires ci in all above-grade walls for all climate zones. American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, defines ci as:

Insulation that is uncompressed and continuous across all structural members without thermal bridges other than fasteners and service openings.

Stuffing insulation between Z-girts is not consistent with ASHRAE 90.1. If a project uses horizontal girts, they should be shimmed from behind so water is free to run down the AWB and not become trapped.

Although sprayed polyurethane foam (SPF) and expanded polystyrene (EPS) are used as insulation in cavity wall assemblies, thermoplastic extruded polystyrene (XPS) is a much more prevalent ci. XPS is a thermoplastic foam rigid insulation board. As a combustible thermoplastic polymer, XPS generally melts and drips prior to ignition when exposed to a fire source.

Due to its fundamental combustion properties, XPS is not used behind combustible claddings in cavity wall systems that must pass NFPA 285 for resistance to fire propagation. In such situations, mineral wool or fire-enhanced polyisocyanurate (polyiso) must be used instead. For noncombustible-cladded NFPA 285 assemblies, XPS is a realistic option, as the masonry or other noncombustible cladding provides adequate fire protection.

XPS also has the highest resistance to water absorption of any type of foam plastic insulation, allowing it to maintain its R-value in wet cavity wall locations. According to the Extruded Polystyrene Foam Association (XPSA), the aged R-value of XPS at 50 mm (2 in.) is R-5.0 per inch @ 24 C (75 F).

Since polyiso is a thermoset plastic, it is less susceptible to burning than XPS, but will char and smolder when exposed to fire. This behavior enables certain types of polyiso, with additives in the foam, to be used behind combustible claddings and pass NFPA 285. Designers should check with the manufacturer to verify the polyiso under consideration is suitable for such applications.

According to Polyisocyanurate Insulation Manufacturers Association (PIMA):

Among all foam plastics, polyiso possesses the highest level of inherent fire resistance due to its unique structure of strong isocyanurate chemical bonds. These bonds result in improved high-temperature resistance (up to [200 C] 390 F, more than twice the temperature resistance of other building insulation foams) which in turn leads to enhanced fire resistance.

It is uncommon to see more than a 76-mm (3-in.) layer of polyiso pass an NFPA 285 test with a combustible cladding. The aged R-value of foil-faced polyiso, per ASTM C518, Standard Test Method for Steady-state Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus, at 50 mm (2 in.) is 6.25—6.5 per inch.

For NFPA 285 compliance, mineral wool offers designers a get-out-of-jail-free card. Offering a heat resistance of 850 C (1562 F) and a melting point of 1177 C (2150 F), mineral wool essentially will not burn. Mineral wool is not limited by thickness, so any thickness of insulation can be installed and maintain compliance. Mineral wool has a flame spread and smoke developed of ‘zero’ per ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials. The R-value of one manufacturer’s exterior wall product ranges from R-4.0 to R-4.3 per inch.

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