What’s different about silicone air water barriers?
When it comes to air water barriers, most of us have a preference. If you work with design firms, you’ll quickly learn that some have a brand preference, and yet another has a completely different preference that gets specified on nearly all projects. When I talk with building product representatives in the AWB space, I hear a lot about product thickness, perms rates, low air leakage results, and warranties. The information can get a little confusing, and knowing the differences in AWB products can lead to some interesting debates.
But what’s different about silicone air water barriers? In general, silicone AWB’s are fluid-applied to sheathing and tested to the same standards as other AWB’s. At first glance, you’ll notice the products offer low air leakage results (typically 0.0002 cfm/ft2 per ASTM E2357), a flexible installation range of 0 – 150˚F, and tolerance to direct UV exposure.
An area that seems a little hazy is the NFPA 285 compliance, and when one product is preferred over another. Then you add in ASTM E84, ASTM E 1354, and all the other technical information, you can come to the conclusion that AWB’s are similar with different formulations and manufacturers.
To learn more, we reached out to the Building Science Guys at dtr Consulting Services for a quick lesson on why silicone AWB’s are unique. It all starts with California Building Code, CBC 1403.5.
CBC 1403.5 Vertical and Lateral Flame Propagation
CBC 1403.5 requires that combustible AWB in certain wall assemblies taller than 40 feet, or higher than 40 feet above grade be tested according to NFPA 285, which can be an onerous burden for some assemblies using plastic foam exterior continuous insulation or for claddings other than brick, stone, concrete etc. of significant thicknesses. See the snip below from the 2016 code (no change in 2019).
The yellow highlight describes the attributes of a combustible AWB that is permitted.
The following are the critical criteria and attributes of the silicone AWBs from our specs, showing that they meet the yellow highlight thresholds of CBC 1403.5 Exception 2.
Silicone air water barriers offer the properties for meeting both requirements.
|Surface Burning||Flame Spread: 10
Smoke Development: 185
NFPA Class A, UBC Class 1
|Oxygen Consumption (Cone) Calorimeter||Effective Heat of Combustion: 9.8 MJ/kg
Peak Heat Release Rate: 97 kW/m2
Total Heat Release: 5.6 MJ/m2
The Oxygen consumption value is below the threshold required to support combustion, but both criteria of surface burning and oxygen consumption (effective heat of combustion) are code requirements. This means that the material will burn if adjacent materials are on fire, but will not burn by itself.
Many building product representatives and design professionals are unaware of the code requirement. Products like GE’s Elemax 2600 AWB and Dow’s DefendAir meet the requirements. They are a critical component of a code-compliant wall assembly.
So, what’s different about silicone air water barriers? Sometimes it’s all about the building code.
To learn more about specifying GE Elemax 2600 AWB, visit 07 27 26 Fluid Applied Membrane Air Barriers.
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