There’s some confusion in relation to fire protection of ducts, AS1668.1 and how it calls up AS1530.4 (2014), says TBA Firefly Technical Manager Gavin W.
AS1668.1 requires all ducts irrespective of function – if they are excluded from protection via fire dampers, they must have the FRL from both External and Internal tests simultaneously. Then, the more severe condition (or lower FRL) wins.
From testing Alkaline Earth Silicate blankets (AES)
- Internal Exposure part of the test for 2 layers of Alkaline Earth Silicate (AES) wrapped ducts usually achieves 240/240/120
- External Exposure part of the test for 2 layers of Alkaline Earth Silicate (AES) wrapped ducts at best achieves 240/240/90 (usually 80 minutes only for insulation - the gas leakage through the layers results in thermocouples inside the duct, on the duct surface to exceed Δ 180 K between 80 and 90 minutes)
So, for Ducts with Fire Resistant Enclosing Construction per AS1668.1, with 2 layers of AES wrap, 240/240/90 is useless for ducts passing through 2-hour (120/120/120) fire separating elements.
Testing of 3 layers has demonstrated that 240/240/120 is achievable with AES wraps for External Exposure.
Therefore, 3 layers of AES wraps will usually be needed when passing through 120/120/120 separating elements in order for the system to be approved by the AHJ.
Specifiers/ Installers/ Manufacturers cannot pick and choose or assign exposure direction based on duct function.
There are no concessions in AS1668.1 to allow for ‘Internal Exposure Only’ or ‘External Exposure Only’, except for Smoke Exhaust/ Smoke Spill, where the duct is solely used for smoke exhaust, and is not a hybrid system. In the case of smoke exhausts, the Insulation Criteria of External Exposure only can be waived and is detailed in AS1668.1 C3.7.2. Only then does the overall insulation component of the FRL revert to the insulation result from Internal Exposure. In this case only 2 layers of AES wrap could be on a duct passing through a 2-hour wall or floor and be certified by the AHJ.
AS1530.4 does not give any concessions either. However, some prefer to interpret the ‘e.g.’ in Section 9.1 as ‘Normative’, and therefore, meaning ‘exhaust ducts need only be tested for internal exposure’. This is simply incorrect.
The ‘e.g.’ is not a concession or directive to ascribe an exclusive direction of exposure and subsequent FRL from testing. The reality is, for 2 hours’ protection for a duct without fire dampers – if going through a 2-hour wall/floor and wanting to use wrap, you are going to need 3 layers of AES wrap, i.e., 152mm minimum thickness including all of the layering laps.
The issue between AS1530.4 (2005) Section 9 and AS1530.4 (2014) Section 9 (ducts), how AES wrap suppliers took advantage of an error, and how this error in the 2005 version was allowed to persist for 9 years is a topic for a separate article.
