“Bush Fire Attack Level” or “BAL” is the term used to describe the level of bush fire risk a home could be subjected to.
Factors that determine a home’s “BAL Rating” include: The area you live in, the vegetation, the distance from the vegetation to your home, and the slope of the land.
There are 6 classifications that BAL ratings fall into:
BAL LOW:
At BAL LOW, no special considerations are needed as this rating indicates no expected exposure to bushfire attack.
BAL 12.5:
At this level there is expected ember attack with heat flux of up to 12.5kW/m² (Heat flux and calculations explained below)
BAL 19:
At this level there are increasing levels of ember attack and burning debris ignited by windborne embers, together with increasing heat flux of up to 19kW/m²
BAL 29:
Increasing levels of ember attack and burning debris ignited by windborne embers, together with increasing heat flux of up to 29kW/m²
BAL 40:
Increasing levels of ember attack and burning debris ignited by windborne embers, together with increasing heat flux of up to 40kW/m² and with the increased likelihood of exposure to flames.
BAL FZ:
Direct exposure to flames from fire, in addition to heat flux above 40kW/m² and ember attack.
What is heat flux?
Heat flux or thermal flux is the rate of heat energy passing through a certain surface. Temperature differences in a given system induce a heat flux. The induced heat flux always flows from the hot to the cold side.
In order for heat flux to exist, it requires, not only a temperature difference, but also a medium through which heat is flowing. Heat can flow through solid materials, gases, liquids, and through electromagnetic waves.
What this means, is that temperatures inside a house can increase enough to start setting internal materials alight, creating a fire within the house as well as outside.
Heat Flux is measured in Kilowatts per metre squared (kW/m²). This relates to the amount of heat energy being exerted on a 1 metre space. 1kW of heat energy is about equivalent of the sun’s heat on a 1 metre space on a summers day.
With the definition of BAL and understanding of heat flux in mind, it’s important to know that each level of BAL requires additional consideration when a house is being built and as such, more work at each level.
These considerations are always changing and being updated, but are a step in ensuring that homes are adequately equiped with the right bushfire defences. Especially when building in the blue mountains.
If you’re unsure of your homes BAL rating or looking at a new build, you can use the RFS website to find out if you’re in a bushfire prone area. And if so, apply to council or contact a qualified BAL assessor to conduct a proper assessment.
Sources:
https://www.bushfireprone.com.au/what-is-a-bal/
https://www.rfs.nsw.gov.au/plan-and-prepare/building-in-a-bush-fire-area/building-after-bush-fire/your-level-of-risk
https://www.sciencedirect.com/topics/chemical-engineering/heat-flux
https://www.greenteg.com/heat-flux-sensor/about-heat-flux/what-is-heat-flux/
https://en.wikipedia.org/wiki/Thermal_conductivity#/media/File:Simple_definition_of_thermal_conductivity.png
https://forum.solar-electric.com/discussion/4150/1-kw-per-square-meter
