Radiant heat (bushfire)
Radiant heat (kW/m²) is the heat flux from a bushfire flame front used by AS 3959 to set BAL. Bands: 12.5, 19, 29, 40, FZ. Drives glazing and cladding spec.
Ask Chalkline about this →Radiant heat in the bushfire context is the heat flux (measured in kW/m²) emitted by a bushfire flame front and felt by a building at a given distance from the fire. It is the primary parameter AS 3959:2018 uses to set the Bushfire Attack Level (BAL) for a building element, which in turn drives the required construction spec for glazing, walls, frames, and openings.
The five BAL tiers and their heat flux:
| BAL | Heat flux | Description |
|---|---|---|
| BAL-Low | < 12.5 kW/m² | Insufficient risk to require special construction. Ember controls only. |
| BAL-12.5 | 12.5 kW/m² | Ember attack + radiant heat from a distant fire. |
| BAL-19 | 19 kW/m² | Increasing radiant heat. |
| BAL-29 | 29 kW/m² | High radiant heat with some short-term flame contact possible. |
| BAL-40 | 40 kW/m² | Severe radiant heat with potential flame contact. |
| BAL-FZ (Flame Zone) | > 40 kW/m² | Direct flame contact expected. The highest-risk tier. |
What radiant heat does to a building:
- Heats wall and window surfaces until they pyrolyse (release flammable gas) and ignite.
- Cracks ordinary annealed glass within seconds at high flux. Toughened or fire-rated glass is required at BAL-29+.
- Ignites unprotected timber (decking, eaves, window frames) without flame contact.
- Melts plastics (rainwater downpipes, fascia trims, plastic skylight glazing) at lower flux thresholds.
Construction requirements escalate sharply at each tier:
- BAL-12.5: ember-attack controls dominate; basic timber framing with fire-stopping is acceptable.
- BAL-19: BAL-rated glazing (Grade A toughened or BAL-rated laminated); non-combustible cladding within 400 mm of ground.
- BAL-29: BAL-29-rated glazing throughout; non-combustible cladding generally; deck framing requires non-combustible materials or treated timber.
- BAL-40: BAL-40-rated glazing (specialised IGU or fire-rated); non-combustible wall framing options expand; very limited timber permitted externally.
- BAL-FZ: full bushfire shutter systems, fire-rated walls (typically 60 min FRL), specialised glazing, no timber externally.
How heat flux is determined for a specific site. Under AS 3959, an accredited Bushfire Planning and Design (BPAD) consultant measures:
- Vegetation type within 100 m to 150 m of the building (forest, woodland, scrub, grassland).
- Slope down to and across the vegetation.
- Fire Danger Index (FDI) for the climate zone.
- Distance from the closest vegetation in each direction.
The standard’s tables and formulas convert these into the radiant heat flux for each face of the building, which then becomes the BAL rating per face.
For builders.
- Multiple BAL ratings per house are normal. A south-facing wall might be BAL-12.5 while a north-facing wall faces directly into a forested slope and rates BAL-29 or higher. Each face is built to its rating.
- Cost scales steeply. BAL-12.5 adds typically $5,000 to $15,000 vs non-bushfire spec; BAL-FZ can add $50,000 to $150,000+. Cost the rating in at quote stage, not after siting decisions.
- Glazing is the dominant cost driver. BAL-29 windows in residential sizes are typically 2-3× the cost of standard double glazing.
Also known as: heat flux, thermal radiation, BAL heat flux.
Category: Approvals / bushfire / building physics.
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Last updated: 2026-05-14. Verified: 2026-05-14.