Performance Solutions for External Fire Spread: The Role of Radiant Heat Screens

This article explores the functionality and regulatory considerations of radiant heat attenuation screens, illustrating how Jensen Hughes can support their integration into performance-based design solutions.

What Are Radiant Heat Attenuation Screens?

Radiant heat attenuation screens are specialised barriers installed on the exterior of window openings. They are designed to reduce radiant heat transmission while permitting natural light and ventilation. Typically composed of stainless-steel mesh within a metal frame (steel or aluminium frame), these screens provide a protective barrier against radiant heat and embers.

Initially developed in the 1990s as an alternative security product to conventional security bars and grilles, these screens offered effective protection against home burglaries. Over time, it was recognised that these screens also had properties that could be utilised in fire safety engineering design, making them valuable dual-purpose products. Today, these screens are widely used in bushfire-prone areas and urban settings, effectively providing both security and fire safety.

How Do These Screens Work?

Radiant heat screens reduce the risk of piloted and non-piloted ignition in two main ways:

• Reducing non-piloted ignition: The stainless-steel mesh reflects and absorbs radiant heat, significantly decreasing the heat flux passing through window openings. This reduces the likelihood of spontaneous ignition of combustible materials inside the building.
• Preventing piloted ignition: The mesh provides a physical barrier that prevents embers from entering the building, reducing the risk of external ignition sources igniting combustible materials within the building.

What Is Piloted and Non-Piloted Ignition?

To understand piloted and non-piloted ignition we first consider the three mechanisms of fire spread: conduction, convection and radiation. 

• Conduction takes place when two items are in contact and heat is transferred from the ignited object to the other.
• Convection takes place when the flame of the fire plume carries sufficient heat to cause another object to ignite. This can either be by piloted or non-piloted ignition.
  • Piloted ignition is achieved by the introduction of an ignition source such as a flame from a burning ember or a spark.
  • Non-piloted ignition occurs when an object’s temperature is raised sufficiently to cause the onset of combustion, which is spontaneous.
• Radiation is heat transfer through electromagnetic waves, such as the heat felt from the sun or from a fire at a distance. It occurs when the intensity of a fire is sufficient to radiate heat onto nearby objects and cause ignition. This can again be by either piloted or non-piloted ignition. 

Fire spread from one building to another via openings is most likely to occur by radiation, as there is usually sufficient separation to limit heat transfer by conduction and convection.

Why Protect Windows? 

Fire-resistant external walls provide little benefit if they contain unprotected openings that allow fire spread. Clause C4D3 of the NCC states that openings in an external wall that is required to have an FRL must be protected in accordance with clause C4D5 of the NCC if the distance between the opening and the fire-source feature to which it is exposed is less than:

• 3 m from a side or rear boundary of the allotment; or
• 6 m from the far boundary of a road, river, lake or the like adjoining the allotment, if not located in a storey at or near ground level; or
• 6 m from another building on the allotment that is not Class 10.

The purpose of this requirement is to prevent fire spread from an adjoining allotment boundary or between buildings on the same site.

What is a Fire-Source Feature?

A fire-source feature is defined in the NCC as any one or more of the following:

• The far boundary of a road, river, lake or the like adjoining the allotment.
• A side or rear boundary of the allotment.
• An external wall of another building on the allotment which is not a Class 10 building.

Can Radiant Heat Screens Be Used as a Method of Protection?

Yes, but this requires a fire engineering performance solution.

Radiant heat screens are not included as a deemed-to-satisfy solution (DtS) under clause C4D5 of the NCC. Clause C4D5 specifies only three methods of protection for window openings:

• External wall-wetting sprinklers
• -/60/- fire windows
• -/60/- fire shutter

Radiant heat screens must undergo case-by-case assessment by a fire safety engineer to demonstrate compliance with Performance Requirement C1P2. The use of radiant heat screens is dependent on:

• The location of the openings relative to the boundary
• The orientation of the openings to the boundary
• The size of the openings
• The number of openings
• The radiant heat screen used. 

Important Considerations and How Jensen Hughes Can Help

When selecting radiant heat screens, several important factors must be considered to ensure effective fire safety:

• Ensure screens are tested per AS 1530.4:2014, particularly Appendix B7.
• Tested systems specify maximum opening sizes and suitable wall constructions.
• Different screens attenuate varying radiation levels. 

Jensen Hughes’ fire safety consultants can assist in interpreting test reports and confirming whether chosen screens match project specific requirements.