Selection of Windows for Bushfire Zones

Requirements

In 2018, Australian Standard (AS) 3959 was updated and republished as AS 3959:2018 Construction of buildings in bushfire prone areas. This Standard is primarily concerned with improving the ability of buildings in designated bushfire prone areas to better withstand attack from bushfire thus giving a measure of protection to the building occupants and the building itself until the fire front passes. Research is ongoing with regards to the effects of bushfires on buildings, the determination of bushfire prone areas within Australian states territories, and construction techniques designed to maximise the performance of buildings when subjected to bushfire attack. The outcomes of this research will be reflected in subsequent editions of AS 3959.

Under the Standard, all new homes and renovations are assessed and rated to one of six Bushfire Attack Level (BAL) categories ranging from low to flame zone. The BALs are based on heat flux exposure thresholds that measure the amount of energy per square metre of radiant heat exposure. Once the BAL has been determined, specific construction requirements must be followed, ranging from ember protection to direct flame protection.

NOTE: Compliance with both AS 2047 and AS 1288 is required.

Bushfire Attack Levels

AS 3959 provides a methodology for categorising building sites into one of six levels of severity expressed as Bushfire Attack Levels (BAL). The description of levels of exposure for each BAL is outlined below.

Bushfire Attack Level
(BAL)
Description of Predicted Bushfire Attack and Levels of Exposure

BAL-Low

There is insufficient risk to warrant specific construction requirements.

BAL-12.5

Ember attack.

BAL-19

Increasing levels of ember attack and burning debris ignited by windborne embers together with increasing heat flux between 12.5 and 19 kWm2.

BAL-29

Increasing levels of ember attack and burning debris ignited by windborne embers together with increasing heat flux between 19 and 29 kWm2.

BAL-40

Increasing levels of ember attack and burning debris ignited by windborne embers together with increasing heat flux and the increased likelihood of exposure to flames.

BAL-FZ

Direct exposure to flames from fire front in addition to heat flux and ember attack.

Compliance Pathways

AS 3959:2018 clause 3.8 (General Construction) provides that window and door systems that achieve a level of performance for any given BAL when tested in accordance with AS 1530.8.1, for BAL–12.5 to BAL–40 or AS 1530.8.2 for BAL–FZ, then satisfy the requirements of that BAL.

The Deemed-To-Satisfy (DTS) requirements of Sections 4 to 9 only apply where a system has not been tested.

Screens for windows & doors

Various provisions within AS 3959 require screening of windows and doors. These requirements can be broadly divided into two categories: radiant heat protection – to reduce the radiant heat load on glazing, or ember protection – to protect openings from burning embers and debris entering.

The provisions for radiant heat protection are primarily associated with low-level glazing, which is glazing within 400 mm of the ground, or other horizontal (or near horizontal) surface. At these locations, where small leaf litter and other flammable debris may accumulate, the window or door system is particularly vulnerable to flame contact and higher radiant heat loads. The purpose of the screening in this instance is to form a barrier to offset direct contact of flames and to attenuate the subsequent radiant heat. Screening used for this purpose must be installed externally, using materials appropriate for the relevant BAL.

Screening is also prescribed by AS 3959 to protect the building from burning embers, which may travel several kilometres ahead of the fire front and attack buildings before the occupants are fully prepared. Windows, which may inadvertently be left open while the property is being prepared, are particularly vulnerable during ember attack. For this reason, it is necessary that all window openings be screened, either internally or externally, using materials appropriate for the relevant BAL. These provisions do not apply to doors.

NOTE: Where a system satisfies the test criteria without screening for ember protection, the requirements of the Standard for screening of the openable parts of windows still applies.

NOTE: Insect screens that are fitted internally, and are completely protected by the closed door, are not required to meet these provisions.

Glazing

Various glass types and thicknesses are prescribed within AS 3959 for the various BALs. In each case, where double glazing is used, the requirements apply to the outer pane only.

A Note on BAL- Low

BAL-Low is described as having insufficient risk to warrant specific construction requirements. Standard window and door products may be used at this level.

Industry Guide

As part of its commitment to raising the built performance standard of windows and doors in Australia, AGWA has published an industry guide on the requirements for windows and doors in bushfire prone areas. For more information, download the guide below.

A Guide to Windows & Doors in Bushfire Prone Areas