Bushfire-proof houses: raising the standards

Researching what happens when you subject a steel framed house to bushfire conditions.

Building houses that can survive any extreme weather events is challenging but particularly so for bushfires. Not only does the house need to be able to withstand in some cases the direct onslaught of flame and high temperatures, but it also needs to be strong enough to maintain its integrity in high winds, with debris flying around and embers being blown onto the house.

The perfect bushfire-proof house would be a concrete bunker with no windows. But given most houses in bushfire-exposed settings are there because their owners have actively sought out a bushy vista, this design is unlikely to win any takers.

Designing houses to withstand bushfire is about balancing not only the bushfire resilience of the house, but also the aesthetic qualities of the house, the aesthetic appeal of the surroundings, and the cost, practicality and availability of materials.

A house in a bushfire may be faced with three ‘waves’ of assault. The first is the radiant heat that arrives just ahead of the fire front itself, which can start to build as much as thirty minutes before the arrival of the flames. Once the flames themselves hit, a house in a flame zone environment could be subjected to temperatures as high as 1000°C for a couple of minutes. But even after this initial front passes, the danger is not over; there is still a threat from residual radiant heat that takes a while to ease and embers that continue to bombard the house for many hours.

Using research to set a new standard

Australia has a building standard for bushfire-prone areas – the AS3959 – which covers everything from sub-floor supports and floors to roofs, verandahs and gas pipes. This standard was most recently updated following the devastating Black Saturday bushfires in Victoria, and sets the standard for each Bushfire Attack Level (BAL) rating.

But now there’s a new standard in town, which is specifically targeted at houses built with steel frames.

Dr Justin Leonard assessing damage with the NSW Rural Fire Service after bushfires in the Blue Mountains, 2013.

The National Association of Steel-Framed Housing (NASH) bushfire standard takes a rigorous, evidence-based approach to defining how steel-framed houses can be designed to avoid destruction in bushfires, that takes into account not just the external cladding or veneer of the house, but the entire wall, floor and roof system. Recent adoption of the standard into the National Construction Code means that people now have a choice as to which building compliance path they take.

“Given we’ve got this frame made out of steel rather than pine, we can design the whole wall system as a barrier to bushfire, rather than relying on the external cladding to protect the frame behind it,” says CSIRO bushfire expert Dr Justin Leonard, recipient of the 2015 Bushfire Building Professional of the Year Award.

“The standard specifies the cladding, the insulation, and what you can line the inside of the house with, along with the steel frame, and all of a sudden you’ve got a house where, if you didn’t put any furniture in it, actually can’t burn.”

The NASH standard is the result of a research collaboration between the National Association of Steel-Framed Housing Inc. and CSIRO, which is serving as a model for the development of bushfire construction standards in other forms of housing.

The collaboration included representatives from a broad range of industries and companies, each of whom brought their expertise to the table, which enabled detailed specification of many different structural elements.

For example, the insulation experts were able to help specify the requirements for insulation that was non-combustible and where needed could act as a barrier to flames and embers. The location and type of insulation was also carefully considered so that the house could meet and exceed the energy efficiency standards for new buildings

Designing houses for the flame zone

The major distinction when designing bushfire protection for steel-framed houses is made between houses designed as being in the ‘flame zone’, and those in the other, lower bushfire attack levels.

For flame zone houses, the roof insulation requirements, for example, are much more stringent: a layer of reflective foil backed glass wool insulation blanket under the roof sheeting. This protects the steel roof trusses if the roof is directly exposed to flames during the bushfire. Houses at lower BAL ratings do not require the same protection.

The standards also require that all external doors in flame zone-exposed houses be built into steel frames, without any glass panels, and be fully-enclosed by smoke and fire-resistant seals.

Much of the research that underpins these standards was done at the Eurobodalla Rural Fire Service Training Facility, where a full scale bushfire burnover facility has been jointly developed by CSIRO, Gameco and state fire agencies. This facility was developed to provide realistic bushfire burnover conditions for everything from fire truck to houses.

The facility has provided valuable new information for the standard; for example, showing that walls needed extra protection in flame zone-exposed houses. As a result, the recommendation is that flame zone houses included a layer of plasterboard between the sheeting and the steel frame.

A steel framed house before (left) and after being subjected to bushfire conditions and the Eurobodalla test site.

Dr Leonard says the collaboration between CSIRO and the steel-framed housing industry came about because the industry association NASH was not only motivated but also had considerable experience in developing building standards for adoption in the national building codes.

Now the opportunity exists to use these same principles for a wider range of house designs, including double-brick, rammed earth, and even straw bales.

Dr Justin Leonard was awarded the 2015 Bushfire Building Professional of the Year Award by the Bushfire Building Council of Australia.