energies Article Bushfire: Retrofitting Rural and Urban Fringe Structures—Implications of Current Engineering Data Glenn P. Costin School of Architecture & Built Environment, Faculty of Science, Engineering and Built Environment, Deakin University, Geelong, VIC 3220, Australia; [email protected] Abstract: Since the 2009 Black Saturday bushfires in which 173 lives were lost, two-thirds of whom died in their homes, the question of what a home prepared for bushfire looks like has been repeatedly raised. The 2019/2020 fires saw us not much further advanced. This paper seeks to consolidate what is known about bushfire behavior, its influence upon structures, and, through this data, infer improved standards of practice for retrofitting rural and urban fringe homes. In particular, the prevention of ember and smoke incursion: the data suggesting the prior as the main mechanism of home destruction; the latter as high risk to sheltering occupant health. The article is framed around a comprehensive literature review, and the author’s own experiences and observations from fire impacted structures in Victoria’s northeast. The article’s import lies in demonstrating how embers and smoke may enter homes otherwise seen to be appropriately sealed prior to the fire’s approach. Included in the findings are developed hypotheses based on thermal expansion, pressure differentials and backdraft; offering defined paths towards future research. In addition, the work provides practical advice towards mitigating the identified issues using retrofit practices based upon the author’s practical experience as a tradesperson and building designer. Citation: Costin, G.P. Bushfire: Keywords: bushfire; retrofit; ember attack; pressure differential; urban fringe; rural housing; pyro- Retrofitting Rural and Urban Fringe tornadogenesis; backdraft Structures—Implications of Current Engineering Data. Energies 2021, 14, 3526. https://doi.org/10.3390/ en14123526 1. Introduction Academic Editors: Mark Luther, Prior to the Black Saturday fires of 2009 Australian bushfire policy could be summed Igor Martek and Mehdi Amirkhani up by the phrase ‘stay or go’. The premise underlying this approach suggests most homes lost to bush fire succumb to ember attack, not the fire front [1–8]. Received wisdom held, Received: 19 April 2021 holds today, that ember attack can be defended against, and thus many homes saved. Accepted: 24 May 2021 The policy, however, was based upon another premise: that homes were prepared, and Published: 14 June 2021 residents mentally and physically capable of such defense. The year 2009, and the loss of 173 lives—two-thirds of whom died in their homes—changed that perspective radically [2,3]. Publisher’s Note: MDPI stays neutral Whilst conceptually ‘stay or go’ still exists, active defense risks to homeowners, with regard to jurisdictional claims in discussed by many including state and territory fire authorities [2,3,9–12] have altered published maps and institutional affil- the underpinning message. Today, Australians are advised to prepare property before iations. the bushfire season, then leave early should a fire start. In conditions categorized as Catastrophic or Code Red (state dependent categories) the advice is to prepare the home and leave before a fire event begins [13,14]. In emergencies, with or without a ‘state of emergency’ declaration, mandatory evacuations may be ordered, though the legalities of Copyright: © 2021 by the author. forced removal from a home property are debatable, state specific and unclear [15,16]. Licensee MDPI, Basel, Switzerland. Occasionally evacuation is not possible; on others, the indicators of potential fire are This article is an open access article low, and communities are taken by surprise. In December of 2015, over 100 homes were distributed under the terms and lost in the Victorian coastal community of Wye River [6]. On that occasion, the McArthur conditions of the Creative Commons Forest Fire Danger Index (FFDI)—the measure by which Australia’s fire danger levels of Attribution (CC BY) license (https:// High, Very High, Severe, etc., are identified—was only 49 or ‘Very High’ [6]. Code Red or creativecommons.org/licenses/by/ Catastrophic is 100+. Fortunately, due to the fire’s approach direction, no lives were lost. 4.0/). Energies 2021, 14, 3526. https://doi.org/10.3390/en14123526 https://www.mdpi.com/journal/energies Energies 2021, 14, 3526 2 of 18 Both before [17] and after [18] that event, the FFDI as a sole indicator of extreme bushfire risk has been considered questionable. This aside, what remains clear is that homes were not prepared adequately. The 2019/2020 fires again exposed this truth, with fires raging, and homes lost, in all states and territories of the country except the capital. NSW and Victoria took the brunt of losses, with over 2800 houses destroyed in those two states alone. Sadly, on this occasion, 34 human lives were also directly lost [19]. Indirectly, a further 417 lives were lost and 4456 hospitalized due to smoke inhalation [20]. Such deaths give rise to another purpose behind retrofitting: air quality. Occasionally the house is the only shelter available; though Dengate [9] notes that some people decide to stay, others have no such option [2,3,10,21]. Retrofitting will not turn an older home into a bushfire bunker, but it will improve its potential for occupant survival. Much retrofitting discussion is about sealing the home against embers, however as identified above, smoke is just as important, studies also identifying in utero growth restriction of babies through the mother’s bushfire smoke inhalation [22,23]. Examples of new homes purposefully built to withstand bushfire attacks without smoke incursion are promising [24]: improving indoor air quality should likewise be a consideration in retrofitting [25]. So what does a home prepared for bushfire actually look like? Specifically, how might we retrofit a home to improve it, and its resident’s, preparedness? When constructing a new home in Australia, performance requirements are found in the National Construction Code (NCC) [26] and the standard AS3959 Construction of buildings in bushfire-prone areas [27]. However, that standard’s veracity is questioned both in this article and by others [28], due to its currently limited interpretation of ember attack mechanisms. Further advice for new and existing homes is given by state and territory fire agency guides on land and property preparedness. Yet, whilst commentary on defensible Australian homes began as early 1945 [1], only recently has discussion focused on retrofitting. This paper explores the efficacy of these approaches, and AS3959, by drawing upon extant literature, current engineering modeling, reported experiences of homeowners and the author’s own experiences. Improved retrofitting actions are proposed based upon these findings; particularly issues arising from potential pressure differentials in extreme bushfire events. 2. Researching Bushfire and Its Influence Upon Structure What causes a structure to burn is best studied through careful analysis of burnt build- ings [1,4–6,29], practical experimentation [30–32], or, when such is problematic (missing evidence, cost, risk), through computer modeling [33,34]. Much of this work has been, or is being, undertaken in facilities around the globe. This article consolidates that material, combining a comprehensive literature review and anecdotal evidence. In reviewing the literature a broad range of terms and phrases were chased through online search engines, academic libraries, Springer, Scopus, Google Scholar and the like. To this was added news reports, blogs, drilled down for quality evidence such as photographs or events witnessed by multiple parties. The main terms used derived from previously understood retrofitting actions and basic bushfire defense systems such as: bushfire sprinklers; intumescent paints; water tanks and bushfire; glazing; ember attack and the like. These terms were followed by those surrounding the hypothesis of pressure differentials, cyclonic winds, pyrogenic winds and such, prior to turning to the specifics of structural components and materials under temperature extremes. Within this core of articles, the hypothesis of pressure differential was explored; seek- ing more information as the data exposed questions, potential answers, new terminology: testing the hypothesis in light of known data and reported experiences. Future, targeted, research was thus developed in pursuit of insights to aid retrofitting of older homes, guide the construction of new homes, improve legislated standards and, perhaps, solve mysteries surrounding why seemingly secure homes are destroyed by bushfire, whilst others, much older and more decrepit, survive. To interpret the insights gained from this exploration, Energies 2021, 14, 3526 3 of 18 an understanding must first be had of the structure, design and context of the typical Australian home; be it rural, or within the rural-urban fringe interface. The Australian Home: A Structural Description The rationale for retrofitting existing dwellings against bushfire attack is clear. Also clear, within the literature, is that ember ingress is the main concern. To understand how ingress occurs requires appreciation of the typical Australian home’s structure. Gen- erally, this is a one or two story timber or steel frame building clad with brick veneer, timber weatherboard, cement sheet or rendered panels of autoclaved aerated concrete or polystyrene. Roof cladding is either corrugated metal sheeting or tiles of concrete or glazed