energies Article Bushfire: Retrofitting Rural and Urban Fringe Structures—Implications of Current Engineering Data Glenn P. Costin   Citation: Costin, G.P. Bushfire: Retrofitting Rural and Urban Fringe Structures—Implications of Current Engineering Data. Energies 2021, 14, 3526. https://doi.org/10.3390/ en14123526 Academic Editors: Mark Luther, Igor Martek and Mehdi Amirkhani Received: 19 April 2021 Accepted: 24 May 2021 Published: 14 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). School of Architecture & Built Environment, Faculty of Science, Engineering and Built Environment, Deakin University, Geelong, VIC 3220, Australia; g.costin@deakin.edu.au 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. Keywords: bushfire; retrofit; ember attack; pressure differential; urban fringe; rural housing; pyro- tornadogenesis; backdraft 1. Introduction Prior to the Black Saturday fires of 2009 Australian bushfire policy could be summed 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, holds today, that ember attack can be defended against, and thus many homes saved. The policy, however, was based upon another premise: that homes were prepared, and 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]. Whilst conceptually ‘stay or go’ still exists, active defense risks to homeowners, discussed by many including state and territory fire authorities [2,3,9–12] have altered the underpinning message. Today, Australians are advised to prepare property before 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 forced removal from a home property are debatable, state specific and unclear [15,16]. Occasionally evacuation is not possible; on others, the indicators of potential fire are low, and communities are taken by surprise. In December of 2015, over 100 homes were lost in the Victorian coastal community of Wye River [6]. On that occasion, the McArthur Forest Fire Danger Index (FFDI)—the measure by which Australia’s fire danger levels of High, Very High, Severe, etc., are identified—was only 49 or ‘Very High’ [6]. Code Red or Catastrophic is 100+. Fortunately, due to the fire’s approach direction, no lives were lost. Energies 2021, 14, 3526. https://doi.org/10.3390/en14123526 https://www.mdpi.com/journal/energies