Award-winning NSW Blue Mountains building designer Mark Davis made the startling claim at the Australian Bushfire Building Conference.

"It is a ticking time bomb," he said.

Together with David Boverman from the NSW Rural Fire Service, he told the assembled crowd about the "trials and tribulations of trying to design, get approval and build a roof that will protect from bushfire and not cause mould and rot in the building".

Mr Davis said he had to raise the issue "so I wasn't designing houses that allow rot and mould to develop making the occupants sick".

"The 'deemed to satisfy' solutions for flame-zone roofs under the National Construction Code can cause condensation to develop in the roof, causing rot and mould and a deterioration in the structure. The colder the climate the more likely it is to occur," he said.

Mr Davis said the topic has been "somewhat controversial ... getting people to acknowledge it has been okay, getting them to do something about it is another matter. The bushfire regulations are very strict and don't allow a lot of wiggle room".

He said some builders "have suspected it" and as a Passive House Certified Designer he had learnt a lot about condensation.

"We have modelled the 'deemed-to-satisfy flame-zone roofs' from the Australian standard using the Upper Blue Mountains climate and it showed varying risks from likely to extreme.

"Being someone who designs a lot in bushfire-prone areas, I realised that the flame-zone details did not manage condensation well, if at all; particularly in cold climates like the Blue Mountains.

"These problems have arisen when owners have discovered it and called for help. So I guess you could say it is a ticking time bomb based on similar examples from around the country."

Conference goers were told the information was backed up by The University of Tasmania and the pair had also looked at German software that models condensation and mould risk.

A 2018 report by University of Tasmania lecturer and designer Dr Mark Dewsbury raised the issue about lack of roof space ventilation - to remove unwanted water vapour - and issues with "thermal bridging between roofing and sarking materials and ... impermeable building wrap".

Titled a "Technical Guide for the Mitigation of Condensation and Mould in Southern Australian Homes", it was produced from research funded by the Consumer, Building and Occupational Services Department of Justice, Tasmania.

It said: "To ensure that water vapour can freely and passively leave the building, all materials that are combined to make an external wall should be vapour permeable".

"The use of vapour-impermeable pliable membranes on the walls of buildings should be avoided. The pliable membrane, (or building wrap) is installed to assist in building sealing and to hold insulation within the wall frame. Internationally, this membrane is also referred to as a vapour control layer, which is required to have climate specific vapour permeable properties.

"Most reputable pliable membrane manufacturers can specify and provide a climatically appropriate building wrap system".

Bushfire expert Dr Grahame Douglas said the problem is not universal, but agreed it can be an issue in the Blue Mountains and other cool climates and could affect walls as well as roofs.

Dr Douglas is the academic course adviser for postgraduate construction at Western Sydney University and a long-standing member of the Australian Standards Committee for Bushfire Construction. He was involved in putting the standard together for the flame-zone, including the roofing system. He also developed the original Planning for Bushfire Protection for the Rural Fire Service in 2001 and then revised in 2006.

"These issues can be complex, but solutions can be found. Mark is really leading the way."

He said other building designers and architects needed to be educated about it, otherwise it would "keep being perpetuated".

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He believes new performance requirements for condensation from the National Construction Code, which came in on October 1, could help resolve the issue but Mr Davis is not so sure.

Changes to the Australian standard are involved and expensive.

Mr Davis said his own solution was designing a new roof that a fire engineer could sign off on.

"My presentation was about the problem itself, and the difficulties in using performance solutions [like more breathable materials] to solve it".

Since giving the speech Mr Davis has been asked to provide more details to architects, building designers, bushfire specialists and a couple of product manufacturers in the field.

The Australian Bushfire Building Event was held on September 14-16. Now in its eighth year, the conference is the only event of its type for professionals in the planning, designing, building, regulating and certification of buildings in bushfire-prone areas.

Mr Davis, of Wentworth Falls, has been recognised both nationally and regionally by the Building Designers Association of Australia. He specialises in healthy and energy-efficient homes and has been working in the industry for 14 years.

The Building Designers Association of Australia is a not-for-profit association which accredits building designers to meet legislative requirements and provides support to designers and their clients.