I'm not sure what "fairly impervious" means but this article looks interesting:

“We know from the experience of the 2009 Victorian bushfires that some people did perish in what they thought were bushfire shelters that they’d built themselves. Whether it was secondary to a wine cellar or it was some sort of building container, shipping container, they died. And it wasn’t necessarily they died because of fire; they suffocated. The oxygen was extracted out of the bunker because it wasn’t air-tight or the heat essentially killed them.”

https://www.gatewaycontainersales.com.au/shipping-containers...