Here is an explanation of a flashover:
"When a flame in a room is not quickly put out, hot gases rise to the ceiling and spread heat throughout the space, fire experts say. Temperatures can rise quickly to as high as 1,000 degrees Fahrenheit, a point at which everything in the room can start burning at once, according to the [US] National Fire Protection Association."
“All the wood, all the seats, all of the decorations and everything else in the room would be heated to the ignition temperature,” said Steve Kerber, executive director of UL Research Institutes’ Fire Safety Research Institute in Maryland. “If you have very combustible materials, like plastics, it happens very fast.”
In a flashover, a flame can spread from a candle to a sofa, then to the carpet and the rest of the room in three to five minutes, depending on the type of materials involved, Mr. Kerber said.
It would burn the people inside too.
Even a firefighter in full protective gear would be unlikely to survive a flashover, according to the
National Fire Protection Association. “Those are temperatures well beyond what you could get away with breathing,” Mr. Kerber
said.
Here is the key message in this article: Flashovers are preventable. Sprinklers can stop fires from reaching that point, Mr. Kerber said, as can adherence to fire codes, including not having flammable material on ceilings in enclosed spaces.
(emphasis by me)
A flashover is an event where an entire room is rapidly engulfed in flames.
www.nytimes.com
This reminds me of the descriptions of the effects of the pyroclastic flows from Mt. Vesuvius that decimated Pompei.