Six youngsters who died in a petrol-fuelled blaze plotted by their father would not have been able to survive the intense heat and thick smoke, the fire service said.
Fire investigators said temperatures in the upstairs bedrooms of 18 Victory Road in Allenton, Derby, would have reached more than 500C (932F) during the blaze and that anyone in those rooms did not stand a chance of survival.
Thick, black smoke would have quickly filled each of the rooms from floor to ceiling, rendering Jade Philpott, 10, and her brothers Duwayne, 13, John, nine, Jack, eight, Jesse, six, and Jayden, five, unconscious in minutes.
From the moment the fire was set in the hallway in the early hours of May 11, the flames would have surged up the uPVC front door, melting the plastic, and quickly progressed up the staircase with the help of varnished wood panelling.
All the upstairs doors were open during the blaze, allowing the smoke to quickly flood the three bedrooms in which the youngsters slept.
An open window at the top of the stairs created a "chimney effect", helping to draw the flames and smoke up the stairs.
The downstairs living room was also severely smoke-logged as the flames surged through on to the ceiling, embers falling onto the carpeted floor and setting light to the curtains.
Firefighters made a desperate attempt to rescue the children in pitch black conditions, carrying them out of the burning semi-detached council house one by one.
Members of the public gallery wept during the trial at Nottingham Crown Court as one of the six firefighters who went into the inferno described the harrowing moment he found "casualties everywhere" upstairs.
Michael Patterson, from Derbyshire Fire and Rescue Service, told the jury the fire created such thick black smoke he had to try to locate the children by touch and described finding them in their beds.
In her summing-up, Judge Kate Thirlwall referred to the "heroic" actions of the firefighters and neighbours.
Discussing the night of the fire, Mrs Justice Thirlwall said: "We have heard from people who were heroic. Firefighters, police officers and, impressively, civilians who did all they could to save those six children. Remember this.
"This truly was a shocking event and it is safe to assume they had never seen anything like this in their lives. You heard a compelling picture from all of them of the thick, black, acrid smoke," she said.
The trial heard how neighbours Daniel and Callum Stevenson, along with Jamie and Darren Butler, desperately tried to get inside the house to save the children but were beaten back by the dense smoke and intense flames.
Giving evidence, a tearful Jamie Butler told the court he ignored police officers who told him to leave the scene: "I wouldn't leave without trying to get those babies out."
Mr Butler, who only stepped back once fire crews arrived, told the court he apologised to Mick Philpott when he saw him at the front of the house.
Prosecutor Richard Latham QC asked him: "What did you apologise for?"
Breaking down on the witness stand, Mr Butler replied: "For not being able to save his kids."
Dave Coss, watch manager at Derbyshire Fire and Rescue Service, was part of the team that investigated the fire.
Speaking at Derby Kingsway fire station, Mr Coss said the children "didn't stand a chance".
"It would have been heavy, thick with smoke," he said. "Visibility would have been non-existent and it would have been very hot. In reality, anyone on that first floor didn't really stand a chance."
Mr Coss arrived at the scene hours after the final flames had been damped down on May 11.
Describing the interior of the house, Mr Coss said: "The upstairs bedrooms and bathroom were blackened by the smoke. The windows were black with smoke. Smoke had come down to floor level. It was thick and dirty everywhere."
He said it was obvious from the outset that the fire started in the hallway.
"With a fire investigation you always go open-minded. You're never led down an alley. You find the evidence and the evidence tells you what happened.
"So from this point of view, from outside looking in, to us there had been a fire. It looked like it had been in the hallway downstairs but we had to confirm that."
Fire dog Freckle helped investigators establish that petrol was used as an accelerant to light the fire. The springer spaniel sniffed out traces of the petrol at what became apparent was the seat of the fire - inside the front door below the letterbox.
In a statement read to the court, Stuart Hamilton, a Home Office-registered pathologist, said the children would have been "rendered unconscious without necessarily having woken from sleep" after inhaling the toxic products of the combustion.
Mr Coss said: "The fire crews tend to work as a family and they help each other out considerably.
"I am sure those who attended that fire went home to their families and reflected on what had happened."