Braydon Barnes was put in his crib on a Friday and left unattended with a space heater on for more than 38 hours, St. Charles police said.
Both parents Kathleen Peacock and Lucas Barnes told police they had used methamphetamine recently. They told officers they had been manufacturing in the home the week preceding their son's death.
The two were home all weekend as Braydon was shut in his room.
Peacock, who is currently pregnant, told police she heard Braydon make noises and knew that she should check on him but didn't.
The space heater has no thermostat and does not turn off automatically.
It wasn't until Sunday morning that the parents checked in on the boy, and found Braydon had died.
Paramedics said the child had been dead for some time.
Autopsy results revealed that Braydon was generally malnourished and thin. His cause of death was hyperthermia due to the extreme heat, which would have "caused extreme discomfort and difficulty breathing as he died."
Both parents are facing criminal charges of child abuse that resulted in death, and of manufacturing a controlled substance in a residence where a child resides.
The living conditions inside the house were so poor, police declared it uninhabitable and have condemned the house.