An incomplete list of the adjectives used to describe the house in The Haunting of Hill House: schizophrenic, insane, hungry. These are words typically reserved for people, not structures of plaster and wood. But by the end of the brilliant Netflix series, it's obvious the house is a sentient being, methodically trapping its inhabitants within its walls.
Nowhere is the house's intelligence more evident than in the so-called Red Room. Despite Hugh Crain's (Henry Thomas) persistent and admirable exertions via crowbar, the door to the Red Room cannot be opened. And yet, six of the seven Crain family members frequently find themselves in the Red Room — they just don't know it at the time. The room disguises itself as a solitary retreat. For Olivia (Carla Gugino), it takes on the shape of a reading room; for Luke (Julian Hilliard), a treehouse. Hugh is the only Crain who never enters the Red Room — and there's a reason why. In fact, it's what makes all the events of Hill House possible.
Compared to other pop culture works about haunted houses, the actual haunting in The Haunting of Hill House proceeds slowly. That's because Hill House's goal isn't to scare the Crain family away — it's to subsume them. The house pursues said goal by "digesting" the Crains, one by one, in the Red Room. The more time a person spends in his or her version of the Red Room, the more the house can prey on them. This process takes time — more time, perhaps, than it would take to flip the mansion, as is the Crains' plan.
Where does the house find that time? By tasking Hugh, a chronic fixer, with an unfixable problem: endlessly spreading black mold. "I can’t flip this house if it’s rotting with mold. We’ll be stuck here," Hugh tells Olivia after assessing the damage. Step one to trapping the Crain family in Hill House indefinitely: Distract the most ghost-skeptical member of the Crain family with a very tangible renovation problem.
Clearly, the house knows Hugh just as well as it knows the rest of the Crains. Hugh's sense of self is derived from his capable, can-do spirit. He's skeptical of ghosts, but he believes in his ability to methodically fix problems– and that stubborn attitude is what ultimately renders his family vulnerable to Hill House. When Hugh stares at the toxic black mold on the wall, he says, "This could ruin us," with dread. Despite his children's talk of hauntings, he is still firmly fixed in the rational world, one in which mold is more dangerous than poltergeists.
One redditor has an apt guess for the actual location of Hugh's Red Room. Studying the house's floor plans, Hugh concludes that the basement mold originates from a leak on the third floor. "I can’t think of a water source that explains it spreading like this,” Hugh tells Olivia. Hugh can't identify the water source because he hasn't seen it — until the Red Room is opened at the end of their stay in Hill House, no one really has. Obviously, the source of the mold must be the Red Room itself. "I've checked everywhere except one room," Hugh tells the inspector (who happens to be played by Bachelor producer Elan Gale). The "one room" Hugh references is the Red Room, which also happens to be covered in mold.
For Hugh, the Red Room is not a place. It's a purpose. The Red Room needs to occupy him, and therefore force the Crains to stay in the house for longer than expected. Imagine if that mysterious leak from the third floor never happened. Imagine if the black mold had disappeared after a one round of treatment. The Crains could have finished flipping Hill House and moved on to their "Forever Home." Instead, the house preys on Hugh's "fixer" sensibility and tasks him with an unending project. While his wife goes mad and his children see ghosts, Hugh focuses on construction. His problem is that he thinks he's still in a rational world. He's Peak Dad.
However, on his last night in Hill House, Hugh awakens to the supernatural reality of the house he'd previously ignored. He witnesses paranormal phenomena, like a dead girl turning into a ghost and the Red Room opening. For the rest of his life, Hugh remains a stalwart believer — the opposite of his son, Steven (Michiel Huisman), who denies ghosts exist. At the end of the series, Hugh sacrifices himself to the house and stays there forever. The Red Room didn't get him in the past, but it certainly gets him in the present.