It's not hard to see how cold might be symbolic in a film about, well, deadness. The bulk of the movie takes place in the fall following Malcolm's death, and the whole atmosphere of autumnal Philadelphia—with falling leaves and clearly less-than-summery temperatures—definitely carries a sense of the upcoming barrenness and lifelessness that winter brings.

Everyone knows that the temperature drops when there's a spirit in the house. That's just Ghost 101. For example, just before Cole sees the ghost of an abused wife in his kitchen, we see a close-up of the thermostat's needle dropping rapidly. Cole's mother complains frequently about being cold, too. She doesn't know it, but that's because Cole's surrounded by a bunch of ghostly visitors.

The heavy cold = ghost symbolism also gives us an early hint that Malcolm is dead. Early on, we see him creep in on his wife while she's sleeping, and she immediately draws the blanket closer around her.

When we don't know better, that gesture kind of plays like she's just mad at Malcolm and drawing away from his attempts at affection. It's all part of Shyamalan's misdirection about marital problems being the reason they aren't talking.

Later, though, we realize that her sudden chill is a big clue that Malcolm is a ghost just like all of Cole's furnace-busting buddies.