Q. Why did my wood door just get bigger?

A. You’re not imagining things. Wood doors can indeed experience seasonal affective disorder. They are porous and, if not protected properly, can contract in dry air and expand in humidity  sometimes considerably  which is why the bathroom and front doors are often the tightest fits in the house.

This so-called “door swellage” is a topic of particular passion for those in the door industry, who look at the world as a collection of spaces separated by hinges, wood and jambs. According to them, too much moisture and not enough water-repellent finish is a combination to avoid at all costs.

“I like to say that a wood door is only as good as the finish that’s on it,” said Duane Berger, a manager of Frank Lumber Door Store in suburban Seattle.

Beyond that, there’s also the location of the door  and of the house itself  to consider. For starters, regions with heavy moisture in the air are trouble spots, Mr. Berger said. “Southern humid states and northern snow states both have big problems,” he said. “And it matters where the door is  southern and western exposures get a lot of sun, and it can bake the finish right off, even if it’s only 30 degrees out. People don’t realize that.”