Dehumidifier or just open a window?

The mistake, and why it is so easy to make

Relative humidity is relative. It tells you how close air is to saturation at its own temperature, not how much water it is carrying. Two bodies of air can both read 70% and hold completely different amounts of water, because warm air can hold far more before it saturates.

That is the whole trap. You check a weather app, see the outside humidity is about the same as your basement, open a window for the day - and the basement gets damper. The outside air was warmer, so 70% out there was a great deal more actual water than 70% down there.

Why a basement is the worst case

A below-grade room is cool - often 60 to 68F through the summer - because the ground around it is. Warm outdoor air at, say, a 68F dew point meets those cool surfaces, and the water it is carrying condenses out onto them. That is the same mechanism that puts beads on a cold glass, applied to your walls and floor.

So the summer instinct - it smells musty down here, let some air in - is precisely backwards. The room feels stuffy because it is humid, and the fresh air is what is making it humid.

When opening a window genuinely wins

• Cold, dry outdoor air. Winter air has a low dew point almost everywhere in the US. Bringing it in and warming it drops the relative humidity substantially - which is why houses get dry in winter rather than damp.
• A dry spell after rain has passed. When the outdoor dew point drops below your indoor one, ventilating is free and fast.
• Removing a burst of moisture at the source. A bathroom after a shower, a kitchen while something boils. That is a large amount of water in a small volume, and getting it out of the building beats condensing it back out of the air. An extractor fan is the better version of this. Bathrooms.
• Air quality rather than humidity. Ventilation does things a dehumidifier cannot - it dilutes odors, CO2 and indoor pollutants. If that is the problem, ventilate.

When the dehumidifier wins

• Any humid warm-weather day, which in most of the US is the entire season you actually have a damp problem.
• Any space that has to stay dry regardless of the weather - a store room, a workshop with tools, a crawl space. A machine holds a setpoint; a window follows the outdoors.
• Anywhere unattended. A humidistat runs the machine when the room needs it. Nobody opens and closes a window on a dew-point schedule.
• Where the window is also a security or rain problem, which is most basements and most ground-floor rooms.

The combination that wastes the most money

Running a dehumidifier with a window open. The machine will work exactly as designed, remove water continuously, never reach its target, and cost you the full running figure every month - because the room it is drying is connected to the outdoors, which is effectively infinite.

That figure assumes the machine reaches its target and idles. With a window open it never idles, so the real number is the 24-hour column. Sealing the space is free and it is the single biggest lever most people have. Running it for less.

What neither of them fixes

Water coming in. If the floor is wet after rain, if there is a white bloom on the block, or if a patch is damp regardless of the weather, that is ingress. Ventilation will not stop it and a dehumidifier will run against it forever. Gutters, grading, a working sump and a plumber come first; the machine is for the humidity that remains afterwards.