How to measure the moisture in your air right now

The only reliable way to know whether you need a humidifier or dehumidifier is to measure your indoor humidity. You do this with a tool called a hygrometer, which costs $10 to $30 and reads the percentage of moisture in the air. You want to aim for 30 to 50 percent humidity indoors — below that and you have too little moisture; above that and you have too much.

Buy a basic hygrometer at any hardware store or online retailer and place it in a central room away from windows, doors, and direct sunlight. Leave it there for 30 minutes so it stabilizes, then read the number. If your home sits consistently below 30 percent, dry air is your problem and a humidifier will help. If it stays above 50 percent, excess moisture is the issue and a dehumidifier will help. Between 30 and 50 percent means your air is already balanced.

Check your humidity at different times of day and in different seasons. Winter air tends to be drier because cold air holds less moisture, even when heated indoors. Summer and spring tend to be wetter. If humidity swings wildly between seasons, you may need different solutions at different times of year.

Key Takeaways

  • A hygrometer is the only tool that tells you whether your air is too dry or too wet — aim for 30 to 50 percent humidity indoors.
  • Humidity below 30 percent causes dry skin, static electricity, and wood shrinkage, and calls for a humidifier.
  • Humidity above 50 percent encourages mold, dust mites, and musty smells, and calls for a dehumidifier.
  • Winter air is naturally drier and summer air naturally wetter, so your needs may change with the season.
  • Some homes need both devices — a humidifier for winter and a dehumidifier for summer or damp rooms.

Signs your home is too dry

If your humidity is below 30 percent, you will notice physical symptoms. Your skin feels tight and itchy, your lips crack, and your hair becomes brittle and prone to static electricity. You may wake up with a dry throat or stuffy nose. These are all signs that the air is pulling moisture from your body and your home's materials.

Dry air also damages your house itself. Wood furniture, hardwood floors, and wooden door frames shrink and warp when humidity stays low for months. Paint and varnish can crack. Drywall joints may separate slightly. If you have a piano or other wooden instruments, they go out of tune. Dry air also makes colds and flu spread more easily because your respiratory tract dries out and becomes more vulnerable to viruses.

A humidifier adds moisture back into the air. The most common type is an evaporative humidifier, which uses a fan to blow air through a wet wick. Ultrasonic humidifiers create a fine mist and are quieter. Warm mist humidifiers heat the water before releasing it, which can feel more comfortable in winter but use more electricity. All of them work best in a single room or small area — whole-house humidifiers are expensive and require professional installation.

Signs your home is too wet

If your humidity is above 50 percent, moisture builds up in ways you can see and smell. You notice condensation on windows, especially in the morning or on cold days. Bathroom mirrors fog up even after you turn off the shower. Closets and basements smell musty. Walls or ceilings show dark spots or discoloration, which is mold or mildew beginning to grow.

High humidity creates ideal conditions for mold, dust mites, and mildew. Mold spores trigger allergies and asthma, especially in children and older adults. Dust mites thrive in damp air and leave behind droppings that cause allergic reactions. Musty smells mean mold is already present, even if you cannot see it yet. Over time, mold damages drywall, wood framing, and insulation, and the repairs become expensive.

A dehumidifier removes moisture from the air by pulling it across cold coils, where water condenses and drips into a tank or drain line. Portable dehumidifiers work well for single rooms or basements. Whole-house dehumidifiers connect to your HVAC system and require professional installation. If you have a basement or crawlspace that stays damp, a dehumidifier is often the fastest way to stop mold before it spreads.

Why humidity changes by season and room

Winter air is naturally dry because cold air cannot hold as much moisture as warm air. When outdoor air at 20 degrees and 50 percent humidity enters your home and is heated to 70 degrees, that same air suddenly feels like 15 percent humidity — even though no moisture left. This is why winter is the most common time to need a humidifier, especially in climates with harsh winters.

Summer and spring bring the opposite problem. Warm outdoor air holds more moisture, and that moisture seeps into your home through walls, basements, and crawlspaces. Air conditioning helps by removing some moisture as it cools, but if your AC is undersized or your home has poor ventilation, humidity can still climb above 50 percent. Basements and bathrooms are the wettest rooms because they are naturally cooler and have less air circulation.

This is why many homes need both a humidifier and a dehumidifier — one for winter and one for summer. You might run a humidifier in the bedroom from November through March, then switch to a dehumidifier in the basement from May through September. Check your humidity in each season before you buy, so you know which device will actually help.

What to do if you cannot measure humidity

If you do not have a hygrometer yet, you can make an educated guess based on what you see and feel. Dry air usually comes with visible static electricity, cracked lips, dry skin, and wood that creaks or warps. Wet air usually comes with condensation on windows, musty smells, visible mold spots, or a damp feeling in the basement or bathroom.

However, guessing is not reliable. A room can feel stuffy for reasons other than humidity — poor ventilation, dust, or temperature can all make air feel "off." And mold can grow invisibly behind walls or under flooring long before you smell it. Buying a hygrometer takes five minutes and costs less than a single tank of gas. It is the only way to know for certain whether you need a humidifier, a dehumidifier, or neither.

Other ways to adjust humidity without buying a device

Before you invest in a humidifier or dehumidifier, try straightforward changes that cost nothing. If air is too dry, take longer showers (steam adds moisture), boil water on the stove, hang wet towels to dry indoors, or run your bathroom exhaust fan less often. If air is too wet, run exhaust fans during and after showers, open windows on dry days to let moisture escape, fix any leaking pipes or gutters, and make sure your basement or crawlspace has a sump pump or drainage system that works.

Improving ventilation also helps. A bathroom exhaust fan that vents to the outside (not into the attic) removes moisture at the source. A kitchen range hood does the same for cooking steam. If your home feels stuffy and humid, poor ventilation is often the culprit — a dehumidifier will help temporarily, but fixing the ventilation solves the problem permanently.

When to call a professional

If your humidity stays above 60 percent even after you run a dehumidifier, or if you see mold growing on walls or ceilings, you have a moisture problem that goes beyond what a portable device can fix. This usually means water is entering from outside — through a leaky roof, cracked foundation, or poor grading around the house. A professional inspector or contractor can find the source and recommend repairs.

Similarly, if your humidity stays below 20 percent even in winter and you have symptoms like nosebleeds or severe dry skin, your HVAC system may be the problem. A heating contractor can check whether your furnace is stripping moisture from the air or whether you need a whole-house humidifier installed on your ductwork. These are bigger projects than a portable device, but they solve the problem permanently rather than temporarily.

Frequently Asked Questions

Can I use a humidifier and dehumidifier in the same house?

Yes. Many homes run a humidifier in winter when air is dry and a dehumidifier in summer when basements or bathrooms get damp. You can also use them in different rooms — a dehumidifier in the basement and a humidifier in the bedroom, for example. Just measure humidity in each space separately so you know which device belongs where.

What humidity level is best for sleeping?

Most sleep experts recommend 30 to 50 percent humidity, the same range as the rest of your home. Some people sleep better at the higher end of that range (around 45 to 50 percent) because it feels less dry on the throat and sinuses. Use a hygrometer in your bedroom to find what feels best for you.

Will a dehumidifier help if I have mold already growing?

A dehumidifier will slow mold growth by removing moisture, but it will not kill mold that is already there. You need to clean visible mold with a bleach solution or mold-killing spray, then use the dehumidifier to keep humidity below 50 percent so it does not come back. If mold covers more than a few square feet or is inside walls, call a professional.

How often should I check my humidity?

Check it once a month, or whenever you notice symptoms like dry skin, condensation on windows, or musty smells. Humidity changes with the seasons, so checking in winter and summer gives you a clear picture of what you need. Once you know your home's pattern, you can adjust your humidifier or dehumidifier schedule accordingly.

Do I need to drain a dehumidifier tank every day?

It depends on how wet your air is and how large the tank is. In a very damp basement, a small dehumidifier might fill its tank in 12 to 24 hours. Most dehumidifiers have a drain hose option so you can run a line to a floor drain or sump pump instead of emptying manually. Check your dehumidifier's manual to see what tank size it has and how often you will need to drain it in your climate.