What Should a Basement’s Humidity Be?

You’ve heard the phrase: “It’s not the heat, it’s the humidity.”

Uttering this inspires knowing nods or annoyed eye rolls, depending on your perspective. Humidity, relative humidity, dew point — isn’t it just hot outside? What do those terms even mean? And what do they have to do with your basement?

Humidity describes the amount of water vapor in the air. When we’re talking about a comfortable humidity level, though, it’s relative humidity that matters. Relative humidity takes into account air temperature, says Kelly Smeltzer, indoor air unit enforcement coordinator at the Minnesota Department of Health.

“We know that the air can hold a maximum amount of water at each temperature,” Smeltzer says. “Relative humidity is the ratio of how much water vapor is in the air compared to how much potentially could be in the air at that temperature.” Smeltzer adds dew point is the temperature when air would hit 100% relative humidity.

But back to your basement. Smeltzer says indoor and outdoor humidity are measured the same way, and it’s important to keep your basement relative humidity levels within specific seasonal ranges. Maintaining an ideal home humidity all year will keep your family healthy and your home structurally sound. Find out if a dehumidifier cools a room.

Why Are Basements Humid?

Basements are humid because they’re underground and often lack great ventilation.

“Since basements are below grade, they have a lot more contact with the soil than the rest of the house,” says Smeltzer. “Water can seep into the basement from the soil, from poor grading around the landscape, and from cracks that develop in the foundation over time.”

Many homeowners also have laundry rooms and bathrooms in the basement, and Smeltzer says these contribute to higher humidity levels simply from regular use. It’s extremely important to ventilate dryers and basement showers outdoors to avoid moisture accumulation.

Finished basements tend to see more humidity-contributing activities like showering, and building materials can mask water problems occurring behind walls and under floors. That’s why it’s important to get any moisture problems under control before renovating.

The Problem of Basement Humidity

Humid basements cause health problems and can wreak havoc with the structure of your home.

“A lot of our common indoor allergens thrive in humid environments,” says Smeltzer. Mold, dust mites and insects love a humid basement, and the allergens they leave can contribute to allergies and asthma.

Molds won’t grow without a source of moisture. A humid basement creates one when warm, moist air meets cool pipes and surfaces. “The higher the relative humidity in your home, the more likely you are to have condensation problems that lead to mold,” says Smeltzer.

Once mold take hold, it’s a production to get rid of it. Molds grow anywhere: drywall, flooring, paint, wood studs, carpet, paneling, clothing and more. Just about anything in your basement can harbor molds.

What Should Basement Humidity Be in Summer?

Summer indoor relative humidity (including the basement) should be less than 60%, Smeltzer says. Inexpensive hygrometers can be purchased at hardware stores to help you keep track of basement humidity and take action if needed.

What Should Basement Humidity Be in Winter?

Winter indoor relative humidity (including the basement) should be between 20% and 40%, with regional variation, Smeltzer says. In the Upper Midwest, cold weather tends to bring lower humidity levels.

Tips for Lowering Basement Humidity

Smeltzer recommends the following steps to ensure your basement stays cool and dry all year round:

• Repair leaks and mop up spills quickly. Mold only takes 24 to 48 hours to grow when there’s a moisture source.
• Use sump pumps to remove water from your basement.
• Slope soil away from your foundation.
• Install exhaust fans in kitchens and bathrooms. Use the exhaust for 30 to 60 minutes after showering in a basement bathroom.
• Use humidifiers sparingly. Dehumidifiers in the summer reduce basement humidity.
• Vent dryers and other appliances outside of the house.
• Don’t overwater plants. Plants hate soggy roots, and the excess moisture can raise the humidity in your home.