What Should Basement Humidity Be? Ideal Levels for Colorado Homes

Basements sit partly underground, against cool soil, so they run cooler and often damper than the rest of the house. That extra moisture is what turns a storage room musty and lets mold grow on drywall, carpet and boxes. Getting humidity into the right range is one of the simplest ways to protect a finished basement. If mold has already started, ReestorianPros can connect you with a local provider for mold removal and remediation in Littleton.

This is general information, not medical or engineering advice.

What humidity should a basement be?

The EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. The CDC's guidance is to keep it as low as you can and no higher than 50 percent all day long. ENERGY STAR describes 30 to 50 percent as generally optimal and notes that higher levels may promote mold.

Is 60 percent humidity too high for a basement?

Sixty percent is the upper limit in the EPA's guidance. A reading that touches 60 percent on a rainy summer day isn't an emergency. A basement that sits there for weeks is. Long periods above 60 percent are when you start to notice a musty smell, damp-feeling carpet and mildew on stored items.

Why basements get humid

• Ground moisture moving through the slab and foundation walls
• Water from outside, from short downspouts, poor grading or sprinklers near the house
• Plumbing leaks, often slow ones at the water heater, laundry or under-slab lines
• Cool surfaces, where humid summer air condenses on cold-water pipes, ducts and walls
• Indoor sources, such as laundry, showers in a basement bathroom or a dryer vented indoors
• A past flood or leak that was never fully dried

How to measure basement humidity

1. Buy a digital hygrometer. They're inexpensive and sold with thermometers.
2. Place it away from the dehumidifier, vents and exterior doors, at about waist height.
3. Check it at different times, after rain, on hot afternoons and in winter.
4. Keep notes for a few weeks. One reading doesn't tell you much; a pattern does.
5. Check problem spots, like storage rooms and closets on exterior walls, which can run damper than the open room.

How to lower basement humidity

Start with water sources before you buy equipment:

1. Move roof water away. The Colorado Geological Survey advises discharging downspouts at least 5 feet from the foundation.
2. Fix grading so soil slopes away from the house.
3. Keep sprinklers back, at least 5 feet from the foundation by the same guidance.
4. Fix leaks, including drips at the water heater, laundry hookups and sinks.
5. Vent the dryer outdoors and use bathroom exhaust fans.
6. Insulate cold-water pipes so they don't sweat in summer.
7. Run a dehumidifier for the moisture that's left.

ENERGY STAR's moisture tips also include keeping gutters clean, avoiding overwatering foundation plantings and repairing leaking outdoor faucets.

What size dehumidifier for a basement?

ENERGY STAR sizes dehumidifiers by how damp the space is and how large it is:

ENERGY STAR says it's better to oversize than undersize, and that ENERGY STAR models use about 20 percent less energy than conventional ones for the same moisture removal.

Where to put it and how to run it

• Close the room's doors and windows while it runs.
• Give it clearance from walls and furniture unless it's a top-discharge model.
• Use a drain hose to a floor drain or sump so it doesn't stop when the bucket fills.
• Watch the temperature. ENERGY STAR notes that standard units may not work well in spaces that regularly drop below 65°F, because the coils can frost.
• Set it around 45 to 50 percent and confirm with your own hygrometer.

Signs your basement humidity is too high

• A musty or earthy smell
• Condensation on windows, pipes or ductwork
• Mildew on stored items, shoes or leather
• Damp-feeling carpet, or carpet pad that never seems dry
• Peeling paint or white mineral deposits on concrete walls
• Mold spots on drywall, baseboards or the backs of boxes (see is black mold dangerous?)

If you see these, read mold in the basement for cleanup and prevention steps.

Basement humidity in Colorado

Colorado's climate is drier than much of the country, so many basements here run on the dry side, especially in winter when the heat is on. ENERGY STAR suggests aiming for about 30 to 40 percent in colder climates during the heating season to avoid window condensation. Summer is different: afternoon storms and irrigation can push a basement above 50 percent, and a finished basement with carpet holds that moisture.

The bigger Colorado risk is usually liquid water, not humid air. A slow leak, a sump pump that overflowed once or runoff seeping through a foundation wall can keep one area damp even when the room's humidity looks normal. If one spot stays musty, check it with a moisture meter. See signs of hidden water damage.

Related reading: Learn how to deal with mold in the basement, and how to spot signs of hidden water damage.

When should you call a mold remediation professional?

Call a professional when:

• Mold covers more than a small area or keeps coming back
• Humidity stays high even with a dehumidifier running
• One area stays musty, which points to hidden moisture
• Someone in the home has asthma, allergies or a weakened immune system
• A past leak or flood was never fully dried

Damp or musty basement in Littleton? Call (303) 879-0950 to reach a local mold remediation provider.