The short version
Keep indoor relative humidity between 30 and 50 percent, as the EPA recommends. In practice that means running a dehumidifier in the basement through the summer, using bathroom and kitchen exhaust fans that vent outdoors, covering bare dirt in crawl spaces, fixing leaks within a day or two, and keeping rain away from the foundation.
Why humidity matters so much here
Charleston sits in a river valley surrounded by hills. Summers are warm and humid, and moist air settles in low areas. When warm, humid air meets a cool surface, such as a basement wall, a crawl space joist, an air conditioning duct or a closet on an exterior wall, it condenses. That steady dampness is all mold spores need to start growing.
That is why so many mold calls in the area involve basements, crawl spaces and closets rather than dramatic leaks. See basement mold removal and crawl space mold.
Measure it first
A digital hygrometer costs little and tells you what you are dealing with. Check a few spots:
| Reading | What it means |
|---|---|
| Below 30% | Dry; can cause static, dry skin and cracked wood |
| 30% to 50% | The target range |
| 50% to 60% | Getting damp; mold and dust mites become more likely |
| Above 60% | High risk of mold and musty odors, especially in basements |
Basements and crawl spaces often read well above the rest of the house in summer.
Basements
- Run a dehumidifier from late spring through early fall, set to about 45 to 50 percent. Use one sized for the space and drain it continuously to a floor drain or sump if possible.
- Keep basement windows closed in humid weather; outdoor summer air adds moisture rather than drying the space.
- Insulate cold water pipes so they stop sweating.
- Keep stored items off the floor and away from walls so air can circulate.
- Watch for water coming in after rain, which is a drainage problem, not just humidity. See sump pump failure if water is getting in.
Crawl spaces
- Cover bare soil with a heavy vapor barrier; dirt releases moisture into the crawl space all year.
- Consider encapsulation and a crawl space dehumidifier if the space stays damp.
- Make sure gutters and grading carry water away from the foundation.
Bathrooms, kitchens and laundry
- Run the bathroom fan during showers and for 20 to 30 minutes afterward.
- Make sure fans vent outdoors, not into the attic, where the moisture causes attic mold.
- Use the range hood when cooking, especially when boiling water.
- Vent the dryer outside and keep the vent clean.
Air conditioning
Air conditioning removes moisture as well as heat. A system that is oversized may cool the house quickly without running long enough to dry the air, leaving it cold and clammy. If your home feels damp even with the AC running, have a technician check the system, and consider a dehumidifier for problem areas.
Fix water problems quickly
Humidity control does not help if water keeps getting in. Dry any leak, spill or overflow within 24 to 48 hours, repair plumbing and roof leaks promptly, and keep gutters clean. Anything that stays wet longer than that is a mold risk. Our guide to mold and your health explains why it is worth the effort.
When it is already too late
If you smell mustiness or see spots on walls, joists or stored items, the moisture has already done its work. A small area on a hard surface can be cleaned; larger areas, or mold inside walls, crawl spaces or ductwork, need professional removal. See mold inspection and mold removal.
Damp homes around the valley
Hillside basements in South Hills and Spring Hill, crawl spaces in Sissonville and Malden and Rand, and river-level homes in Kanawha City are the most common places for humidity-driven mold. For help with mold, water or fire damage, see the restoration services across the Kanawha Valley.
Call to be connected with an independent local provider for a mold inspection.
Humidity and mold FAQs
What humidity level prevents mold?
Keeping indoor relative humidity below 50 percent, and ideally between 30 and 50 percent, makes mold much less likely to grow.
What size dehumidifier do I need for a basement?
It depends on the size of the basement and how damp it is. Larger, damper basements need higher-capacity units; check the manufacturer’s sizing chart for the square footage and conditions.
Will opening windows reduce humidity?
In dry weather, yes. In humid summer weather, open windows usually bring more moisture into the house, especially into basements.
Can mold grow without a leak?
Yes. High humidity and condensation on cool surfaces are enough for mold to grow, which is why basements and crawl spaces are common trouble spots.
