There’s nothing mysterious about basement humidity issues. Chronic humidity accumulates in the basement due to chronic moisture sources down there. Another non-mysterious fact is that basement humidity problems frequently don’t stay in the basement. Consequences of a persistently damp, humid basement directly below your floor—such as toxic mold growth—can impact the upstairs environment, too.   

Here are 8 things to know about basement humidity and ways to deal with it:

• A basement dehumidifier can lower humidity but doesn’t address  ongoing causes of it.
• Sources of basement moisture/humidity fall into three categories: Rain and resultant infiltrating groundwater, influx of humid outdoor air that triggers basement condensation and moisture sources originating inside the basement.
• One inch of rainfall deposits over 1,200 gallons of water on the roof of an average-size residence. Unless it’s collected in functional gutters and discharged at least four feet from the house through downspouts, roof runoff soaks into the ground adjacent to the house and may then seep into the basement, contributing to high humidity.
• Soil surrounding the foundation should be slightly graded to divert pooling water away from the basement during rain.
• In certain locales, naturally occurring groundwater may also seep through pores and tiny cracks in the concrete basement floor and walls, elevating basement humidity. A sump pump with an air-tight cover installed in the basement floor relieves under-slab pressure that drives seepage. A perforated drain pipe installed around the perimeter of the basement footing also collects and removes groundwater.
• Plumbing pipes routed through the basement should be checked for seepage and leaks. Cold water pipes “sweat”condensation in a humid basement, adding more moisture to the air. Insulate cold pipes with foam insulation sleeves.
• An unvented clothes dryer placed in the basement is a common humidity source. Dryer exhaust should be vented directly to the exterior through an approved fireproof dryer vent.
• Avoid frequently ventilating a basement with outdoor air in a humid climate. Humidity becomes trapped and accumulates in the enclosed basement space. If a basement requires ventilation, consider adding an operating ductwork vent to admit cool or warm air from the house HVAC system.