What Causes Musty Smells in a Crawlspace?

A musty smell is the signature odor of mold (mildew). Mold grows when humidity stays above 60% and there's a food source (wood, paper, dust). In a crawlspace, the moisture comes from three main sources: the ground (water vapor from damp soil), the air (humid outside air entering through vents), and bulk water (leaks or poor drainage).

The smell you notice in your home is crawlspace air — carrying mold spores and volatile organic compounds — being pulled up through the floor by stack effect. Up to 50% of the air on your first floor originates in the crawlspace, so a musty crawlspace means a musty home.

A bare dirt crawlspace floor releases gallons of water vapor into the air every day, even when the soil looks dry. This is the most common moisture source. The fix is a heavy-duty vapor barrier sealed across the entire floor and up the walls, which stops the vapor at its source.

Older building codes required vented crawlspaces, thinking outside air would dry the space. Modern building science has proven this wrong — in Ottawa's humid summers, warm moist outside air enters through the vents, hits cool surfaces, and condenses. The result is high humidity and mold. The fix is to seal the vents with insulated covers and condition the space.

If water is physically entering your crawlspace — through the foundation, from poor grading, downspouts that discharge too close, or a high water table — no amount of sealing will fix the smell until the water entry is stopped. This may require exterior drainage, downspout extensions, grading correction, or an interior drainage system and sump pump.

Encapsulation addresses all three sources at once: a vapor barrier stops ground moisture, sealed vents stop humid air entry, and (if needed) drainage and a dehumidifier handle bulk water and residual humidity. Existing mold is removed before sealing. The result is a dry, clean crawlspace — and a home that no longer smells musty. Most customers notice the odor is gone within days.