Mold in the crawl space

Why do crawl spaces get mold?

Because they’re damp, dark and rarely visited. Many older Los Angeles houses sit on raised foundations with a crawl space between the soil and the floor. Moisture builds up there from several sources:

• Damp, exposed soil. The California Department of Public Health lists damp, exposed dirt in crawl spaces as an outdoor source of the dampness mold needs.¹
• Plumbing leaks. Supply and drain lines run under the floor, and a slow drip there can go on for months because nobody sees it.
• Drainage toward the house. CDPH also lists ground that slopes and drains water toward a building, including from downspouts.¹ The EPA recommends making sure the ground slopes away from the foundation and keeping gutters clean.² Hillside lots are especially prone after winter storms.
• Condensation on ducts and pipes. The EPA says ducts in crawl spaces should be properly sealed and insulated to prevent condensation.³ Uninsulated cold water lines can sweat too.
• Blocked foundation vents. Vents covered by planters, soil or storage cut off the airflow that lets the space dry out.

What are the signs of crawl space mold from inside the house?

Most people never see it. They notice the effects upstairs:

• A musty smell that’s strongest near the floor, in closets or in rooms over the crawl space
• Floors that feel soft, bouncy or cupped
• Cabinet bases or baseboards that swell near plumbing walls
• A water bill that crept up with no obvious leak inside

Our guide to a musty smell in the house covers how to narrow down where an odor is coming from.

Is some mold in a crawl space normal?

Some staining on old framing is common, and much of it may be historic: a leak that was fixed long ago leaves its mark on the wood. What matters is whether the wood is still wet today.

Moisture readings answer that. We compare readings on stained joists and subfloor with readings on clean, dry framing elsewhere in the same crawl space. If they match, the staining is likely old. If they don’t, water is still getting in.

Location helps too. Staining concentrated under a bathroom or the kitchen points to plumbing. Staining along one side of the house, near the downhill or downspout side, points to drainage. Staining spread evenly over the whole subfloor points to general dampness from the soil and poor airflow.

What can you check yourself?

If you can open the access hatch safely, look in with a flashlight:

1. Standing water or mud on the soil
2. Drips or white mineral crust on pipes, especially under bathrooms and the kitchen
3. Wet or sagging insulation, if the floor is insulated
4. Ducts that are disconnected, crushed or dripping
5. Foundation vents blocked from the outside

Take photos of anything you see, with the date. If you call a plumber or a drainage contractor, those photos save time.

Don’t crawl in if you’d have to squeeze through tight spaces, if there’s standing water near wiring, or if you see signs of animals. That’s what an inspection is for.

When should you get a crawl space inspected?

When there’s a sign of moisture and you can’t see or reach the source, or when you’re buying an older home on a raised foundation. A general home inspection may only look in from the hatch. If you’re in escrow, see our page for home buyers.

You probably don’t need one if you’ve found an obvious drip, a plumber has fixed it, and the area is drying out with no smell left behind. If you want confirmation that it dried, a moisture and humidity assessment can compare readings in the affected area against dry ones.

What does a crawl space inspection include?

Our crawlspace and attic inspection covers the crawl space where it’s safely accessible: moisture readings on framing and subfloor, a check of supply and drain lines, signs of standing water and drainage problems, and photos of every finding located in a written report. If part of the space can’t be reached safely, the report says so. Added to a mold inspection it’s $250; a standalone visit is quoted by size and access, and the price is confirmed before booking.

Who fixes crawl space mold?

Whoever fixes the water, then a remediation contractor if the growth is extensive. A plumber handles leaks, a drainage or grading contractor handles water running toward the house, and an HVAC contractor handles sweating ducts. Cleaning wet framing without fixing the source doesn’t last; the EPA’s core advice is that the key to mold control is moisture control.²

We don’t do remediation or repairs. The report gives any contractor what they need to quote, and post-remediation clearance checks the result independently. For the other hidden space in the house, see mold in the attic.