Mold vs. mildew: how to tell the difference in your Marin home

Deep dive · Updated October 17, 2026

"Is it mold or mildew?" is one of the most common questions we hear. The honest answer is that mildew is a kind of mold. The word usually means mold that grows flat on the surface, while "mold" covers fuzzier or deeper growth too. The difference matters less than what's underneath: both mean there's too much moisture, and both should be cleaned up and the moisture fixed (EPA).

The short answer

• Mildew is the everyday name for mold that sits on top of a surface: the gray or white powdery film on a shower grout line, a window sill or a damp leather jacket. It's usually easy to wipe off.
• Mold is the broader term. It includes the same surface growth but also fuzzy, slimy or dark patches that grow into drywall, wood, carpet and insulation, where you can't simply wipe them away.
• Color doesn't tell you much. Mold can be black, green, white, gray or orange. "Black mold" isn't a separate diagnosis. See what black mold really means.

What mildew looks like

• Flat, powdery or slightly fuzzy, white to gray, sometimes turning darker
• On hard or non-porous surfaces: tile, grout, caulk, glass, painted trim, vinyl shower curtains
• Wipes off with a damp cloth and household cleaner, and the surface underneath looks normal

What mold looks like

• Raised, fuzzy or slimy patches, often green, black or brown, sometimes in spreading spots
• On or in porous materials: drywall paper, wood framing, ceiling tiles, carpet backing, insulation
• Comes back quickly after cleaning, or leaves a stain or soft spot behind
• Often with a musty, earthy smell, even where you can't see growth

Quick checks you can do at home

1. Wipe test: does it come off a hard surface completely with soap and water? That's surface growth you can usually handle.
2. Press test: is the material underneath soft, crumbly, warped or stained? Growth has gone into the material.
3. Comeback test: clean it and watch. If it's back in days, the moisture source is still there.
4. Smell test: a musty smell in a closet, crawlspace or behind furniture often means hidden growth.
5. Size check: the EPA says a homeowner can usually clean an area under about 10 square feet; bigger areas may need a professional (EPA).

You don't need a lab test to tell mold from mildew. California's public health department says that if you can see or smell mold, the priority is fixing the moisture and cleaning up, not testing (CDPH).

Where each shows up in Marin homes

• Bathrooms in older homes in San Anselmo, Ross and Mill Valley, where there's no exhaust fan or a weak one. Gray mildew on grout and caulk is common; black spots on the ceiling paint or behind the toilet tank mean moisture is soaking into drywall. See bathroom mold and ventilation.
• Fog-belt homes in Mill Valley, Sausalito and West Marin, where summer fog keeps window frames, closets on north walls and stored clothes damp. Mildew on leather, shoes and window sills is typical; mold on closet walls is a sign the wall itself is cold and damp. See fog-belt mold prevention.
• Crawlspaces under redwood-shaded homes in Larkspur, Fairfax and Kentfield, where wood framing can grow mold that you smell upstairs before you ever see it. See crawlspace mold in Marin.
• Vacation and second homes in Stinson Beach and Inverness that sit closed up for weeks. See vacation home mold.

Cleaning: what you can do yourself

For small areas on hard surfaces, the EPA's advice is simple (EPA):

• Fix the water problem first, or the growth will come back.
• Scrub hard surfaces with detergent and water, then dry them completely.
• Wear gloves, goggles and an N-95 respirator while you clean.
• Never mix bleach with ammonia or other cleaners; the fumes are toxic.
• Porous materials like ceiling tiles, carpet and drywall that have grown mold may need to be thrown away.
• Don't paint or caulk over mold. It will grow through.

When to call a professional

• The area is larger than about 10 square feet, or keeps coming back
• It's in an HVAC system, inside walls, under floors or in the crawlspace
• It followed a flood, sewage backup or a long roof leak
• Anyone in the home has asthma, allergies or a weakened immune system

In Marin, the most common reason a "mildew" problem turns into a remediation job is a fog-belt crawlspace or a slow roof or plumbing leak that's been feeding the growth for months. If you're not sure, a mold inspection will find the moisture source.

We can connect you with an independent, licensed local mold inspection or remediation provider for free. Check any contractor's license at CSLB.

Preventing both

• Run bathroom fans during showers and for 20 to 30 minutes after, and wipe down glass and tile.
• Keep indoor humidity under about 50 to 60 percent, especially in fog season. A dehumidifier helps in closets and lower floors. The EPA recommends staying below 60 percent (EPA).
• Let air move: pull furniture a few inches off exterior walls and don't pack closets on north-facing walls.
• Dry leaks within 24 to 48 hours. After a storm, follow our drying and mold-prevention timeline.
• Check the crawlspace once a year for damp soil, standing water or a musty smell.

Related reading

• Mold remediation in Marin: the complete guide
• Mold inspection vs. testing
• Black mold: what it really means
• Fog-belt humidity and mold

Sources

• EPA: A brief guide to mold, moisture and your home
• EPA: Mold cleanup in your home
• California Department of Public Health: Mold FAQ
• CSLB: license check

Sources checked October 26, 2026.

Frequently asked questions

This guide is general information, not legal, insurance or medical advice. Sources are linked in the text. Marin Mold Removal is a referral service and doesn't perform the work.