Mildew or Mold on Your Walls? How to Tell, and Who to Call

Mildew is a surface growth. It sits on top of the paint, it cleans off, and the wall underneath is fine. Mold grows into what it lives on. In Florida, treating more than 10 square feet of it is legally mold remediation, and that is where it stops being a cleaning or painting job.

So the first question is not which cleaner to buy. It is which one is on your wall, because the answer changes who you call.

How to tell them apart

Start with how it looks and feels. The University of Rhode Island’s housing guide puts it simply: mildew is a surface fungus that looks white, gray or yellow and feels powdery. Mold is a deeper fungus that looks black, green or fuzzy, and it can spread fast.

	Mildew	Mold
Color	White, gray, yellow or light brown	Black, green, sometimes other colors
Texture	Flat and powdery	Fuzzy, raised or slimy
Where it lives	On the surface of the paint	Into the drywall, wood or caulk
After cleaning	Gone, wall looks normal	Stain or soft spot left behind, or back within weeks

A smell that will not clear, even after you air the room out, is another sign that something is growing where you cannot see it.

There is also a simple test for the most common mix-up, which is not mildew versus mold at all. It is mildew versus dirt. Sherwin-Williams’ test: put a few drops of household bleach on the spot. If it bleaches away, it is probably mildew. If it does not, it is probably dirt.

None of this is a lab result. It is enough to tell you who to call first.

Why Sarasota walls get it so often

A mildewed wall does not mean a dirty house. Sherwin-Williams notes that mildew forms on surfaces that stay damp or get little or no direct sun. Our climate supplies the damp for much of the year.

Indoors, it gathers where air does not move:

• Bathrooms where the fan is small, broken or not run long enough
• Closets on outside walls
• Behind a headboard or dresser pushed flat against an outside wall
• Ceilings and corners near an air handler

Outdoors, it shows up on north-facing walls, under deep overhangs, and on the bottom of walls that a sprinkler head hits every morning. Mildew on stucco has its own page: mildew on stucco.

Painting over mildew does not work

This is the mistake that costs people a second paint job. Sherwin-Williams lists painting over a surface that still has mildew on it as one of the causes of mildew in paint. The growth is still alive under the new coat, and it comes back through.

The order that works:

1. Kill and remove the growth. Sherwin-Williams’ method is 1 part liquid bleach to 3 parts water, left on the surface for 10 minutes, then rinsed off well. Never mix bleach with ammonia.
2. Let the wall dry fully. Not dry to the touch. Dry all the way through. In Florida humidity that can take longer than you expect, and paint on a damp wall fails from behind.
3. Fix the reason it was damp. The fan, the sprinkler, the furniture against the wall. Skip this and the rest is temporary.
4. Prime and paint with a mildew-resistant product. Sherwin-Williams recommends a paint that contains agents that inhibit mildew growth on the paint film.

Step 3 decides whether you are doing this again next year.

Where it becomes a licensed job

Florida regulates mold work under Chapter 468, Part XVI of the Florida Statutes. Two terms matter:

• Mold assessment is the inspection that finds the mold and decides the scope.
• Mold remediation is the removal, cleaning or treatment of mold on more than 10 square feet (Florida Statutes 468.8411).

Under Florida Statutes 468.8419, only someone who meets the training and licensing rules may perform or offer mold remediation. The same law protects you in a second way. A company that assessed your home for mold generally cannot remediate it within 12 months, and a remediator’s company cannot assess a home it remediated in the last 12 months. There is an exception for certain certified general contractors. If one company offers to find your mold and then fix it, ask about that rule.

The EPA’s homeowner guide matches the 10 square foot line. If the moldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, most people can clean it themselves. If there has been a lot of water damage, or mold covers more than 10 square feet, the EPA points to professional guidance (EPA, Mold Cleanup in Your Home).

In plain terms:

• Surface mildew on a painted wall: a cleaning and repainting job.
• A small patch of real mold, under about 10 square feet, on a hard surface: the EPA says most homeowners can clean it. Painting comes after it is gone and the wall is dry.
• More than 10 square feet, or mold inside drywall, insulation or a wall cavity: a licensed mold remediator.

Mold remediation is not one of our services. Mildew is different: on exterior jobs we treat mildew as part of the prep before any paint goes on.

What paint can and cannot do

Mildew-resistant paint slows growth on a wall that dries out between wettings. It is worth using in bathrooms, laundry rooms and on north walls.

It cannot beat a wall that stays wet. Against a constant moisture source, the additive is overwhelmed, which is why the sprinkler head and the bathroom fan matter more than the paint can.

There is also no paint that fixes mold. Paint over mold is a cover, not a cure. It does nothing about what is growing in the drywall, and it can hide a problem a buyer’s inspector would want to see.

Keeping it from coming back

These hold up best, roughly in order of what they do for the effort:

Check where the bathroom fan goes. Researchers at the Pacific Northwest National Laboratory say bath fans should vent outdoors, not into the attic, a crawlspace or between floors, and suggest looking in the attic to make sure. A fan that vents into the attic moves the moisture instead of removing it, and it can condense on the roof framing and grow mold there. Run the fan well after the shower ends, not just while it runs.

Pull furniture a few inches off outside walls. A dresser or headboard flat against an outside wall makes a cool, still pocket. That is a common reason for growth in a bedroom corner with no leak.

Re-aim the sprinklers. Any head that sprays the wall instead of the bed keeps the bottom of that wall damp on a schedule.

Watch for a house that feels cold and clammy at once. That points at the AC not pulling enough moisture out of the air. It is a question for an AC company, not a painter, but it can be the root cause.

Use mildew-resistant paint where the room needs it. Last on the list on purpose. It helps a wall that dries out. It will not save one that never does.

When to get an assessment first

Call a licensed mold assessor before anyone paints if:

• A musty smell stays in a room that looks clean
• Growth comes back within weeks of a thorough clean
• There has been a known leak, flood or storm water in that wall
• The area is bigger than a bath towel laid flat, which is close to the 10 square foot mark

If it turns out to be surface mildew, or the remediation is done and the wall is dry, the repaint is ordinary interior painting work. We give free estimates to homeowners in Sarasota and Manatee County. For bathrooms and closets that need a regular deep clean, see house cleaning.