Signs of Mold in Your House

Which signs matter most?

Washington DOH points out that in most cases you can see or smell mold, which is why lab testing is rarely the first step. A practical checklist, roughly in order of how often each clue turns out to be real:

1. A musty, earthy smell, especially one that is strongest in one room or when the heat first kicks on.
2. Visible growth: spots, speckles or fuzzy patches in black, green, gray, white or brown.
3. Water stains on ceilings or walls, often with a darker ring at the edge.
4. Paint that bubbles, peels or flakes, and wallpaper that lifts at the seams.
5. Drywall, flooring or baseboards that feel soft, spongy or swollen.
6. Heavy condensation on windows or walls that persists for weeks.
7. Musty-smelling clothes, shoes or boxes stored in a closet, garage or basement.

One sign alone is a reason to look more closely. Two or three together, especially a smell plus a water clue, usually mean there is moisture and growth somewhere you have not found yet.

What does a mold smell tell you?

A musty odor means something is damp enough for mold or bacteria to be active. It does not tell you how big the problem is. A whole-house smell after the furnace starts in October often points to the crawl space or ductwork, because air moving through the house pulls from below. A smell confined to one closet or bedroom points to a nearby wall, window or roof leak. If you suspect the heating system itself, EPA advises not running an HVAC system you know or suspect is contaminated, since it can spread mold through the building.

Does the color of what you see matter?

Less than most people think. Many common molds are dark, and others are white or pale green. CDC says it is not necessary to determine what type of mold you have; all indoor mold should be cleaned up and the moisture fixed. Our page on black mold facts versus fear covers why the color label causes so much worry and so little useful action.

Also look for what is easy to mistake for mold: soot from candles along cold wall corners, efflorescence (white, chalky mineral deposits on concrete), and plain dirt. Efflorescence on a basement wall is still a sign of water moving through the concrete, so treat it as a moisture clue even though it is not mold.

Which water clues should you look for?

Mold follows water, so the water clues are often more useful than the mold itself. Seattle's rental inspection checklist, written for landlords but useful for any homeowner, lists the kinds of conditions that fail an inspection: a leaking roof, exterior walls or windows that let in water, walls, floors or ceilings that are soft, spongy or wet to the touch, missing or broken bathroom and laundry fans where they are used in place of an openable window, and leaking plumbing. Standing water in a crawl space is flagged as an advisory item. Walk your own house with that list in mind.

A stain that grows after each storm, a ceiling that sags slightly, or a baseboard that has pulled away from the wall are all signs that water has been getting in for a while. Wall cavities can hide a lot of growth behind clean-looking paint; our page on mold inside walls explains what happens when a wall needs to be opened.

Where should you look in a Seattle-area home?

Older houses with vented crawl spaces, daylight basements cut into hillsides, and attics with bath fans that dump moist air under the roof each have their own weak spots.

For more on the first three, see our pages on crawl space mold, attic mold removal and basement mold in Seattle, along with the longer guides on crawl spaces in Seattle-area homes and attic mold in Seattle houses.

Why do signs come and go with the seasons?

Seattle's pattern of mild, wet, gray months from October through April followed by dry summers means many signs are seasonal. Windows that stream with condensation in January may be dry in July, and a musty basement can smell fine by August. That summer lull is misleading: the moisture source is usually still there and returns with the rain. EPA suggests keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, and a simple humidity gauge is one of the more useful tools a homeowner can own here. Our guide on why Northwest homes get so much condensation covers ventilation and heating habits that help.

What are the signs after a leak or flood?

After any water event, timing is the main sign to watch. EPA and Washington DOH both say to dry water-damaged areas and items within 24 to 48 hours, and EPA's post-disaster guide says that if a flooded home could not be dried in that window, you should assume mold is growing. In December 2022 a king tide pushed the Duwamish River over its banks in South Park and flooded homes and businesses; in a case like that, growth often starts inside walls and under flooring well before anything is visible. King County's flood services publishes flood warnings and floodplain maps for the county's major rivers, which is worth checking if your home sits in a river valley.

Can mold cause symptoms that point to a problem?

Be cautious about using symptoms as a detector. EPA says mold exposure can irritate the eyes, skin, nose, throat and lungs, and people with asthma or weakened immune systems should stay away from moldy areas. Washington DOH adds that most molds do not harm healthy people, while people with allergies or asthma may be more sensitive. Those same symptoms have many other causes, so if anyone in your home has health concerns, talk with a health professional rather than treating them as proof of mold.

You found signs. What should you do next?

1. Find and stop the water: fix the leak, reroute the fan, clear the gutter, or dry the space.
2. Measure the area. EPA and DOH say a homeowner can usually clean an area under about 10 square feet with an N-95 respirator, gloves and eye protection. Larger areas, hidden growth, or anything involving sewage are reasons to bring in a professional.
3. Skip the test kit for now. EPA says sampling is usually unnecessary when mold is visible, and there are no federal limits to compare results against; read when a mold test is worth the money first.
4. If you smell it but cannot find it, a visual mold inspection in Seattle with moisture readings is usually the best next step. North of the city, the same applies if you are looking for a mold inspector in Everett or help in Shoreline.
5. Before hiring anyone, confirm the company is registered with L&I as a construction contractor using L&I's Verify a Contractor tool. Washington has no state mold license or certification.

What if you rent?

Public Health - Seattle & King County advises tenants to notify the landlord in writing about leaks or moisture, and to report the underlying water or maintenance problem to building or code enforcement, since those offices will not respond to a mold complaint by itself. Under RCW 59.18.070, after written notice the landlord generally has 10 days to begin repairs for conditions like this. In Seattle, SDCI Code Compliance handles complaints when a landlord refuses repairs. This is general information, not legal advice; our guide to dealing with mold as a Seattle renter covers the steps in more detail.