Mold, damp and window condensation in San Francisco: causes, fixes and who pays

It usually starts on the window. On a cold January morning water runs down the inside of the glass and pools on the sill. By March there is a gray-black line along the bottom of the frame and a smell in the closet that no laundry run gets out. Nothing is leaking. The apartment is making its own water.

Here is where the damp comes from, what fixes each cause and who pays when you rent. Water coming through a ceiling right now? Read the first 48 hours after a leak.

#Start with what you see

Tap what you're seeing. Each answer gives the usual cause, the fix that lasts, and who usually pays under the California rules below. If you own the place and live in it, that last part is you.

#Why the west side of the city runs damp

Ask anyone who has rented in the Outer Sunset. San Francisco Planning's history of the neighborhood mentions its foggy climate and sandy soil, and most of its houses went up between 1925 and 1950: stucco-clad, packed onto 25-foot lots, garage below and living space above. The Richmond, across Golden Gate Park, sits on the same stretch of ocean.

The physics is plain. Warm air holds more water than cold air. A shower, a pot of pasta, a rack of laundry and two people asleep behind a closed door all add water to the room, and when that air touches a surface colder than its dew point, the water comes out as droplets. In winter the coldest surfaces are the glass, the outside corners and the backs of closets, so that is where the water, then the mold, appears.

Fresh air helps most on dry days. On a clear afternoon, windows open on two sides of the flat swap wet air for drier air; on a dripping foggy morning the incoming air is nearly as wet as what you're removing, and a dehumidifier does more. A small hygrometer in the worst room settles it.

#Condensation, or a leak?

On a wall the two look alike and need opposite fixes, so sort them out before you scrub. Timing is the tell.

Pencil a line at the edge of any stain and look again after the next storm. A line that moves means a leak, and that record helps whoever comes to fix it.

#Bathroom ceilings: the fan is the fix

Spots over the shower show where the steam goes. Interior bathrooms with no window are common in flats and apartment buildings, and a window nobody opens in January is not much better. California lists the lack, or improper operation, of required ventilating equipment as a substandard condition in Health and Safety Code 17920.3.

For new homes, California's green building code, in the 2025 edition in force since January 1, 2026, asks for each bathroom fan to be ENERGY STAR rated, ducted outside and run by a humidity control adjustable between 50 and 80 percent (CALGreen 4.506.1). It is a good spec for an old building too: a fan wired to the light stops when you leave, with the room still full of steam.

Two checks before anyone blames the tenant. Where does the duct end? A fan blowing into the attic or between floors just moves the water. And does it pull? Hold a square of toilet paper to the grille with the fan on; if it doesn't cling, the fan or duct is failing.

Then clean the spots with soap and water, let the ceiling dry and repaint. California's health department tells tenants to skip bleach, ammonia and vinegar, which can trigger asthma, and EPA says never to paint over mold. A ceiling that must be cut out is drywall and plaster repair; the finish is interior painting.

#Running a drier apartment

Most of the fix is habit, and none of it needs a contractor or the landlord's permission:

• Run the bathroom fan or open the window for every shower, then squeegee the glass and tile, as the state health department advises.
• Cook with lids on and the range hood running, if it vents outdoors.
• Dry laundry in a dryer vented outside, never on a rack in a closed bedroom.
• Keep beds and dressers a few inches off outside walls, and leave closet doors on those walls ajar.
• Heat the whole flat a little rather than one room a lot. A shut, cold back bedroom is where condensation settles.

Choosing a dehumidifier for an apartment

In a fog-belt flat or a garage-level room, a dehumidifier earns its floor space. ENERGY STAR sizes them by how damp the space is, in pints removed per day:

Close that room's doors and windows while it runs, keep it clear of walls and furniture, and use a drain hose if a drain is close. ENERGY STAR notes that capacity ratings have changed, so compare new models with new models.

#Cleaning mold yourself, and when to stop

EPA's rule of thumb: a moldy area under about 10 square feet, roughly 3 feet by 3, is usually a do-it-yourself job. Scrub hard surfaces with soap and water, dry them completely, and consider an N-95 respirator. Moldy ceiling tiles and carpet may have to go. Anything bigger, or back after a careful cleanup, is a building problem; EPA suggests hiring someone experienced with mold and checking references.

• Heard this?You need a mold test to know what to do.Tap for the factsWhat's trueCalifornia's health department doesn't recommend testing, such as measuring spores in the air; results can't tell you the health risk. EPA calls sampling usually unnecessary when mold is visible.
• Heard this?Black mold is the dangerous kind.Tap for the factsWhat's trueCDPH says the evidence doesn't justify singling out some molds as toxic. What tracks with health problems is water damage, dampness, visible mold and mold odor, whatever the color.
• Heard this?Bleach kills it for good.Tap for the factsWhat's trueMold bleached or painted over without fixing the dampness is likely to grow again, CDPH says. Its tenant advice is soap and water, with baking soda for scrubbing.
• Heard this?A dehumidifier will dry out a leak.Tap for the factsWhat's trueIt pulls water from the air, not from water arriving through a wall. A stain that grows after rain or never dries needs the leak found.

#Who pays: what California law says

General information, not legal advice. Building problems are the owner's, household habits the renter's, and three sections draw the line.

Under Civil Code 1941.7, the landlord's duty to repair mold starts only once the landlord has notice, and not when the tenant is violating Civil Code 1941.2, which includes keeping the unit clean and properly using its plumbing, gas and electrical fixtures. The state's mold booklet for renters gives the plain example: a code officer may find that mold came from a tenant not using the bathroom ventilation.

Getting it fixed in San Francisco

1. Photograph it, with dates

   The mold, the window, the fan grille, any stain, each with the date you first saw it.

2. Tell the landlord in writing

   What, where, since when, and what you think causes it. The repair duty starts with notice, so keep a copy.

3. Make access easy

   The landlord may enter to repair mold with the notice California's entry law requires. Answer quickly.

4. If nothing happens, call 311

   SF.gov sends mold and indoor air quality complaints to the Department of Public Health's Environmental Health branch through 311, or (415) 701-2311 from outside the city. Leaks and broken fans can go to Building Inspection, online or through 311.

Looking at a new place? Landlords must give prospective tenants the state's mold booklet before signing (Health and Safety Code 26148, in force since January 2022, CDPH says) and disclose in writing known mold that poses a health threat (26147). At a viewing, look for what the booklet flags: no working bath fan or opening window, a dryer with no outside vent.

#Fog between the panes

Haze you can't wipe off from either side means the seal on a double-pane unit has failed. InterNACHI, the home inspectors' association, blames thermal pumping: sun heats the gas between the panes, it swells and shrinks every day, and the seal cracks. A defogging service clears the moisture but leaves the seal broken.

The usual repair is a new sealed glass unit in the same frame. In a rental, that is the owner's job. Replacing whole windows in San Francisco raises permit questions, covered on the window replacement page.