Black Spots on the Window Sill: Condensation, a Leak, or Both?

By John Calhoun, owner of Mold Remediation Gettysburg PA · Updated October 24, 2026

Nobody buys a house hoping to spend January wiping window sills with a rag. Yet here we are. Black spots on a sill or the bottom of a window frame mean water is sitting there long enough to grow something, and the first job is figuring out where that water comes from. Either it’s condensing out of the room’s air onto cold glass, or it’s getting in around the window from outside. The cleanup is easy. Picking the right cause is what keeps it from coming back.

Condensation or a leak?

The EPA describes the condensation side plainly: moisture condenses on cold surfaces, and drops forming on the inside of a window is the classic example. ENERGY STAR explains why: water condenses when the glass is colder than the dew point of the indoor air. A leak is different. Water gets past failed caulk, worn glazing putty, missing flashing or a cracked sill, and runs into the wall. A moisture meter on the drywall under the window, compared with the wall next to it, usually settles it in a minute.

Cleaning a sill the right way

For growth on a sound, hard surface like painted trim, vinyl, aluminum or glass, the EPA’s advice is to scrub it off with detergent and water and dry it completely. Check the gasket around the glass too. Don’t paint or caulk over mold; the EPA says that clearly, and paint over growth tends to peel. If the wood is soft when you press a screwdriver into it, cleaning won’t fix it. That section of sill or trim needs repair.

Old paint matters here. In Gettysburg borough, the Census Bureau’s latest estimates put about 87% of homes as built before 1980, and window sills and sash are where old paint gets worn and scraped. The EPA’s lead-safe rules apply to contractors who disturb paint in homes built before 1978, so if a sill needs scraping or replacing, ask about it.

Fixing condensation

The EPA’s moisture guidance: raise the temperature of cold surfaces where moisture condenses, with insulation or storm windows, and it notes interior storms work better than exterior ones. Bring the humidity down with exhaust fans that vent outside and a dryer vented outside. For indoor humidity, the EPA’s ideal is 30 to 50 percent, and its moisture page carries ASHRAE’s maximums, which drop as it gets colder: about 30 percent at 10°F and lower below that. Wall corners do the same thing for the same reasons, covered in winter condensation mold on walls.

Newer, tighter houses can have the opposite problem from old drafty ones. I see newer homes sealed up tight with oversized heating and cooling systems that never run long enough to dry the air. That’s in tight houses and oversized HVAC.

Old windows and historic district rules

A lot of downtown Gettysburg sits in a historic district, and window replacement on a historic house there goes before the borough’s review board first. Before you price new windows, consider whether repair, new glazing putty, flashing or interior storms would do the job. The downtown and Old Town page covers the review process, and historic homes and mold covers what changes in a very old house.

The window is a clue, not the culprit

After years of this, here’s what I’ve learned: the window tells you about the whole house. Spots on every window means too much moisture inside. Spots on one window means water outside getting in. Either way, it can all be fixed.

John Calhoun here, from Mold Remediation Gettysburg PA. If a window keeps growing spots or the wall under it reads wet, I’ll find out which one you’ve got. Read about mold remediation in Gettysburg, PA or see the services I offer. Housing age for your part of town is on the Gettysburg housing data page.