TL;DR
Black spots in a window corner are surface growth feeding on condensation that pooled and never dried. Detergent and water removes most of it, and drying the area afterward keeps it gone. Growth on a flexible rubber seal often stains permanently, because the material is porous.
Why it always starts in the same corner
Window glass is the coldest interior surface in a room in winter, so warm humid indoor air condenses on it. That water runs down the pane and collects at the bottom, where the glass meets the frame.
The sill corner is where it stops. It is the low point, it is shaded by the frame, and it gets the least air movement of any part of the window. Water that pools there in the morning is often still there in the afternoon.
Give that corner a day or two of moisture, plus the dust and debris that collect in a track, and you have the food and water surface growth needs. The condensation side is explained in why windows get condensation in cold weather.
Source: published building science guidance on condensation and surface moisture
Cleaning it off, step by step
- Open the window and ventilate the room. Do this before you spray anything.
- Wear gloves. Eye protection is sensible if you are working overhead or spraying.
- Vacuum the track first. Loose debris and dust come out with a crevice tool. Wetting it first just makes a paste.
- Wash with detergent and warm water. A few drops of dish soap on a microfiber cloth removes most surface growth. Cleaning is what removes it; a disinfectant sprayed onto a dirty surface does very little.
- Work the corners with a soft brush or a cotton swab. An old toothbrush reaches the frame joint where a cloth cannot.
- Rinse with a clean damp cloth, so no detergent film is left to hold dust.
- Dry it completely. This is the step that decides whether the spots come back next week.
- Clear the weep holes. Vinyl frames have small drainage openings along the bottom of the exterior track. Blocked weep holes trap water in the frame, and a bent paper clip clears them.
If detergent leaves staining behind, hydrogen peroxide on a cloth, left a few minutes then wiped and dried, is a reasonable second pass on hard surfaces. Test in a hidden spot first, particularly on painted wood.
The chemical rule that matters here
Never mix a bleach-based cleaner with anything else in the same container or on the same surface.
This warning belongs on window work specifically, because many glass cleaners contain ammonia. Cleaning the glass with one and then treating the frame with a bleach product means you can generate chloramine gas in a small space you are leaning into. It is a real hazard, not a theoretical one.
The same applies to bleach with vinegar or any acid, which produces chlorine gas, and to hydrogen peroxide mixed with vinegar in one bottle. Use one product at a time, rinse between products, ventilate, and never combine them.
Do not scrub the wrong material
Vinyl and uPVC frames take mild detergent well. Avoid abrasive pads and scouring powders, since scratched vinyl gives growth more surface to grip and the marks are permanent.
Painted wood cleans with detergent and a soft cloth, and it should not be saturated. Water sitting on a wood frame is what caused the problem in the first place. If the paint has blistered or peeled, moisture has been in the wood for a while.
Rubber and vinyl gaskets are the ones that disappoint people. Flexible seal material is porous, and growth roots into it rather than sitting on it, so the black speckling can usually be lightened but not fully removed. Do not attack it with abrasives or solvents, because a damaged gasket leaks and costs far more to replace than the staining costs in appearance.
Keeping it from coming back
Cleaning treats the symptom. The corner will regrow unless it stops being wet.
- Wipe the condensation. A cloth along the sills on a foggy morning takes under a minute per window and is the highest-value habit on this list. The routine version is in window condensation and sill cleaning.
- Open blinds and curtains during the day. Trapped air behind a heavy curtain runs colder than the room, which makes condensation worse.
- Run exhaust fans properly during and after showers and cooking, and lower indoor humidity in cold weather. Several windows fogging every morning means the house is running too humid for the outdoor temperature.
- Keep the tracks clear. Debris holds moisture and blocks drainage.
When it is not a cleaning job
Surface growth on a frame is ordinary cleaning. Some things are not.
Stop and get a professional opinion if the frame or the wall beneath it is soft, spongy, or crumbling, if growth spreads onto the wall or into the drywall around the window, if it returns within weeks despite proper drying, or if the area covered exceeds roughly 10 square feet. Mold remediation is a separate licensed trade, and the threshold is covered in when cleaning mold is not enough.
Water between the panes of the glass itself is a failed seal on the glass unit, which needs a window professional and has nothing to do with cleaning. The difference between surface growth and something deeper is in mold spores versus surface mildew.
Cleaning reduces what is present on the surface. It does not treat, prevent, or cure any condition, and symptoms someone connects to a damp room belong with a doctor.
FAQ
How do I get black mildew off a window sill?
Ventilate the room, vacuum loose debris from the track, then wash the area with dish soap and warm water on a microfiber cloth, using a soft brush in the corners. Rinse with a clean damp cloth and dry the surface completely. Drying is what stops it returning.
Why do black spots keep coming back on my window frame?
Because that corner keeps getting wet. Condensation runs down the glass and pools at the bottom of the frame, where there is no airflow to dry it. Cleaning removes the growth, but it regrows within weeks unless the condensation is wiped or the indoor humidity comes down.
Can I remove mildew stains from a rubber window seal?
Usually only partly. Flexible gasket material is porous, so growth roots into it rather than sitting on the surface, and the staining is often permanent. Clean it gently with detergent and accept some discoloration. Abrasives and solvents damage the seal, which causes leaks and costs far more.
Is it safe to use bleach on window frames?
Only carefully, and never alongside another product. Many glass cleaners contain ammonia, and bleach plus ammonia produces chloramine gas in the small space you are leaning into. Detergent and water removes most surface growth, so bleach is rarely necessary on a window in the first place.
What are the small holes in the bottom of my window frame?
Weep holes, which drain water out of the frame track to the outside. When they clog with dust and debris, water collects in the frame instead of draining, which keeps the bottom of the window wet and feeds growth. A bent paper clip clears them.
Does mildew on a window mean I have a mold problem?
Not necessarily. Surface growth in a window corner is a condensation problem and is normal in a humid house in cold weather. It becomes a bigger issue if it spreads onto the wall, if the frame or drywall feels soft, or if it returns quickly despite drying.
The whole-house equivalent of this routine is covered by house cleaning services.
Windows are one of those jobs that quietly slip a whole season, and the corners are the part that suffers for it. If you want them handled properly as part of a regular clean, get a quote and we will build them into the visit.