Black mold on a window sill is one of the few damp problems with a single, reliable cause: the glass is colder than the dew point of the air in the room, so water condenses on it, runs down, and soaks into the sill. Clean the sill and the mold comes back, because the cleaning never touched the reason the sill was wet.
The fix has two halves — lower the moisture in the air, and raise the temperature of the glass. You almost always need the first one. Whether you need the second depends on your windows.
Why the window, and not the wall?
Glass is the coldest surface in most rooms. A single-glazed pane in winter sits within a couple of degrees of the outdoor temperature. A modern double-glazed unit is much warmer, but the edge of the pane and the frame — where the spacer bar bridges the gap — stay noticeably colder than the middle. That is why the mold appears as a band along the bottom of the glass and on the sill below, not in the centre of the window.
Air holds a fixed maximum amount of water at any given temperature. Cool it below that point — the dew point — and the surplus has to leave the air as liquid. The window is where that happens first.
Work out whether your window will sweat tonight
Measure the room temperature and relative humidity with a hygrometer, then read the dew point off the table. If the glass surface is colder than that number, you will get condensation.
| Room temp | 40% RH | 50% RH | 60% RH | 70% RH |
|---|---|---|---|---|
| 64°F (18°C) | 39°F | 45°F | 50°F | 54°F |
| 68°F (20°C) | 43°F | 49°F | 54°F | 58°F |
| 72°F (22°C) | 46°F | 52°F | 57°F | 61°F |
A worked example. Your bedroom is 68°F at 60% RH, so the dew point is 54°F. On a night when it is 35°F outside, single glazing will sit around 40°F and stream with water. Double glazing might reach 55°F in the centre and stay dry — but the edge, a few degrees colder, will not. That is your mold band.
Drop the same room to 45% RH and the dew point falls to about 46°F. Now the double-glazed edge is comfortably clear, and even the single pane only sweats on the coldest nights. Nothing about the window changed — you simply moved the target.
Where the moisture is coming from
A bedroom window that sweats every morning is usually a bedroom problem, not a house problem. Two sleeping adults release roughly 1–2 pints of water vapour overnight through breathing and perspiration. In a closed room with no ventilation, that is enough to lift relative humidity by 15–20 points by dawn.
| Source | Water released | Typical timing |
|---|---|---|
| Two adults sleeping | 1–2 pints | Overnight |
| Shower (10 min, unvented) | ~0.5 pint | Morning spike |
| Drying laundry indoors | 2–5 pints per load | Over 12–24 hours |
| Cooking without an extractor | 1–2 pints | Evening |
| Unvented gas or paraffin heater | Roughly its own fuel weight in water | While running |
If the sweating window is in a bedroom and you also dry laundry on a radiator in that room, you have found your answer before buying anything.
Cleaning the sill properly
Do this once the moisture problem is under control, otherwise you will be repeating it in a month.
- Open the window and wear gloves and an N95 mask. Disturbing dry mold sends spores into the air.
- Wipe the growth away with a damp disposable cloth first. Do not dry-brush it.
- Wash the sill and the bottom of the frame with detergent and warm water, then dry it thoroughly. Detergent and physical removal do most of the work.
- If you want a biocide afterwards, use a proprietary mold cleaner and follow its contact time. Never mix bleach with ammonia or with other cleaners.
- Check the sealant bead between sill and frame. If mold is growing inside soft silicone rather than on it, the bead has to be cut out and replaced — surface cleaning will not reach it.
Painted timber sills that have gone soft or dark under the paint are absorbing water. Those need drying out and repainting, or the growth simply returns from underneath.
Stopping it coming back
Lower the humidity
Target 40–50% RH in winter. That is low enough to keep the dew point below most double-glazed surfaces and still comfortable to live in. A hygrometer on the windowsill itself, rather than across the room, tells you what the glass is actually facing.
- Run the bathroom extractor during the shower and for 15 minutes after, with the door shut.
- Use a lid on pans and run the cooker hood.
- Dry laundry outdoors, in a vented tumble dryer, or in one room with a dehumidifier running — not spread across the house.
- Leave trickle vents open. They cost a little heat and save a lot of condensation.
- Leave a gap between furniture and external walls so air can move.
If those measures are not enough — and in a tightly sealed modern flat they often are not — a dehumidifier sized for the room will hold the level. For a single bedroom, a small unit run overnight is usually sufficient; see our sizing calculator for the capacity you need.
Warm the glass
The other half of the equation. Heavy curtains and blinds pulled tight actually make window condensation worse, because they cut the glass off from room heat and let it fall further. Leaving a gap at the bottom, or opening the curtains earlier in the morning, helps more than it sounds like it should.
Longer-term options, in rough order of cost: secondary glazing film for the winter, replacing failed double-glazed units (a misted pane has lost its seal and its insulation), and upgrading to units with a warm-edge spacer bar, which is specifically designed to fix the cold-edge problem that causes this pattern of mold.
When it is not condensation
A few signs point somewhere else. Staining that spreads up the wall above the window, or appears on the outside face, suggests a failed seal, cracked render or a blocked gutter overflowing onto the reveal. Damp that is worse after rain than after a cold night is water ingress, not condensation. And a tide mark rising from the floor is a rising-damp or plumbing question, not a window one.
If the affected area is larger than about 10 square feet, if it keeps returning after the humidity is genuinely controlled, or if anyone in the household has asthma or is immunocompromised, get a professional assessment rather than working through it yourself.
Frequently asked questions
Is black mold on a window sill dangerous?
Most dark growth on sills is Cladosporium or Aspergillus, not the Stachybotrys chartarum that gets called “toxic black mold” in the press, and you cannot tell them apart by eye. A small patch on a hard sill is a nuisance rather than an emergency for most people, but any mold can trigger symptoms in asthmatics and allergy sufferers, so remove it promptly and fix the cause.
Will a dehumidifier stop window condensation?
Usually yes, provided it is sized for the room and the moisture source is indoors. Dropping a 68°F room from 60% to 45% RH lowers the dew point by roughly 8°F, which is often the difference between a wet pane and a dry one. It will not help if water is entering through the fabric of the building.
Why does only one window in the house get mold?
Either that window is colder than the others — north-facing, single-glazed, or a unit with a failed seal — or the room it is in produces more moisture and gets less ventilation. Bedrooms and kitchens are the usual culprits.
Does bleach kill mold on a window sill?
On a hard, non-porous sill, diluted bleach will kill surface growth, but detergent and physical removal do the same job with less risk. On porous timber, bleach mostly removes the colour while the mold remains in the material, which is why the stain seems to come back quickly. Never combine bleach with ammonia-based cleaners.
How quickly will it come back?
If the sill keeps getting wet, visible growth can reappear within one to three weeks. See how long mold takes to grow for the full timeline. Keep the surface dry and it will not return at all.