If the black dots on your sill cover less than about one square metre and the sill is a hard surface like vinyl, painted wood or metal, you can clean it yourself with unscented dish soap, warm water and two rags. Dry it right away. Then look at where the water came from, because that's what decides whether the dots come back next week or stay gone until spring.
First, is it small enough to do yourself?
Health Canada uses a very practical measure. In its video on getting rid of mould, an environmental health officer explains it like this:
A good way to figure out what a small area of mould is considered, is to take a standard garbage bag, fold it in half and that's about 1 square meter. If the area of mould fits under that, then you can clean it yourself.
Health Canada's full guide to moisture and mould indoors adds two details. Small also means no more than three patches, adding up to one square metre or less. A single patch bigger than three square metres needs a qualified professional to find the cause. A typical sill, even one with spots along its whole length, sits well inside the small group. If the black runs down the wall under the window or across the ceiling above it, that's a different job.
Where the water sits tells you why it's there
Before you scrub, look at the window first thing in the morning, before anyone has opened the blinds or wiped anything. Natural Resources Canada explains that moist indoor air turns to water or frost when it touches a cold surface, and that the risk grows as it gets colder outside or more humid inside. Where you find the water says a lot.
| Where you see the water | What it usually means | What helps |
|---|---|---|
| Drops on the room side of the glass, running into the bottom corners | The air inside is too humid for the cold glass | Lower the humidity and let warm air reach the window |
| Wet frame or wet weather seals, with the glass mostly clear | Moist air is cooling at the coldest edge of the window | The same, plus drying the frame and sill every morning |
| Fog between the two panes that you can't wipe | The seal of the glass unit is broken | Replacing the sealed glass unit. Cleaning can't reach it |
| Fog on the outside of the glass around sunrise | A cool, calm, humid night on an efficient low-e window | Nothing. It's outside and dries on its own |
| Water pooling in a sliding track | Condensation running down and collecting in the lowest part of the frame | Keeping the track clean and dry (see below) |
The first two rows are where nearly all sill mould comes from. The water runs down, sits in the corner overnight, and dust in that corner gives the mould something to grow on.
Cleaning the sill the way Health Canada describes it
Health Canada lists window sills among the washable surfaces that can be properly cleaned, along with wood, glass, plastic, vinyl and tile. The method is simple, and the order matters.
- Open the window a little if the weather allows, and ask anyone who shouldn't be in the room to leave. Health Canada names infants, children, seniors, pregnant people and anyone with asthma or another breathing condition, and recommends keeping pets away too.
- Put on safety glasses or goggles, a well-fitting disposable N95 mask and household gloves.
- Fill one bucket with warm water and a little unscented dish detergent, and a second bucket with clean water. Have two clean rags ready.
- Move plants, frames and anything else off the sill. Wipe those items too if they stood in the wet corner.
- Scrub the sill with the soapy rag, working into the corners and along the edge where the sill meets the frame.
- Wipe the same area quickly with the rag from the clean bucket.
- Dry it completely, straight away. A dry towel works, and a small fan pointed at the sill helps in a cold room.
- Seal the used rags in a plastic bag and take the bag outside.
If the sill is painted wood and the dark marks are under the paint rather than on it, wiping won't reach them. Health Canada's point about walls applies here as well: painting over mould does not kill it, it only hides it for a while.
Why the bleach recipe stays in the cupboard
Many guides you'll find this month tell you to spray bleach and water on the sill and leave it for ten or fifteen minutes. Health Canada says the opposite, in bold, in its guide: do not use bleach to clean up mould. For a hard sill, detergent and water and fast drying is the whole method.
The other common tip is to use bleach and vinegar one after the other, or vinegar mixed with something else for extra strength. Don't. Health Canada's household chemical safety page says never to mix household chemical products, because some mixtures give off dangerous gases. In a small room with the window only open a crack, that's the last thing you want. One product, then plain water.
Two spots that need a different touch
The sliding track
Tracks on sliding windows and patio doors collect grit, dead insects and the water that runs down the glass. Vacuum the dry dirt out first with a crevice tool, because wet grit turns into mud that's harder to lift. Then wipe the track with the same soapy rag wrapped around a flat stick or an old card, follow it with the clean wet rag, and dry it. If your frame has small slots along the bottom, keep them clear of dirt and don't pack them with soap.
Caulking that stays grey or black
If the line of caulk where the frame meets the sill is still dark after a good scrub, the mould is usually inside the caulk, not on top of it. More scrubbing just wears the caulk thin. That bead needs to be cut out and replaced once the area is fully dry, which is a small repair job rather than a cleaning one.
What stops the morning drip from October to March
Cleaning removes what's there. These habits keep the water from building up again, and each comes from Health Canada's checklist for windows or from Natural Resources Canada:
- Keep indoor humidity between 30% and 50%. A hygrometer costs little and shows you the number. If it reads above 50% and the windows are wet, Health Canada suggests a dehumidifier. We wrote about the same little gauge in our piece on salt season and entryway floors, since floors care about the number too.
- Leave blinds and curtains open, at least during the day. Heavy curtains trap cold, wet air against the glass.
- Keep furniture away from the heating vent or baseboard heater under the window. NRCan explains that heat sources sit under windows on purpose, because air moving across the glass keeps it warmer.
- Clear deep sills. A row of plants, books or photo frames blocks that same air and lets cool air pool at the glass.
- Run the range hood when you cook and the bathroom fan when you shower, and keep the bathroom fan going for at least 30 minutes afterwards.
- Dry the sills and frames every morning while condensation season lasts. It takes one towel and two minutes.
- Turn on the heat recovery ventilator if your home has one.
- Be aware that a rack of wet laundry in the bedroom raises the humidity in exactly the room where the windows are coldest.
When the spots come back anyway
Health Canada's guide says mould that returns after cleaning is usually a sign the moisture source has not been removed, and that it's time for professional help. If the sill grows dots again within a week or two even with the humidity in range, look for water coming in from outside: a leak at the frame, a gap in the outside caulking, or a broken seal. Those are questions for a window or building professional, and no cleaner will fix them.
Window sills are on the list for our window cleaning, inside and out, and every deep clean takes in the sills along with ledges, corners and door panels. If the bathroom window is the worst one in the house, our bathroom deep cleaning page shows what else gets done in that room.
