What is the black stuff around my window sill?

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The windowsill is one place where black mould can grow easily as it is exposed to a lot of water and debris during rainstorms. Black mould on the windowsill is unsightly and can cause health problems as they pathogens from the mould can travel into the home when a lot of wind comes through the window. However, black mould is easily removed and prevented with just a little effort from the homeowner.


Black mould, also referred to as just mould, comes in a variety of colours; such as black, white, red, orange and yellow. There is also a distinct odour, usually a musty or earthy smell, that comes with the presence of mould. This characteristic often helps homeowners locate mould when it's hidden from view. When it is in the windowsill, the odour may not be noticeable if it's left open all the time.


Large amounts of standing water and humidity causes black mould to grow in abundance. When there isn't enough ventilation present, black mould will grow where the highest level of water sustenance are located. Enough moisture within the windowsill, though not plainly visible, can be enough for black mould to grow. According to the Toxic Black Mold Information Center, 55 per cent humidity or greater for a long stretch of time is enough moisture to trigger a black mould outbreak on windowsills and other surfaces.

Cleaning Up Black Mold

When cleaning up black mould, it is best to put on a pair of rubber gloves first to protect your hands. Fill either a bucket or a spray bottle with warm water and bleach. The bleach will kill the mould quickly and allow you to wipe it away. Spray the mixture directly on the mould and let it sit for five to 10 minutes. Wipe the area using a disposable cloth or a paper towel. Spray more of the mixture as needed. Once you have removed all evidence of the mould, dry the area completely to prevent a reoccurrence.


Keep the window open after you cleaned, to help air out the room and allow for good airflow to help dry everything out. To prevent mould from coming back, keep all areas of the home as free of moisture as possible. Clean up any standing water or condensation as quickly as possible. Using a dehumidifier during times of humidity will limit the potential for mould growth.

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