What is the black stuff on windows?

In short, the black stuff on windows is mould. Yes, mould can grow on windows. It doesn’t grow on the glass but on the dust and debris that may accumulate on the glass. The typical condensation in the winter months provides the necessary moisture for mould spores to grow on the organic material which is dust and debris.

But wait a minute! If there is mould on my windows does that mean that I have mould in my home?

Mould doesn’t always grow on walls and ceilings and floors. Mould can grow anywhere it finds food and moisture. Mould spores are natural and are found in the outdoor environment in abundance. Mould is nature’s way of decomposing organic material. Mould spores come into the home through doors, windows, Air exchange systems, air intakes etc. 

If a house is not well-ventilated then the mould spores will linger and with the right environment will create an issue. It is important to keep the indoor air clean.

Window with condensation
Heavy condensation on a window

How do I keep the indoor air clean?

Everything starts with the regular cleaning of all surfaces, ventilators, windows and carpets. With carpets, it is crucial that regular deep cleaning is done. Carpets can keep a lot of dust, pollen and spores in them.

If your home has a forced air system, it is important to replace the filter every three months, and the humidifier pad every 18 to 24 months and clean the duct system every 3 to 4 years. The HVAC system is what redistributes the indoor air therefore your first line of defence.

Dirty filter
Dirty air filter

Can I do all these things myself?

Most of them you can but if you don’t know how or don’t have time, hire professionals such as Revision Home Services to do it for you. We also collaborate with Indoor Air Consultants that can evaluate your home and test for mould and other air pollutants.


  1. Can I use vinegar to clean the mould of the windows?
    No. Vinegar is a known household cleaning product but not for mould.
  2. Will mould reappear on the windows?
    Yes, it will if the windows are not kept clean and the indoor air has a high concentration of mould spores.
  3. Does vacuuming the carpet remove allergens?
    Not unless you are using a special vacuum. 
  4. Do indoor plants contribute to mould growth?
    If the indoor plants are kept healthy and the debris removed from the soil then there is no reason for that to happen. Everything in moderation of course. An indoor jungle is not a good environment.