The appearance of mould on window joinery is due to what's commonly known as "damp": too much moisture in the air and on the window, creating advantageous conditions for fungi (this is what mould is) to grow. This is in turn caused by lack of sufficient ventilation, sometimes caused by incorrectly fitted windows.
High humidity in the room and large temperature differences between the indoor and outdoor temperatures cause condensation: water vapour becomes liquid on the surface of the window or window frame. This water, in turn, remains, for example, on frames, seals or in a layer of poorly made insulation, creating ideal conditions for mould to grow.
Sometimes DIY home remedies, used for years by our mothers and grannies, turn out to be the most effective, and at the same time the least invasive and the safest not only for the window but also for the household members. Commercial products and strong chemicals can be highly corrosive and negatively affect, for example, the insulating properties of seals, not to mention the adverse effect on the respiratory system of the people inhabiting the house. Even some detergents, incorrectly used, can dull the gloss on the window frame (depending on materials, for example on uPVC windows). Therefore, it is most sensible to first reach for home-grown ways of dealing with mould.
The first and most effective solution for getting rid of mould on the windows is a solution of water with vinegar in a ratio of 2:1. Spray on the treated surface, and wait approximately 2 hours, then rinse thoroughly with warm water. There should be no trace of the mould left. Acid kills the fungus in this case.
Other options include using salt or tea tree oil: both are known as effective fungicides. It is also worth using those to wash the windows and window joinery, from time to time, to prevent further mould growth.
If the problem persists, commercial products can help, but be careful when using them as they can irritate skin and airways. Also, never mix bleach with acidic products (e.g. vinegar) as it can cause dangerous vapours.
The installation conducted with due care and according to correct principles ensures optimal insulation and ventilation. Unfortunately, it often happens that windows are fitted by incompetent "tradespeople", which causes various defects during later use. Tensions and stresses can be the result of poor installation, which leads to warping of the joinery or the formation of thermal bridges, which in turn might lead to condensation on the window or in its close vicinity, and this, as we already know, creates ideal conditions for fungi and mould developing.
It is important not only to remove the mold itself, but also the reason for its growth. One of the simplest solutions here is maintaining good ventilation of the rooms, and especially airing the rooms if the fixed ventilation is not ideal.
Main ways to avoid damp and resulting issues include: