Mould on windows

mould on windows 1

Mould on windows is usually one of the first sigh of deeper problems with condensation. The problem is usually most prominent in winter months when the temperature difference between the inside and outside of the window is greatest. Other factors such as reduction in natural ventilation, drying of clothes indoor can greatly amplify the mould on window problem.


Mould is mostly found inhabiting the rubber or silicon seal around the double glazed window panes, on putty, or window frames themselves and occasionally it can also be found to the glass surface of the window. Glass surface of the window is in most cases very resistant to mould contamination, however if the windows are not cleaned frequently the accumulation of small invisible layer of dust, skin cell and other debris allows the mould to take hold.

Causes of mould on windows

Single pane windows are the most likely to be attacked by moulds, fortunately few homes have such windows these days. Double and triple glazed windows are usually very resistant to mould. The reason why mould start to grow on window and surrounding window structure is as usual condensation. Warm moisture loaded air comes into contact wit relatively cold surface of the window pane where it cool rapidly and sheds water in the form of condensation. Condensation on the windows does not necessarily means that the indoor air humidity is very high. It only tells you that the temperature difference between the worm indoor air and cold air close to the windows surface is great enough to condense water.

There are two possible ways to reduce condensation of the windows. You can either reduce the amount of available moisture in the air by having extractor installed or use dehumidifier. The other way is to decrease the temperature difference between the indoor air temperature and window temperature. This can usually be achieved only by installation of a new double glazed window system.

Type of mould found on windows.

Mould on windows is usually an aesthetics concern to homeowners than a health issue. Stachybotrys, the mould spore typically referred to as "toxic mould" or "black mould" is rarely found on window surfaces, as the growth conditions on window surfaces (frequent changes in moisture and lack of suitable nutrition sources) are usually not adequate for supporting toxic black mould (Stachybotrys) growth. Toxic mould require good , usually cellulose rich growing material, plenty of water and long enough time to develop. Based on mine experience and numerous analysis typically encountered mould are Cladosporium and Alternaria moulds however a number others can also be present.  

Preventing mould on windows

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Due to the impervious character of the windows structure none of the chemical treatment whether professional or domestic will have long lasting effect on keeping mould from windows. The reason is that the active chemicals cannot penetrate the surface of the materials and are gradually washed away. To reduce the chance of mould getting hold on your windows try to :

The mould growth on windows rarely extends below the top surface of the window base plate sill, and it is not necessary to remove the window sill unless the water damage and mould growth is very extensive such that the base plate has deteriorated and is not structurally sound.

Condensation and subsequent mould growth on window surface is usually not much of a problem. If however water damage and usually brown staining occurs at the base the window may be suffering from a more serious structural problem. Any damage to the window can potentially lead to serious indoor mould contamination and structural damage. Additional investigative work is almost always required if mould contamination growth or water staining is observed below the window sills.