Mold on Windows
by Building Inspector and Indoor Air Specialist, Dan Schilling
© Copyright 2003 Residential Inspections LLC, All Rights Reserved
Is a little mold on the windows anything to be concerned about?
If you take the time to look, in northern climates, you will see mold growing on most indoor windows where the glass meets the window frame. Sometimes it can be a valid health concern and other times it may only represent a minor cleaning project.
How can I tell if there is mold growing on my windows?
Many people are totally unaware that they have mold growing on the windows of their homes. The mold usually grows right at the joint between the glass and the window sash frame along the bottom edge of the windows. Sometimes the color of the mold matches the color of the window, thus making it visually imperceptible by the untrained eye. Other times the amounts of mold may be visible, but misconstrued to be insignificant. This is because the total cumulative effect of the mold on all of the windows is not being considered. The amounts of spores being emitted into the air can be in the millions per day from window mold alone. Lastly, there are indeed cases where the mold is visibly thick and an obvious concern. Usually when people have large amounts of mold growing on their windows it is also indicative of potential mold contamination in other areas in that same indoor environment.
Does the type of mold matter?
Yes, but not necessarily enough to justify the expense of testing the mold in a laboratory. While some types of mold that have been discovered growing on windows are certainly worse than others from a heath standpoint, basically all molds should be treated with the same degree of respect and be promptly and properly removed.
Is the mold eating the windows?
Not typically. While wood framed windows can be destroyed by mold feeding on the cellulose fibers of the wood, for the most part, window mold does not feed on the windows themselves. Usually the mold that is discovered growing on windows is feeding on common house dust that settles on the bottom of the window sashes. The make up of house dust is primarily dead skin cells sloughed off by the human occupants of the building. This dust, being organic, will quickly rot when exposed to the moisture that condenses on window glass during cold weather and will subsequently become a delicious meal for even the most discriminating mold. This is also why we see more molds growing on the windows in bedrooms and bathrooms because that is where we dress, undress, shower and sleep. It is during the process of changing clothes that most of our skin cells are sloughed off, additionally, we create plenty of moisture when we shower and while we sleep through breathing and evaporation through our skin. After the mold starts to grow, it no longer requires moisture but can survive well on the humidity in the ambient air all year long.
What can I do to control window mold?
Humidity control and good house cleaning practice are the best defenses against window mold. During the winter months, indoor relative humidity levels should be kept approximately 30%, certainly no higher than 50%. Even under the best indoor humidity conditions, you should expect to see some moisture condense and collect on the glass when the outdoor temperatures drop down below freezing. Make a practice of wiping these areas dry often during cold whether. This wiping action will also help to keep the dust from collecting in these areas, although it is good practice to keep these areas clean all year long as well.
What if I already have mold on my windows?
If you already have a window mold condition, be certain to clean the mold properly. If the window frames are made of non-porous materials such as metal or vinyl, a simple solution of chlorine bleach and water will suffice for the cleaning and sanitizing. (Keep in mind that specific safety precautions must be taken when using bleach and when exposing oneself to mold contaminated areas.) If the windows are made of wood, a professional biocide should be used to penetrate the wood surface to help kill the subsurface root base of the mold during and after clean-up. This will help prevent the mold from recurring soon afterward.
When cleaning mold on windows, be sure to look the windows over carefully. Mold can grow not only where the glass meets the sash frame but it can grow on the inside of the wood pieces that cover crank and gear mechanisms, inside of the tracks on sliding window systems, inside window sill areas, and on storm windows.
with those you care about.