Mold On Windows: Causes And How To Fix

Mold growing around windows is a common site in many homes, with small patches of green, black, or blue mold growing either in the corners or the surrounding walls.

It’s not something that anyone wants in their home, it’s ugly, it smells and it has the potential to be hazardous to your health. So, why does mold grow around windows in the first place, how can you remove it, and importantly, how can you prevent it from happening again?

In this article, we answer all these questions and more, so for everything you need to know about mold growth on windows, keep reading.

Condensation is the number one cause of mold growth occurring around windows or on wooden or PVC window frames. Keeping the ambient humidity of your home lower than 55% and regularly cleaning windows will help prevent mold growth. Should it occur, white vinegar and baking soda are effective methods of removal.

How to stop mold growing on windows

Why is mold forming on or around your windows?

Mold and mildew growing on or around your windows is common because the environment provides exactly what mold needs to live and grow.

Mold is a fungus, and though it is a complex organism, its needs are fairly basic. All it requires are a source of moisture, a source of nutrients, and preferably an ambient temperature between 60-80 degrees Fahrenheit, as this is the range it grows the fastest.


You may be wondering what possible nutrients could be lurking on and around your windows. The truth is, mold feeds on decaying organic matter, and a good source of organic matter within a property is dust.

Dust is made from shed skin cells, bacteria, dead bugs, pollen, and many other components. These ingredients supply mold with all the nutrients it could need, therefore if your window hasn’t been cleaned in a good while, dust could have settled and become a perfect place for mold to set up home.


Moisture is plentiful around windows, as warm, humid air will condense on cooler surfaces, and as the temperature outside is generally cooler than the internal temperature of a property, the moist air will condense on the cooler surface of the window pane.

Once the humid air starts to condense, it drips down the pane and forms into pools in the corners and bottom of window frames. You will often see mold starting to grow in these spots, as it has a near-constant supply of moisture.

There could be many additional reasons why you have excessive moisture within your home, including:

  • Burst pipes
  • Leaks
  • Drying laundry indoors
  • Rising damp

It is important that you investigate these additional causes of excessive moisture to rule them out of the equation, or you could spend time tackling the symptoms without addressing the true underlying cause.


Mold can grow in a variety of temperatures and at varying speeds. Many strains, however, will grow at their fastest rate between 60-80 degrees Fahrenheit. This also happens to be the range at which most American households set their thermostats for many months of the year.

In cooler temperatures, mold can still grow, it simply does so at a slower rate, in warmer temperatures it tends to dry out and hibernate, but has the ability to grow once again when the appropriate moisture levels return.

Lack of airflow

Airflow is an important factor when it comes to mold growth, as a room with poor ventilation will keep moist air trapped in one place, making it more likely that it will condense into pools of liquid.

A flow of fresh air also prevents mold spores from spreading and settling in one area, making it more difficult for them to set up a colony. Modern homes are more prone to airflow issues due to increased insulation combined with building and design techniques that make them more airtight for energy efficiency.

How to identify mold on windows

Identifying mold around a window should not be difficult, as it usually appears in small spots or clusters of slightly irregular circles, often in dark colors such as brown or even black. This mold will form on either the frame, window sill, or on the sealing strip (rubber seal that is usually black or white).

The only time it might be tricky to determine if what you are dealing with is actually mold is if the window frames themselves are dirty. In this case, it may be difficult to know what is mold and what is dirt.

The good news is, there is a simple test to see if what you are looking at is indeed mold. The test involves dipping a swab (cotton bud), in one part bleach to three parts water, then dabbing it onto the affected area. The bleach will kill mold, so if the area begins to lighten, you know you are dealing with mold, any other reaction would mean you are more likely dealing with simple dirt accumulation.

An even more simple test is to simply smell the room. If you walk into a room you suspect of having mold and are immediately met with a musty scent that reminds you of a basement or attic, there is a very good chance you could be dealing with a mold issue.

*Do not put your nose next to the mold patch to do this, as the spores can be hazardous to your health.

What kinds of mold grow on windows?

Whilst there are thousands of potential strains of mold that could be growing in your house, such as pink mold, black mold, white mold, mildew, and so on, there are several in particular that are most commonly seen growing on or around windows, which are:

  • Penicillium
  • Aspergillus
  • Stachybotrys
  • Altenaria

Each of these molds has a slightly different appearance and comes in different colorations. The best way to identify which particular strain you are dealing with, is by using a lab at home testing kit.

You can also look out for telltale signs of particular strains to try to determine which you are dealing with, but this can be tricky. To make that a little easier, here is each strain with some of its particular characteristics.


Penicillium strains are most commonly seen with blue-green, gray-green, or olive colorations, although yellow and pinkish colors have also been seen. Its texture is often compared to that of velvet or powder.


Aspergillus strains are often seen in darker shades, which is also why it is commonly mistaken for black mold, (which we will discuss shortly). It can be seen in yellow-green, brown, and black colors, again with a texture comparable to velvet.


Stachybotrys (and especially Stachybotrys chartarum), is what is commonly referred to as black mold. As you would expect, its color is usually very dark and can include browns, very dark greens, and of course, black, with a texture most often described as being shiny when dry, and slimy when wet.


Alternaria strains are more often seen outdoors but can live within a property if there is sufficient moisture, this is why it is often seen within bathrooms and around any location with high moisture levels. If condensation is a regular issue, this mold may be able to successfully breed within your property.

It often appears in dark, grey-green, and dark brown colors, with a downy or wool-like texture.

As explained, unless you are an expert in molds, it will be extremely difficult to tell which strain of mold you are dealing with.

Using a home testing kit will certainly help you decide how to tackle the strain.

*Please remember to wear safety equipment when carrying out any testing, such as a breathing mask, and eye protection such as goggles and rubber gloves to prevent any contact with mold or its spores. As we will discuss shortly, some of these strains have the potential to be quite harmful to humans and animals.

Are the molds harmful?

Molds most likely found on and around windows and window frames can be hazardous to humans and animals. This is because many of the strains that grow in this environment have the ability to produce mycotoxins as a defense mechanism whenever they are disturbed or feel threatened.

Of the molds listed above, Stachybotrys chartarum (often referred to as black mold), is often cited as being the most harmful, as it is deemed to be the most highly toxic, but there is little to say that this strain is any more harmful than others that produce mycotoxins. Toxins released from any strain can cause a myriad of health complaints, even in healthy individuals.

The most common symptoms of mold exposure are:

  • Wheezing
  • Shortness of breath
  • Dry cough
  • Tight chest
  • Nausea
  • Fatigue
  • Skin irritation

Healthy individuals will often suffer from these symptoms if exposed to mycotoxins over a long period of time, or in large doses. However, people with suppressed immune systems or allergies can suffer from some of the more serious symptoms even when exposed to small amounts.

How to clean mold on a window

The good news is, that much as mold can grow easily on a window, it can also be removed just as easily. Before we begin discussing how to clean the window of mold and mildew, it’s important to note that if you are attempting to do so yourself, you should always wear protective gear, such as goggles, a breathing mask, and rubber gloves to prevent coming into direct contact with the mold or its spores.

You also have a number of options when it comes to mold removal, so I have listed these in easy steps. Each is equally as effective, so you can simply choose your preferred method.


Vinegar is a powerful cleaning product and an excellent remover of both mold and mildew. This is due to the acidity level of the vinegar being high enough to kill mold, but not so strong that it would damage or stain the surfaces on which it is applied. To clean mold and mildew off a window using vinegar, use the following steps:

Step 1. Take a spray bottle and fill it with undiluted white (distilled), vinegar. Liberally spray the affected area and allow it to sit for at least one hour to allow the vinegar time to kill the mold.

Step 2. Using a damp sponge or cloth to wipe away the dead mold and vinegar residue. If you are unsure if all the mold is dead, repeat the process.

Step 3. Using a mix of warm water and detergent, wipe clean the surface where the mold was to remove any last mold residue and remove the scent of vinegar.


Bleach is a powerful cleaning agent that has the ability to kill mold, however, it only performs well on nonporous surfaces, so PVC windows can be cleaned with bleach, whereas wooden window frames require the use of other methods. To use bleach to clean mold on nonporous window frames, use the following steps:

Step 1. After putting on rubber gloves and making sure you are working in a well-ventilated area, mix half a cup of bleach to half a gallon of water.

Perform a patch test on a small area of the window frame by applying some of the bleach solutions to a hidden part of the frame and leaving it for twenty minutes.

If the patch test is successful and there is no staining or discoloration, you can complete the next steps.

Step 2. Liberally spray the affected area with the bleach solution and leave for twenty minutes.

Step 3. Using a soft-bristled brush, gently work the bleach solution into the mold using circular motions. If the solution has dried up, apply some more to moisten the area you are working.

Step 4. Take a dry, clean cloth and wipe away any mold residue as well as any leftover bleach solution.

Baking soda paste

By mixing baking powder with vinegar to create a paste, the acidity of the vinegar assists in killing mold, as well as causing a reaction that creates oxygen at a rapid rate, helping to lift the stains caused by mold. To clean mold and mildew from a window using a baking soda paste, use the following steps.

Step 1. Mix one tablespoon of baking soda with a few drops of vinegar until a thick paste is formed.

baking soda paste to kill mold

Step 2. Spread the paste onto the mold-affected area and leave it to dry fully.

baking soda paste on window to kill mold

Step 3. Once fully dried, wipe away the baking powder and dead mold residue using a clean cloth.

How to prevent mold from growing on windows

Cleaning mold from a window isn’t a complex process, but preventing it from occurring in the first place is certainly the best option. It’s also advised to use these methods to stop the problem from reoccurring if you have had to deal with the problem before. To prevent mold from growing on windows, use the following methods:

Reduce humidity levels

High humidity levels lead to condensation, which in turn provides mold with all the moisture it needs to use for hydration, so by taking this necessary component away, mold will not have the opportunity to grow.

High humidity is a common problem in many homes, but there are some simple ways that it can be reduced.

Installing bathroom and kitchen extract fans for example will help to remove moisture from the air as soon as it is created, as well as keeping windows open to allow moist air to escape when cooking or bathing are great examples of how you can quickly reduce the humidity in a property.

There are also simple changes you can make, such as not drying clothes within the home if at all possible, and making sure your property is properly insulated can help reduce internal humidity levels.

Keeping hygrometers in several rooms of your property can help you keep an eye on the ambient humidity of your home. If it begins to creep up towards or past 55%, you’ll know you need to start using preventative measures.

Keep the windows clean

If mold growth around windows is a particular concern, then keeping them as clean as possible will help to reduce the likelihood of it being able to form.

Without the layers of dust and organic material that can build up on a window, mold will have no source of nutrients, and so it simply will not be able to live in such an environment.

Cleaning the windows once or twice a month with anti-bacterial sprays or vinegar will be sufficient to prevent mold from growing.

Increase airflow

Airflow (or lack of it), is a big issue in new-build properties, as they are heavily insulated. Much as insulting a property and making it energy efficient is clearly a good thing, making a building almost airtight causes a great deal of humidity to build up with nowhere to go.

It is for this reason that if living in a newer build property, it is important that you install extract fans where possible, open windows when you can, add trickle vents to windows, and use simple tricks like keeping internal doors open to allow humidity to move from one room to another.

The exception to this rule would be to keep a bathroom door closed after showing to allow the moisture to be extracted through the extract fan first. Once the fan has done its work, the door can be opened once again.

Purchasing windows that feature trickle vents is also an easy way to increase airflow through a property with little risk of temperature drops or increased heating bills. A trickle vent allows the homeowner to adjust the amount of fresh air that can pass through the vent. Even a small amount will increase the internal pressure within a property which can help to force out stale and humid air through any small gaps in the building, or through vents in other parts of the property.

Consider a dehumidifier

Dehumidifiers are a good option for situations where doors and windows cannot be opened due to very cold weather, or if drying clothes outside is also not possible.

Cooking and cleaning as well as drying clothes indoors will create a great deal of airborne moisture, so if they cannot be avoided, using a dehumidifier to capture as much of this moisture as possible is a good alternative.

Double-glaze your windows

Double glazing your windows will not only help you save on heating bills but could also save you money and time on fixing your mold issues.

High-humidity air will naturally begin to condensate and pool on the coolest parts of a property (known as thermal bridging), these tend to be poorly insulated brick walls or window panes. Double-glazed windows have a thin layer of argon gas between the frames, this gas is a very poor conductor, and so prevents heat from escaping.

When the internal window panes remain at the same temperature as the rest of the property, moisture will not begin to condense there, making the environment less suitable for mold growth.

What about black mold on window sills?

Black mold can certainly occur on or around windows and window sills, and it is for this reason that if you are planning on cleaning and removing the mold yourself you wear the correct protective equipment.

Breathing masks, goggles, and rubber gloves should always be worn before attempting to handle any kind of mold.

The methods described above would be just as effective for killing and removing true black mold (Stachybotrys chartarum) as well as other strains. However, there are also plenty of black mold-specific products available to treat the situation.

When to call an expert

If mold issues are contained to one or two windows or parts of the property, you may be able to tackle the problem yourself, but if there are large amounts of mold growth (patches covering several square meters), or you find that the mold returns just as quickly as it has been cleaned, you may be dealing with a larger issue that needs professional attention.

Undiscovered leaks, poor insulation, rising damp, etc can all cause mold growth issues within a property, and mold remediation experts would be able to find the root cause of the mold issues within your property.


Mold growth around windows and on window frames is very common, especially in new build homes. Keeping the humidity levels below 55% and maintaining the cleanliness of your property will help to reduce the chance of mold growth. If it does still occur, white distilled vinegar and baking soda are good natural options for removal, alongside black mold-specific products.

Chris Walker

Chris Walker has struggled for several years with mold after buying his own property. After finding the solutions to several issues around his home, he decided to create this site in order to answer as many questions about mold and mildew as possible to help others dealing with the same problems.

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