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Mold on window sill: what causes it and how to fix it

Condensation, dirt, and humidity can all lead to pesky mold on your window sills. Thankfully, there are many simple ways to resolve this problem.
Published on January 17, 2024
Written by
Marie Cookson
Based in Manchester, Marie is one of the writers at HouseFresh and our resident dust expert. She works together with our in-house researchers and our Managing Editor to produce in-depth articles offering practical advice on how to tackle indoor air quality issues.

Mold on your window sills not only looks bad, but the spores can affect your health, too. Luckily, there are plenty of safe, natural, and cost-effective ways to remove it, with white vinegar and baking soda among the most effective.

While bleach is another effective option, be sure to take extra care. Above all, never mix it with other cleaning solutions that contain ammonia, as the combination produces toxic gas.

To keep on top of the situation, you can take daily preventative measures, such as using a dehumidifier to ensure that moisture levels remain low.

While mold can grow virtually anywhere in your home, window sills are some of the most likely sources. Trapped warm air, condensation, and dirt combined with water spills and leaks make for an ideal breeding ground for mold to thrive.

If you aren’t sure you have a mold problem, you can carry out various tests. Most of the time, however, you’ll be able to see it on your window sill — and smell the pungent odor. You may also experience symptoms such as a runny nose or breathing difficulties, as inhaling mold spores can impact your health and exacerbate allergies or respiratory conditions.

Acting quickly to clean the mold, together with preventing further outbreaks, is key to controlling the situation. Let’s have a look at how you can ensure your window sills remain mold-free.

Five natural ways to clean mold on a window sill

There are plenty of simple, eco-friendly ways to clean the mold from your window sill. Since inhaling or touching mold spores can be a health hazard, be sure to wear protective clothing, such as waterproof gloves, a mask, and safety glasses.

1. Spray with white vinegar

Powerful and natural, distilled white vinegar is one of the best remedies for getting rid of unsightly mold. Thanks to its acetic acid content, vinegar is highly effective at destroying mold spores and eliminating those unpleasant, musty odors as well.

  1. Take a clean, empty spray bottle and pour in 1 cup of distilled white vinegar.
  2. Spray the solution on the mold, thoroughly saturating the area.
  3. Let sit for at least 1 hour.
  4. Use either a disposable towel to wipe away the mold or scrub with a nylon brush or toothbrush.
  5. Allow the window sill to dry out, then wipe it clean.

2. Use tea tree oil

You might be aware that tea tree oil is an effective antibacterial agent for your skin, helping to keep pimples at bay. But it’s also a potent tool to fight against mold spores. As a natural fungicide, tea tree oil can destroy mold while also preventing the spores from coming back.

  1. Fill a spray bottle with a ratio of 1 teaspoon of tea tree oil to 1 cup of water.
  2. Shake the mixture well.
  3. Saturate the moldy area and leave for 1 hour.
  4. Wipe the oil away with a damp towel or microfiber cloth.


You could add rubbing alcohol to the tea tree oil as the alcohol will help carry the oil onto the moldy area.

3. Apply grapefruit seed extract

Grapefruit seed extract contains mold-fighting compounds called polyphenols. This means that it’s highly effective as both an antibacterial agent and disinfectant. Like with tea tree oil, using grapefruit seed extract also helps to prevent mold from returning in the future.

  1. Fill a spray bottle with 10 drops of the extract for every 1 cup of water.
  2. Spray the affected area and let it sit between 10 minutes to 1 hour, depending on the severity of the mold.
  3. Wipe away with a damp towel or cloth.

4. Utilize baking soda

If there’s one thing you should always have in your cupboard, it’s baking soda. It’s great for cooking, cleaning, and removing odors and is super useful to have for all cleaning purposes. Its mildly alkaline nature means that it acts as a natural disinfectant and prevents mold from flourishing.

  1. Mix 2 tablespoons of baking soda with 2 cups of water and shake in a spray bottle until it’s completely dissolved.
  2. Spray onto the moldy area and let it sit before scrubbing with a small brush.
  3. Rinse the area and allow it to dry.


You can apply the solution one more time and leave it to dry, as this will act as a protective layer, preventing further mold growth.

5. Try lemon juice

If the mold on your window sill is fairly mild, lemons can help as their citric acid content is both antibacterial and antiseptic. Plus, there’ll be a lovely, fresh aroma infusing your space!

  1. Juice 3-5 lemons and pour over the moldy area.
  2. Let it sit for 5 minutes.
  3. Wipe away with a damp cloth or use a small brush for problematic areas.


For optimal mold removal and cleaning, you can add some dish or hand soap plus water to the lemon juice.

Safety tips when using bleach to clean up mold

Bleach is another effective option for cleaning mold, but it’s harsh, dangerous, and toxic. Be sure to follow the following do’s and don’ts to protect yourself.

1. Always follow the manufacturer’s instructions 

According to the CDC, it’s important to follow all guidelines and instructions when using bleach or any other biocide (a substance that kills living organisms).

2. Ventilate the space 

Breathing in mold spores can cause various health problems, so ensure there’s adequate air circulation. The CDC recommends ventilating the area by opening doors and windows to let clean, fresh air inside and reduce the risk of inhaling spores. You could also install a fan as an additional protective measure.

3. Don’t mix chlorine bleach with ammonia 

According to the EPA, chlorine bleach should NEVER be mixed with another cleaning agent or detergent that contains ammonia. This is because a toxic, poisonous gas will be released in the process, causing serious, possibly even fatal, harm if inhaled.  

4. Wear protective clothing 

As mentioned earlier, protective gear is vital, and it’s especially important when you’re cleaning with bleach. Gloves, a face mask, and eye protection are essential items, but you could also wear old clothes that you don’t mind tossing away afterward.  

5. If possible, run an air purifier with plenty of activated carbon while you clean

When you’re getting rid of one toxic substance like mold with another, such as bleach, it’s vital to mitigate any adverse effects. Air purifiers help as they remove harmful pollutants while pushing clean, sanitized air back into your space. Be sure to choose a device with HEPA and activated carbon filters, as these are the most effective at cleaning the air.

6. Don’t allow children or pets into the space 

Young children and animals can be more vulnerable to toxic fumes. As the Wisconsin Department of Health Services states, be sure to keep them away until you’ve finished.

7. Avoid eating or drinking during the cleaning process

You should also never eat (including chewing gum), drink, or smoke during the cleaning process as you may be putting your health at risk.  

According to the Wisconsin Department of Health Services, you should always avoid direct contact with bleach. Eating, drinking, chewing gum, and smoking may mean that you accidentally consume some of the bleach while doing so.

8. Don’t forget to dispose of cleaning items after use 

Don’t wash and reuse the cloths, sponges, and scrubbing brushes, as they’ll contain both mold and bleach residue. Be sure to throw them in the trash straight away.

How to prevent mold from growing on a window sill

One surefire way to combat mold on your window sill is to prevent it from growing in the first place. Follow these daily steps:

1. Keep the window sill clean and dry

Window sills can easily accumulate dust, dirt, and hair — all of which provide substance and an ideal environment for mold to grow and thrive. As the EPA advises, regular cleaning will help to prevent mold from recurring, so aim to give your window sill a quick wipe down on a daily basis.

2. Try to reduce condensation

Condensation is a common problem, particularly in the winter when it’s freezing outside but warm and humid inside. Wiping down your windows with a dry microfiber cloth every morning (or after taking a shower) can help eliminate moisture and inhibit mold growth.

3. Use a dehumidifier

The CDC advises keeping humidity levels in your home low and no higher than 50%. Investing in a dehumidifier will definitely help ensure moisture levels remain minimal, thereby preventing mold from flourishing.


Humidity changes throughout the day, so it’s worth buying a hygrometer from your local home improvement store so you can keep checking humidity levels.

4. Open windows

Fresh air can aid ventilation and reduce the chance of mold thriving, but it’s important to consider the weather first. Sometimes, keeping the windows closed is a better option. This is because rain or snow can bring damp, moist air inside and potentially encourage mold to grow. So it’s best to only open windows (between 15-30 minutes) when the weather’s dry.

5. Fix leaks or spills quickly

Whether you accidentally spill a glass of water on your window sill or discover a leak, it’s crucial to act quickly so that mold doesn’t have a chance to grow. According to the EPA, if you dry areas that are wet or damp within 1-2 days, the risk of mold is significantly reduced.

6. Switch on exhaust fans

Your bathroom window sills may be prone to mold due to the excess humidity. If you have a fan, be sure to turn it on while showering.


The same applies to your kitchen. Cooking and running the dishwasher produces moisture, so remember to switch on the exhaust fan during these activities.

7. Add mold inhibitors to paint

As recommended by the CDC, consider adding mold inhibitors to paints, which you can buy at most home improvement stores.


Another option is to purchase mold-resistant paint. This type of solution contains a fungicide that helps to destroy mold spores while actively discouraging mold growth.

Commonly asked questions about mold on windows

It’s important to take extra care when cleaning mold from wooden window frames, and definitely avoid bleach as it’s corrosive. Also, keep in mind whether the frames are painted, stained, or varnished. In most cases, however, a mild detergent or washing-up liquid works well.

For additional information about how to remove mold from wood and other materials, check out our handy infographic.

Vinegar works well on these kinds of frames. As described earlier, simply fill up a spray bottle with white vinegar. Then, saturate the area, letting it sit for a good hour, before wiping clean with a damp towel or sponge.

Unfortunately, yes. According to the EPA, mold produces allergens and irritants that can cause allergic reactions in some people. These include sneezing, a runny nose, and red or itchy eyes. People with asthma are especially sensitive to the effects of mold, as exposure can lead to an attack.

Wrapping up

Window sills can often be plagued by mold, particularly in areas like bathrooms and kitchens where there is high humidity and poor ventilation. Tackling the problem sooner rather than later means that you can stop the situation from getting out of control and safeguard your health.  

Remember that before you reach for the bleach, try some store cupboard staples like white vinegar, baking soda, and even lemons. But if you do feel bleach is necessary, always take the proper safety precautions and never mix it with ammonia.

When it comes to futureproofing your window sills from mold, be sure to keep them clean and dry, wipe away condensation, and fix any leaks or spills quickly.

Reducing overall humidity is another surefire way to prevent mold, so consider investing in a good dehumidifier to keep moisture levels under control.


About the author

Marie Cookson

Based in Manchester, Marie is one of the writers at HouseFresh and our resident dust expert. She works together with our in-house researchers and our Managing Editor to produce in-depth articles offering practical advice on how to tackle indoor air quality issues.