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How to get rid of and prevent mould around windows

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Black mould around windows can compromise the appearance and longevity of the panes and frames, leading to potential structural issues over time.

The fungus Stachybotrys chartarum, typically simply known as black mould, also poses a severe risk to health if left unchecked. While small mould patches are relatively easy to stub out, they can quickly spread. 

Maintaining the health of your windows means understanding what causes this fungus and taking preventive measures, such as cleaning to prevent build-ups. 

Read on to discover why black mould appears and how to stop it in its tracks. 

What is black mould and is it harmful?

Black mould, known scientifically as Stachybotrys chartarum, is a species of fungi that is particularly concerning due to its potential health impacts and the damage it can cause to buildings and structures.

Alternaria and Cladosporium mould are also relatively common in houses, though not as dangerous or feared as black mould, which is your quintessential interior mould. 

Appearance and characteristics

Black mould often manifests as a dark green or black, slimy substance that forms in patches. It’s often dotted and patchy, radiating from windows or damp corners of rooms. 

While black mould often forms on walls or ceilings, it can also form on fabric and upholstery.

fungus on the windows
Black mould appears on windows and in the surrounding areas (Adobe)

Destructive properties

Black mould doesn’t just sit on the surface – it infiltrates porous materials and can weaken the structural integrity of wood. Wooden window frames are particularly vulnerable to black mould. 

Mould around windows can also attack sealants, which are used to prevent air and water from entering through the gaps in windows, and can also degrade when exposed to this mould. This can lead to drafts, leaks, and further moisture problems, perpetuating a cycle of mould growth.

Health impacts

Health-wise, the impact of Stachybotrys chartarum ranges from minor throat and nasal irritation to headaches, fatigue, memory problems and severe respiratory issues. 

Psychological effects such as depression and anxiety are also frequently reported. 

With chronic exposure, symptoms often include coughing, wheezing and skin irritation. Those with allergies or existing health problems, such as asthma, are particularly vulnerable to the effects of black mould. 

Respiratory failure is an acute health impact of black mould. In 2022, a coroner stated that mould exposure caused the death of two-year-old Awaab Ishak in 2020, which led to “Awaab’s Law,” which enforces additional responsibilities for landlords to tackle mould issues. 

While rare, Awaab’s death is one of several involving mould-induced respiratory failure in the UK. 

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Why does black mould around windows occur?

Like many other fungi, black mould thrives in damp environments. 

When it starts forming, it can be quite thin and almost transparent, which makes it hard to notice. Then, it starts to become thicker and darker as it spreads. 

The role of moisture

The primary reason for mould formation around windows or damp corners of rooms is excessive moisture. This is why it is most often seen in bathrooms and kitchens.

This moisture can arise from both internal and external sources. Internally, everyday activities like cooking, bathing, and even breathing can increase indoor humidity levels. 

Externally, heavy rain and high-humidity climates can introduce the moisture that mould thrives in. 

Windows, being the barrier between the internal and external environment, often become the collecting point for this moisture.

Ventilation issues

Poor ventilation exacerbates moisture problems, as humid air doesn’t have a chance to circulate with dryer air outside. 

In homes with inadequate ventilation systems, humid air gets trapped inside, leading to condensation, especially on cooler surfaces like windows. 

Condensation is particularly evident during colder months when the temperature difference between the inside and outside of the home widens. 

Window sills as mould hotspots

Black mould around windows often forms on window sills that act as shelves where moisture can collect, especially if they’re not regularly cleaned or if the window is often left open during wet weather. 

Given their horizontal surfaces, water from rain or condensation can sometimes pool on the windowsill, providing the perfect breeding ground for mould. Exterior mould by windows can rapidly spread indoors, particularly when the windows are left open during moist, humid conditions. 

Furthermore, materials used in some window sills can provide organic matter for the mould to feed on, making them even more susceptible to mould growth.

Age and state of windows

Older windows or those not properly installed may not be as effective in insulating against external moisture or in preventing condensation. 

Well-fitted A+ double– or triple-glazed windows are considerably less vulnerable to mould. Moreover, some forms of uPVC and other synthetic materials repel mould. 

Damaged sealants, cracks, or gaps can also allow more moisture to seep in, making the windows more prone to mould formation. Black mould on uPVC window sealant should be quickly removed before penetrating neighbouring structures. 

How to get rid of mould on windows

Fungus and mold on the wall and window inside the apartment. The problem of ventilation, dampness, cold in the apartment
Fungus and mold on the wall and window (Adobe)

It’s certainly possible to rid windows of mould, especially if you catch it early. 

Regularly cleaning and maintaining windows – particularly wooden windows – will keep mould at bay. 

While minor mould issues might be manageable with everyday household cleaners, many purpose-made mould cleaners are available, some of which are formulated for black mould. 

Here’s a step-by-step guide to removing black mould from windows:

Step 1: Choose an effective cleaning solution

First, don rubber gloves and a face mask to protect from airborne spores released during cleaning. 

Washing up liquid and warm water is relatively effective at removing black mould, but more robust options are available. The problem is, if almost any spores remain, the mould will likely return.

  • Hydrogen peroxide: Combine one part of 3 per cent hydrogen peroxide with two parts water in a spray bottle. This mixture effectively breaks down the mould and can be used on various surfaces without leaving harmful residues.
  • White vinegar: Vinegar is another effective, non-toxic option that you can use undiluted or diluted, depending on the severity of the mould.
  • Mould and mildew remover: These specialised products are formulated to tackle mould and mildew. Always follow the product’s guidelines. 

If you’re wondering how to get rid of mould on window sealant specifically, then be sure to use a cleaning fluid that won’t attack or warp the mould, such as diluted vinegar or washing-up liquid. 

Step 2: Apply cleaner and let sit

Spray your chosen cleaning solution generously over the mould-affected area. 

Allow it to sit for at least 10 minutes to penetrate and break down the mould at the cellular level, or follow the product guidelines. 

Step 3: Wipe with care

high humidity in the house. hand wipes off water condensation from plastic window glass in the room. home moisture

After the cleaner has soaked into the mould, use a clean, soft cloth or sponge to gently wipe away the mould. 

Take care, as aggressive scrubbing will send spores into the air, which could contaminate other areas of your home. 

Step 4: Rinse and dry the area

Once you’ve wiped away the mould and cleaning solution, rinse the surface with a clean cloth dampened with water. 

Removing residual cleaner is essential as it might harm the surface over time. 

After rinsing, take a separate dry cloth and thoroughly dry the area to prevent future mould growth. Using a fan or a dehumidifier can speed up the drying process.

Step 5: Safe disposal of cleaning materials

Proper disposal is crucial when dealing with mould – the cloth or sponges used for cleaning will likely be covered with mould and spores. 

Seal these materials in a plastic bag before discarding them to prevent any spread of mould spores.

Step 6: Preventive measures

After you’ve cleaned the area, use a vacuum cleaner with a HEPA filter to suck up any remaining airborne mould spores. 

You can also consider applying a mould-resistant primer to the window frame for extra protection. This acts as a sealant that makes the surface less hospitable to mould, thus reducing the chance of it returning.

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Getting rid of mould on window sills

Window sills are particularly vulnerable to mould due to their exposure to external weather conditions and their propensity to retain moisture. 

Window sills with cracked or chipped paint allow water to seep in, vastly increasing the risk of mould formation. 

Here’s how to clean them effectively:

Regular cleaning

Regular cleaning is the first line of defence. Use appropriate cleaners that are specifically designed to combat mould and mildew. 

Specially formulated anti-mould cleaners often contain antifungal agents that clean and prevent mould growth. Always remember to check the product guidelines to ensure it won’t damage your windowsill material. 

Addressing sealant issues

Over time, sealants can degrade, creating crevices and cracks where moisture can gather and mould can flourish. 

If the sealant on your window sill shows significant mould growth, replacing it could be wise. 

Carefully use a scraper or utility knife to remove the old, mould-infested sealant. 

Once removed, clean the area thoroughly and allow it to dry before applying a fresh layer of mould-resistant sealant. Ensure a smooth application without gaps to prevent future moisture accumulation.

uPVC sealant care

For window sills sealed with uPVC sealant, regular cleaning is a must. However, it’s crucial to avoid using harsh or abrasive chemicals that can damage the UPVC material and worsen matters. 

Instead, opt for gentler cleaning agents such as regular washing up liquid and vinegar.

In addition, consider using a protective spray or polish specifically designed for UPVC materials. These can provide an added layer of protection against moisture and mould growth.

Window-specific care

Wooden windows

Due to its porous and organic nature, wood can be a magnet for various mould species if not adequately protected. 

Old or poorly maintained wooden windows are vulnerable to many types of mould and rot. However, if you regularly paint and varnish your wooden windows, they can remain strong and rot-free for decades. 

  • Sealing and painting: Regular inspections are necessary for wooden window frames to check for any chipping or cracks in the paint. Repainting and sealing the windows can act as a barrier against moisture and mould. When choosing paints and finishes, opt for those with antifungal properties for maximum protection.
  • Ventilation: Older wooden windows might have gaps or may not shut tightly, making them vulnerable to moisture seepage. Ensure that they are well-fitted and that any issues are rectified immediately.

Casement and sash windows

Given their design, casement windows have various nooks and crannies that allow mould to seep in. 

This is also true for sash windows, which have sliding mechanisms that could harbour excessive moisture. 

  • Routine inspection: Casement windows’ complex structure requires regular inspection for moisture or mould. Pay special attention to the hinges, seals and corners.
  • Regular cleaning: Ensure routine hinges, sills, and frame cleaning. Lubricate the mechanical parts periodically to prevent moisture accumulation.
  • Seal check: The seals around casement windows are crucial for keeping out moisture. Periodically check them for wear and tear and replace them if needed.

Other types of window mould

Mould is a broad term encompassing a wide variety of fungi that can grow inside and outside homes. 

While black mould (Stachybotrys chartarum) is one of the most well-known due to its potential health implications, many other mould species can also grow on windows and other surfaces. 

Common types of mould found on windows

  • Aspergillus: This mould is ubiquitous and is present in various environments. When formed on windows, it manifests as white or yellowish patches. Some species can produce allergens, leading to respiratory issues.
  • Cladosporium: Often found on wooden window frames, Cladosporium is greenish-black and can grow in cooler areas. It’s commonly responsible for respiratory problems and skin reactions.
  • Penicillium: Recognisable by its blue or green colour, this mould can be found on window surfaces that water has damaged. It spreads rapidly and can cause respiratory issues, especially in those with allergies or compromised immune systems.
  • Alternaria: With a dark and woolly appearance, Alternaria can swiftly colonise window sills and frames, especially after water damage. It’s one of the most common moulds and can cause allergic reactions.
  • Aureobasidium: Often found behind wooden window frames or painted surfaces, it appears pink and black. It can cause respiratory and skin issues.


Windows act as a barrier between the interior and exterior of a home and, thus, are consistently exposed to changes in air and moisture. 

As a result, they’re a prime candidate for mould growth, which often originates from windows or window sills before spreading to other parts of the room.

Black mould is the most notorious form of mould, though it certainly isn’t the only type worth being aware of. All forms of mould inside your home should be viewed as a health risk and removed promptly. 

Removing small quantities of mould is relatively straightforward – but you have to catch it early. 

Periodically check your windows and clean them regularly to keep mould at bay. If you own wooden windows, maintain them correctly to prevent mould and rot. 

Always take black mould inside your home seriously, as it can lead to devastating health consequences if left unchecked. 

Frequently asked questions about mould on windows

Absolutely. Black mould, or Stachybotrys chartarum, produces toxic compounds, called mycotoxins, that can lead to a range of health problems. 

While not everyone will exhibit symptoms upon exposure, many individuals may experience allergic reactions, respiratory issues, or even more severe health complications. Those with compromised immune systems, the elderly, and children are especially vulnerable.

By finding out how to prevent and manage mould around windows, you should be able to stop small mould problems from risking health problems. However, some properties are notoriously hard to clear black mould without professional assistance. If you’re a renter, contact your landlord to discuss any mould problems. 

Yes, using a dehumidifier can significantly deter mould growth on windows. Mould thrives in moist and humid environments, and a dehumidifier reduces the overall moisture level in your home. 

Some studies suggest that maintaining indoor humidity below 60 per cent can substantially reduce the risk of mould formation.

Conversely, if the interior humidity of a home is consistently above 80 per cent, then mould is considerably more likely to form and spread. 

In most cases, the answer is (luckily) no. Black mould on windows typically affects the surface areas and can be cleaned. By learning how to stop mould on windows and acting fast, you should be able to stop any small mould patches from spreading. 

However, if the mould has penetrated deeply into wooden frames or the integrity of the window has been compromised, consider a replacement. Always evaluate the extent of the damage before making a decision.

Mould does not discriminate among window types, but certain windows are more susceptible due to their design or material. 

Given their organic nature, wooden windows can be more vulnerable if not adequately treated and sealed. 

Similarly, older windows, especially single-pane designs, can foster condensation, which is a precursor to mould growth. Windows in bathrooms or kitchens, where humidity tends to be higher, can also be hotspots.

Conversely, modern A+ and A++-rated windows are much less likely to conduct moisture and should reduce interior humidity levels.

Sam Jeans


Sam is an experienced writer whose expertise lies in home improvements and renewables, as well as technology, where he is especially interested in the world of machine learning and AI. He has written for Vested, Age Times, and the Royal Mint.

For the Independent Advisor, Sam writes about windows and solar panels.