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Black Mould Problem in Bathroom – Why?

Black Mould in my Bathroom

Black Mould – Causes

First of all, remember that in the UK it is ‘Black Mould’ not ‘Black Mold’ as they spell it in the USA, as we explained in our article – Mould or Mold previously. We also recommend you read that article after this too as it has very useful information on the health and safety considerations of exposure to mould too.

We always carry out a 5 step risk assessment with black mould problems. One of the most common places is to find black mould in bathrooms or adjoining rooms, but why is this and what can be done if you have a mould problem?

Firstly, lets remember that mould often goes hand in had with a water leak so remember to check you don’t have a water leak that is causing mould. We explain in our article on signs of a water leak in your property.

Black Mould Bathroom Wall
Black Mould Bathroom Wall

If you have a water leak or need help with trace and access services, contact us today.

Let’s remember that mould spores are commonly occurring in properties, they just need the right conditions to grow and expand in your home or business. Think of mould spores like seeds that need water and soil to grow, in comparison black mould spores need the following conditions to grow:

  1. The existence of elevated moisture / humidity
  2. The right temperature (room temperature is often good)
  3. The right (organic) material to grow upon
  4. Little or no air movement / ventilation
  5. Lack of direct sunlight
  6. Lack of disturbance / movement

Some of those things on that list will give you an indication as to why black mould forms in bathrooms in particular but let’s look into that in particular, especially focusing on two things on that list – relative humidity / moisture and temperature. On that topic, we have a great guide to desiccant dehumidifiers.

Black Mould in my Bathroom
Black Mould in my Bathroom

Black Mould – Bathroom Test

We ran a test in a property over a full 24 hours of normal use in a typical UK property. Over that period we monitored the temperature and relative humidity conditions in each room for comparison. Here are the results of that test, which helps to show why black mould is common in bathrooms.

The rooms that were monitored were – bedroom, bathroom, living room and kitchen.

This test took place in early December in a centrally heated property which was occupied over the full day. The chart below shows temperature across the four rooms:

Black Mould - Temperature
Black Mould Test – Temperature

As you can see, despite the weather outside the property being close to zero (the test took place soon after Storm Arwen in Winter 2021) the temperature never dipped below 17°C which was in the bedroom, where the radiator thermostats were set to a lower temperature. Interestingly, the kitchen had the highest average temperature of 20°C, next was the Living Room at 18.9°C, Bathroom at 18.6°C and finally the bedroom at 18.2°C. So not huge differences between each.

We have ran a separate test looking at how kitchen extractor fans help with controlling moisture and mould in kitchens, associated with cooking etc, we also have a similar and informative guide to bathroom extractor fans.

You can see on the chart how this varies during the day, cooling during the night then warming up as the central heating came on at 6:45am and then later in the afternoon for the evening.

Next we will look at the relative humidity across those four rooms:

Black Mould - Humidity
Black Mould Test – Humidity

So, as you can see, this is dramatically different to the temperature comparisons. You can also see how it started very high because someone had a bath before bedtime, this dramatically increased the relative humidity in the room, and that ‘lingered’ for many hours afterwards! Similarly, when someone had a shower in the same room later in the day, there was a big increase too.

Although the property had good ventilation, it did not have a PIV Unit.

The really interesting comparison is between the bathroom and kitchen. Although kitchens can have many sources of moisture, the peaks are not nearly as high as in a bathroom from a bath or shower. In fact, you can see on the green line where breakfast is being prepared, then the kettle being boiled during the day at different times and cooking in the evening. But those spikes are nowhere near those for the bathroom showing why black mould is more common in bathrooms and rooms nearby.

We have looked previously into:

In each of those articles we explained how condensation and damp causes can go hand in hand with a mould problem, including black mould. We have a separate article on the different types of damp.

Here is a quick table showing averages, minimum and maximum humidity by room:

Black Mould - Humidity Test
Black Mould – Humidity Test

Black Mould on Walls & Ceilings

So clearly the bathroom had the highest maximum, minimum, average and range of humidity! Not only that, but much higher than other rooms in the house. So, bearing in mind that moisture / humidity is a key component in allowing mould growth, the bathroom provides the best conditions for black mould growth in many homes. The moisture from bathrooms collects on walls and ceilings as the humid air comes into surfaces which are at a sufficiently low temperature to cause dew point to be reached.

We explain this further on or dew point calculator page, which is a very useful tool. We also explain this a bit more in our guide to a cold spot on walls.

We also have a separate article about how infrared thermometers help with ceiling water damage too.

When we get asked to do a mould or condensation survey at a property, we look especially at bathrooms and, in turn, the walls and ceilings in those rooms. Ensuite bathrooms can be a problem too, especially because they are more likely to not have windows which can bring sunlight and ventilation, which as we mentioned earlier can contribute to mould growth – we explain this on our page about dew point charts.

Black Mould Removal

Clearly, now that we understand why black mould occurs in bathrooms in particular (although not exclusively), the clear question is what is the best black mould removal process?

Remember as we said in our mould or mold article, do not use bleach on black mould. Bleach can actually give a superficial result making you think the problem has gone away when in actual fact the bleach could make the situation worse.

Black Mould Removal
Black Mould Removal – Process

First of all, bearing in mind our earlier comments (and as we always say), the first thing is to look at the possible contributing causes to mould and what you can do about it. So look to see if you have the things on that list and understand how this contributes to your black mould problem.

After that, if you have a bad mould problem or would like a mould survey to look into the above things in more detail, get in touch with a mould removal specialist such as ourselves. We can help identify the cause, make suggestions as to how to remedy it and offer mould removal too.

We use specialist antimicrobials which are specially designed to kill mould. These are highly effective and also have the benefit of having a beneficial residual effect to help prevent black mould.

Contact us today if you need help with a black mould problem in your home., is brought to you with Rainbow Restoration

On the subject of mould and humidity, we discussed it in an interesting recent article about silica gel and how it works, including an interesting experiment. Similarly, we have a really insightful guide explaining the UK’s Beast from the East phenomenon.

See also our informative guide to Shower Drain Leaking Tips

Is black mould harmful?

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Black mould can be harmful, especially to those who are vulnerable or have pre-existing issues with their respiratory system. The reason for this is that it can negatively effect air quality, in particular from mould spores being present in the air as the black mould grows and spreads. If it is not controlled, it can get progressively worse too. Mould remediation experts can help with black mould with both short and longer term solutions.

Can I remove black mould myself?

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If you have bad black mould in your home, especially if it is persistent or covers more than just a small area, you should consider the services of a mould remediation expert. Not only will they be able to remove the mould, but more importantly inspect your property to see what the cause of the mould is. This is very important, otherwise if conditions stay the same, then the mould may return again causing further challenges.