Bathroom Condensation: Clear Test Results

Bathroom Mould & Condensation

Bathroom condensation is one of the key factors in bathroom mould problems. As we said in our article regarding mould on walls, mould needs a moisture source to thrive, grow and spread. That can of course be a water leak, however, in your bathroom condensation can be a big factor. In bathrooms, condensation, damp and mould problems often go together.

We have written extensively on the subject of condensation on windows and, most commonly, people have that particular problem in their bathroom, especially as bathrooms clearly have a lot of water sources which, in turn, means that relative humidity in bathrooms is higher than in other parts of properties. To show that, we ran a test…

Bathroom Condensation – Test

To help to demonstrate how bathroom condensation can form after having a bath, we ran a test in a residential property. Here are the details of the test:

• Over a two hour period we monitored relative humidity and room temperature before, during and after a bath being run and emptied.
• We took relative humidity readings of the air every minute during that time
• We did this for both the bathroom and just outside the bathroom for a control comparison, and to see if having a bath impacted humidity in other rooms.
• Two separate hygrometers were used
• The bathroom door remained closed during the bath time
• The bathroom was a modern, well ventilated bathroom and the test was carried out in November when the temperature outside was around 10°C.
• The test was between 4:30pm and 6:30pm

This is the timeline of events during the test:

• 4:30pm test starts and room left for 30mins
• 5:00pm door closed to rest of house
• 5:15pm started to run the bath (took 15mins)
• 5:30pm bath full, and person had a bath
• 5:45pm person ended bath and emptied it
• 6:00pm person left bathroom opening door
• 6:30pm test period completed (2 hours)

Hygrometer readings were taken automatically every minute during the test. If you are interested, we also did a test to compare Bath vs Shower Condensation, which had interesting results!

We also have a separate test we did which showed that, with one quick trick, you can actually help to stop condensation when having a bath by amazingly reducing humidity or steam by 45%!

A useful accompaniment to this is our dew point chart explanation.

What is a Hygrometer?

A hygrometer is a calibrated instrument which takes readings of the amount of water vapour in the air, the temperature and combines them to give a relative humidity reading. Remember the definition of Relative Humidity is – “the amount of water moisture / vapour in the air, expressed as a percentage, relative to what that air is capable of holding at a given temperature“

So, because measuring relativity is dependent on temperature, all those different readings are needed to monitor the effect of having a bath on relative humidity, which in turn can have an impact on bathroom condensation. We used hygrometers in our silica gel packets test, which is fascinating too.

Here are the results of todays test…

Bathroom Humidity – Test Results

The two charts below show the hygrometer temperature and relative humidity readings for inside the bathroom during the test period (top – blue chart) and outside the bathroom (bottom – yellow chart).

As you can see, before the test started the temperature in the bathroom was slightly below the outside room (a landing in this case) but the relative humidity was actually slightly higher.

Once the bath started to run the relative humidity (and bathroom condensation) rose dramatically from about 70% to above 90%. Notice how it actually rose a bit more to 95% as the person got out of the bath. That 95% means that the air (at that temperature) is holding 95% of the moisture it is capable of holding. In other words, it is almost saturated.

It is interesting to note that running the hot bath raised the temperature in the room, which is to be expected really. But also, it is interesting to see that (even with a good bathroom extractor fan and the door open) the relative humidity in the room stayed elevated and took some time to fall. Even then, 45 mins after the bath it was much higher than the control just outside the room (over 80% vs just under 65% RH).

Another thing to note is that, interestingly, the control (just outside the bathroom) hygrometer’s relative humidity readings did not change hardly at all. This is clearly helped by the negative air pressure that the extractor fan in the bathroom has – this means that air will flow into the bathroom from outside (via gaps in the door etc) not the other way around. This makes a big difference to stopping steam and bathroom condensation spreading to other rooms! This is one of the reasons that bathroom extractor fans run for a few minutes even after the light is turned off, for residual benefit.

The final thing to note is that, somewhat surprisingly, when the door was opened the relative humidity in the room outside only raised slightly, but still nowhere near seen in the bathroom.

As you might expect during this test, bathroom condensation levels raised during the test but because we followed our own advice on how to reduce condensation, it returned back to normal soon after. This is important because, as you can see from the test results above (even with a modern well ventilated bathroom) you will naturally get some condensation with a bath, especially in winter. The key thing is to make sure that it does not linger around for long times as that can cause mould growth, especially if bathroom condensation moisture is absorbed into a material that mould can grow on.

We have a more detailed guide on what causes mould which and another looking at anti condensation paint, both are worth reading too.