If you want your bathroom to last a long time, you must have good mechanical ventilation (electric extractor fan)! And you must use it all the time when you use a shower with the windows closed!
There’s a lot of misconception when it comes to extractor fans, it is infuriating how many contractors recommend not having an extractor fan; this is either because they don’t understand the principles behind the requirements for an extractor fan, either they simply try to avoid fitting one due to the complexity of the job (it requires multiple trades and expensive tools).
We try in this article to explain why you need an extractor fan and how much mechanical ventilation you’d need for your bathroom. There are three reasons why extractor fans are necessary for modern bathrooms: newer bathrooms produce more steam, use more wood-based furniture and the walls/ceiling are less breathable.
As a summary, we recommend either a very powerful timer extractor fan directly above the shower ducted outside if the situation allows it (with ducting less than 3 metres), or a 150mm core drilled extractor fan with humidistat & timer. If these options don’t work, then you should have at least a 100mm humidistat & timer extractor fan! You must use them all the time you take a shower with the windows closed!
A little bit of physics
You might be used to this term from the weather forecast, the UK generally has high levels of humidity levels. This tends to be measured in percentages, most of the times you’ll see values between 70-80%, but sometimes it can go up to 100%. What does this mean? The air can contain water as vapours, but there’s a limited amount of water a unit of air can contain until it reaches saturation.
So what happens when the humidity level reaches 100%? The air can no longer contain the water and it has to condensate somewhere. The air tends to condensate on colder surfaces first, so you’ll see things like cars full of water in the morning.
This is what happens in your bathroom when you take a shower. For a while the air can contain the water vapours and from a certain point it starts to condensate. Pretty straightforward until now, just common knowledge.
The temperature matters!
This is a slightly less known fact, but the amount of water in the air is temperature-dependent. The warmer the air, the more water vapours the air can contain; this is because the air has more energy to keep the water vapours in vapour-state. The opposite it’s true, the colder the temperature, the faster water will start to condensate.
If you’re interested in more details in the relationship between air humidity and temperature, read this Mo’s Corner article. Basically, at 30°C, the air can contain roughly double the amount of water compared to 20°C. This explains why during sunny days, everything feels dry while during cold days you feel how humid everything is.
This relationship between temperature and air explains why water condensates on a cold surface (like basin or toilet). The same principle works for dehumidifier; a dehumidifier is basically an air conditioner system where water condensates on the cooler parts inside.
Temperature also affects the saturation pressure of water
This is just a fancy explanation for the fact that the warmer the water, the more steam will produce at the same volume. It’s pretty self-explanatory, but if you need more details you can check this wikipedia article.
Now we have the basic principles necessary to explain why you need an extractor fan.
The main obstacle we get when we recommend an extractor fan is the fact that the customer didn’t have one before and it was fine. But while it was fine in the past, it doesn’t mean it will be the same for a modern bathroom.
Newer bathrooms produce more steam
The main source of steam is the shower. Modern showers produce much more steam compared to a few years ago. Both electric and gas boiler showers are much more powerful compared to the past. A gas combi boiler can produce 30 KW+ in a domestic setting, meaning that the water can be hotter & at a much better flow than before; this will produce much more steam compared to an old boiler.
Getting back to our previous explanation, if you take a warmer shower, you’ll have more steam. Newer showers also have much better flow, meaning that cumulatively a modern shower produces 2-3 times more steam than old showers!
Modern bathrooms tend to have a lot of storage (out of MDF, which is a wooden based product). You have a basin for the sink, you might have a cabinet for the toilet cistern or other additional storage. This is the weakest point in your bathroom, having too much moisture will eventually damage the items.
There are no completely waterproof options for cabinets, except for some plastic cabinets that are fitted inside showers. They handle some humidity reasonably well, but too much will surely damage them in time.
Walls/ceiling are less breathable
In the past, the walls/ceiling allowed a lot of water vapours to pass through. Current standards don’t allow for this as much, modern buildings have a vapour layer control membrane behind the plasterboard and use moisture-resistant plasterboard in kitchens and bathrooms, which have water repellent liners. The new standards are needed since vapours that escape through walls/ceiling affect the structure of the property.
The best way to see this if you have UPVC panels on the ceiling/walls. This combination doesn’t allow for much vapour to escape through walls/ceiling, this is the moment when you’ll truly experience condensation if you don’t have a good extractor fan.
Opening the windows instead of extractor fan
This is a commonly used ventilation method and it works sometimes. We take two cases and discuss them, one where opening the windows is a good idea and one where it isn’t.
When opening the window is a good idea
Opening the window helps, but not always. An example when this would work is when outside is warmer than inside and the relative humidity is less than 100%, then you can simply open the window and get rid of the moisture. This is an extreme example, it can be a combination of temperature and relative humidity when you can open the window and this would help.
But even in this case, sometimes the ventilation is not fast enough. As we mentioned earlier, modern showers can produce much more steam, meaning that by the time the outside air enters the bathroom, some condensation has already formed.
When opening the window is a bad idea
An example where opening the window doesn’t work is when outside is colder and the relative humidity is 100%. This is the other spectrum of an extreme example, it can be a combination of temperature & humidity when opening the window is a bad idea as well. In this particular case, it’s actually worse to open the window, no matter how counterintuitive it might seem.
What extractor fan do you need?
Now that we’ve explained why you need an extractor fan, in our previous section, we mentioned that opening the window works only if it can replace the air fast enough with new fresh air at less than 100% humidity. So the rate of extraction rate has to be equal or better than how much vapours (steam) the shower produces, so it’s not related to the size of the bathroom as much as it is related to the amount of steam the shower produces.
Extractor fan above shower
This is our preferred option if the conditions allow it to be fitted. When the extractor fan is just above the shower it’s better since it allows for less vapour to travel in the rest of the room.
The problem with this type of extractor fans? You need to duct outside and the longer the ducting, the more it loses effectiveness. Also, you’ll need to have access above the shower to duct it outside. If it’s vertical ducting, then a condenser trap connecting to the waste system it’s also necessary!
Ideally, we would use this if we can get outside in 2-3 meters at most. Less then this and even the most powerful extractor fan would struggle. You might have seen the hotel ventilation systems or the HVAC systems that almost any US house has installed with a similar concept running tens of metres. The difference is that they use reducing diameters to keep the flow of air constant (from large ducting slowly getting to the very small passage of air) + they have huge HVAC ventilators that cost thousands.
150mm humidistat/timer extractor fan
This is the second preferred option if there’s enough space for it. Although it’s only 50% larger than the 100mm counterpart, it extracts around 3 times as much air.
It’s important where you fit the extractor fan. Ideally you’d have it above the shower, as in our previous section, but if this is not possible then it should be as much as possible in the path between the door and shower.
You don’t want the steam to reach too many places in your bathroom, but rather dispose of it as quickly as possible. Another important aspect is that the air travels from underneath the door, the building standards actually insist on having almost 20mm free space underneath the door.
A 150mm is a little bit harder to fit than 100mm since it requires a special core drill machine and takes much longer, but the 3 times more extraction rate will make the difference between a dry bathroom or a wet one.
Other things to consider
- If you can’t have the above, use a 100mm humidistat extractor fan. This passes the minimum standards requirements, but you should aim for more if you can.
- If it’s a core drilled extractor fan, you should have a humidistat/timer one. The extractor fan will work for a certain amount of time (when you use the toilet to eliminate bad smells) or until the humidity level reaches a certain level, whichever comes later. It is wired to the lights, so this is what sets the timer on.
- You can further lower the humidity in your bathroom by heating your property more. Remember that the extractor air extracts air from underneath your door, so that specific room’s temperature/relative humidity would also have an effect on relative humidity.