Bathroom mould is a very common problem, and ventilation is not always the answer. In fact, in some cases, it can be counter-productive.
The most common examples of problematic bathrooms are shown in the diagrams above. In the first example, the extraction fan is on, but it has nowhere to draw the air from. This means that the fan just can’t do it’s job. In the second example, there is either no fan, or the occupants don’t use the fan consistently, so the bathroom relies on passive or natural permanent ventilation, either built in to the walls or via an open window. Permanent, passive ventilation to the outside is not effective, it introduces significant air leakage to the building envelope, which impacts both heating and cooling costs. Outside air can be more humid than the air inside the bathroom, this further compounds the problem.
When ventilating a room, it is important to consider not only where you are pushing the moisture rich air, but also where you are going to get the extra air from to push the moisture through the fan. A door vent, close to the floor, is extremely effective because it is providing a source of air for the fan that can travel the height of the room, not just across the ceiling, and pulling air from the house means you can leverage off leakage in the whole house building envelope as opposed to adding more leakage directly in the bathroom which contributes to additional air leakage in the building envelope as a whole (Impacting building energy efficiency).
Ever wondered how humidity attaches to surfaces???
As an example the humidity level is the same? Even though it looks like there is more moisture in the air? Warmer air can store more water than cold air. When you have a warm house with cold uninsulated surfaces, the moisture in the air gets kicked out and rests on those cold surfaces that cool the air.
In summary, the following 5 crucial factors need to be considered to stop bathroom mould from forming:
1. Provide a means for your bathroom fan to get air. This will increase air flow and make the fan work much more efficiently. Make sure that the source of air for your ventilation is closer to the floor so that air isn’t just moving along the top of the bathroom ceiling.
2. Remove human error and use a ceiling fan with a humidity sensor
3. Make sure both the walls and ceiling of the bathroom are well insulated. This reduces the occurrence of cold surfaces, where moisture in the air can condense back into water. This is depicted in the “Humidity Moisture” diagram shown below.
4. Position the fan and source air in the path of where the humidity is being produced. Putting a door grill in, can improve the amount of air a fan moves by 33%. Eg. Instead of a fan moving only 45l/s with the door closed, installing a door grill will improve the flow and achieve 67l/s.
5. Ensure your fan is ducted to outside of the roof, either through the eave soffit or better yet straight up through a tiled or steel cladded roof system. You don’t want humidity being dumped into your attic where it could damage insulation and cause damage to the building.
Once these factors are remedied, the following will occur:
- Your ceiling fan will run more effectively, i.e. it will move more air in a shorter amount of time and will come on and turn off automatically when required due to the humidity sensor.
- Your building envelope will lose less air because the fan will only run as long as necessary to get humidity down to an acceptable level. This will help with cooling and heating bills.
- Your bathroom mirror will be less likely to fog up because the air flow is crossing the entire height of your bathroom, and removing moisture much more quickly.
- Finally… Stopping bathroom mould!
By John Konstantakopoulos